ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2019

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Social Sciences 164

The perception of actions andinteractions

And the importance of context

JOSHUA JUVRUD

ISSN 1652-9030ISBN 978-91-513-0597-4urn:nbn:se:uu:diva-379490

Dissertation presented at Uppsala University to be publicly examined in Sal IV,Universitetshuset, Biskopsgatan 3, Uppsala, Friday, 3 May 2019 at 13:00 for the degree ofDoctor of Philosophy. The examination will be conducted in English. Faculty examiner:Professor Denis Mareschal (Birkbeck, University of London).

AbstractJuvrud, J. 2019. The perception of actions and interactions. And the importance of context.Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of SocialSciences 164. 102 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0597-4.

The perception of actions and interactions is a dynamic process linked with perceptualprocesses, the internal and external states of the individual, prior experiences, and the immediateenvironment. Given these differential contexts, it is very likely there are differences in howinfants perceive, interpret, and respond to actions. The present thesis took a developmentaland individual differences approach to understanding action perception and processing ininfancy. The overarching aim was to understand the development of action perception and howindividual differences contribute to the perception and processing of actions. More specifically,individual differences included the capacity to which variations in a child’s context can affectthe development of action perception. Study I demonstrated that, like adults, infants coulddifferentiate between physically possible and physically impossible apparent motion paths,as evidenced by pupil dilation. This perception may be related to the context of whether themotion was performed by a human figure or an object. Study II found that in the context of amore complex social interaction, infants differentiated between appropriate and inappropriateresponses to a giving action. Furthermore, infants’ individual differences in perceiving a givingaction were related to their own giving behaviors later in childhood, suggesting possiblespecialized mechanisms. Study III took an integrative perspective on context and demonstratedthe joint impact of internal and external emotional contexts for infants’ subsequent selectiveattention during visual search. Infants’ visual attention was affected by previous exposure to afacial emotion and by the mothers’ negative affect. The results of these three studies demonstratethat given differential environmental contexts and experiences, there are differences in howindividuals perceive and interpret actions and interactions. Together, this thesis proposes anintegrative role of context in perception and demonstrates that perception can never be trulydecontextualized.

Keywords: action understanding, action perception, motion, perception, context, environment,eye-tracking, pupil dilation

Joshua Juvrud, Department of Psychology, Box 1225, Uppsala University, SE-75142Uppsala, Sweden.

© Joshua Juvrud 2019

ISSN 1652-9030ISBN 978-91-513-0597-4urn:nbn:se:uu:diva-379490 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-379490)

To Laila

My companion, my therapist, my comfort, my joy, and my best friend

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Juvrud, J., Bakker, M., & Gredebäck, G. Context dependent per-

ception of apparent motion in 12-month-old infants and adults. Under review.

II Juvrud, J., Bakker, M., Kaduk, K., DeValk, J., Gredebäck, G, & Kenward, B. (2018). Longitudinal continuity in understanding and production of giving-related behavior from infancy to child-hood. Child Development, 90(2), e182-e191.

III Juvrud, J., Haas, S., Fox, N., & Gredebäck, G. How infants view the world: The functional role of emotional context and maternal affect. Submitted.

Reprints were made with permission from the respective publishers.

Additional scientific work not included in the thesis

1. Juvrud, J., Rennels, J.L., Kayl, A.J., Gredebäck, G., & Herlitz, A. (2019). Attention during visual preference tasks: Relation to caregiving and face recognition. Infancy.

2. Juvrud, J. & Gredebäck, G. (2019). Teleological stance: Past, present, and future. Developmental Science.

3. Juvrud, J., Gredebäck, G, Åhs, F., Lerin, N., Nyström, P, Kastrati, G., & & Rosén, J. (2018). The immersive virtual reality lab: Possibilities for remote experimental manipulations of autonomic activity on a large scale. Frontiers in Neuroscience, 12, 305.

4. Gredebäck, G., Lindskog, M., Juvrud, J., Green, D., & Marciszko, C. (2018). Action prediction allows hypothesis testing via internal forward models. Frontiers in Psychology.

5. Rennels, J. L., Juvrud, J., Kayl, A. J., Asperholm, M., Gredebäck, G., & Herlitz, A. (2017). Caregiving experience and its relation to perceptual narrowing of face gender. Developmental Psychology.

6. Juvrud, J., & Rennels, J. L. (2016). “I don’t need help”: Gender differ-ences in how gender stereotypes predict help-seeking. Sex Roles, 1-13.

7. Kaduk, K., Bakker, M., Juvrud, J., Gredebäck, G., Westermann, G., Lunn, J., & Reid, V. M. (2016). Semantic processing of actions at 9 months is linked to language proficiency at 9 and 18 months. Journal of Experimental Child Psychology.

8. Bakker, M., Kaduk, K., Elsner, C., Juvrud, J., & Gredebäck, G. (2015). The neural basis of non-verbal communication—enhanced processing of perceived give-me gestures in 9-month-old girls. Frontiers in Psychology, 6.

Contributions

The contribution of Joshua Juvrud to the studies was as follows:

Study I: Designed and planned the study in collaboration with main supervi-sor and co-authors. Created the stimuli, collected the data, performed the sta-tistical analyses, and wrote the manuscript with contributions from main su-pervisor and co-authors. Study II: Designed and planned the study in collaboration with main super-visor and co-authors. Created the stimuli, collected ½ of the data, performed the statistical analyses for the eye-tracking tasks, performed the correlation analyses, and wrote the manuscript with contributions from main supervisor and co-authors. Study III: Designed and planned the study in collaboration with main super-visor and co-authors. Created the stimuli, supervised two master’s students in data collection, performed data analyses in collaboration with a co-author, and wrote the manuscript with contributions from main supervisor and co-authors.

Contents

Introduction ................................................................................................... 13 Perception of actions ................................................................................ 15

The detection of motion ....................................................................... 16 Motion processing: Biological and non-biological motion ................. 17 Apparent motion .................................................................................. 21 Missing pieces ..................................................................................... 23

Goal-directed actions ................................................................................ 23 Development of goal-directed action perception ................................. 24 Social interactions and theory of mind ................................................ 25 Longitudinal links to theory of mind ................................................... 27

The gap between early understanding and later Theory of Mind ............. 28 Teleological Stance .............................................................................. 28 Statistical Learning .............................................................................. 28 Embodied Account .............................................................................. 29 Integrative perspectives ....................................................................... 30

The role of context ................................................................................... 31 Cultural context ................................................................................... 32 Emotional context ................................................................................ 33

Long-term consequences related to actions and interactions ................... 35 General cognitive-development ........................................................... 35 Missing links ........................................................................................ 39

Remaining questions ................................................................................ 39 Eye-Tracking ............................................................................................ 40

Pupil dilation ........................................................................................ 41 Current study ....................................................................................... 42

Aims .............................................................................................................. 43

Method .......................................................................................................... 44 Overview of the studies ............................................................................ 44

Participants .......................................................................................... 44 Measures .............................................................................................. 46 Apparatus ............................................................................................. 53

Summary of Studies Included in the Thesis ............................................. 54 Study I .................................................................................................. 54 Study II ................................................................................................ 58

Study III ............................................................................................... 65

General Discussion ....................................................................................... 69 How do infants differentially process simple human versus non-human motion? ..................................................................................................... 69

Core knowledge ................................................................................... 70 Top-down modulation of motion perception ....................................... 70 Why is there no motion path differentiation with an object stimulus? 71 What the pupil indicates: Potential neural underpinnings ................... 72

What are the long-term consequences of individual differences in early action understanding? ............................................................................... 74

Does early action-experience drive the understanding of others’ actions? ................................................................................................ 75 The importance of giving-related behavior ......................................... 75

How do differences in infants’ environmental and internal contexts influence their perception of the world? ................................................... 77

The emotional context ......................................................................... 78 An integrative approach to the impact of emotional contexts ............. 79

The importance of context ........................................................................ 80 Moving forward with context .............................................................. 80

Theoretical Implications ........................................................................... 81 Final Thoughts .......................................................................................... 82

Acknowledgments......................................................................................... 84

References ..................................................................................................... 87

Abbreviations

EEG ERP LMM PANAS SECDI STS

Electroencephalogram Event-related potential Linear mixed model Positive and negative affect schedule Swedish early communicative devel-opment inventory Superior temporal sulcus

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Introduction

We live in a world that is in motion. For humans, much of the motion we observe is not simply random movements in time and space. Rather, it is ex-hibited through actions structured by intentions, goals, and extensions of an individual’s thoughts and feelings. In a social event, our own actions can also join together with the shared goals of another person to form a social interac-tion. It is often said that “our actions define us”; and from a developmental psychology perspective, this is at least partially true. Developmental psychol-ogy demonstrates that actions and interactions are dynamic events that are in-tricately linked with perceptual processes, the internal and external states of the individual, prior experiences, and the immediate environment (Jacobs, Pinto, & Shiffrar, 2004; Thelen & Smith, 1996). Together, these form the con-text for which an action occurs.

The overarching aim of this thesis is to understand the development of ac-tion perception and how individual differences contribute to the perception and processing of actions. This is achieved by first investigating basic motion perception and what it reveals about mechanisms for perceiving different types of motion (Study I). I next examine how individual differences in in-fants’ own experiences shape processing of the world in the both the short- and long-term (Study II & Study III). Therefore, a particular interest for this thesis was also to understand action perception and processing in relation to various contexts of the child.

The introduction begins by reviewing what we know so far about the de-velopment of motion and action perception, beginning with how motion is immediately perceived at its most basic level. This is an important background for Study I. Next, I will describe the theories for how basic motion becomes organized and interpreted as goal-directed actions, and how simple actions develop to complex actions in a social context. This background is important for Study II, in which I explore how individual differences in action pro-cessing at 9-months predicts later action behavior at 2 years. Finally, I provide background on the various social factors that might influence the processing of social interactions, such as the emotional context of the child, which is in-vestigated in Study III.

Throughout this thesis, I make a differentiation between action perception and action processing. I define action perception as the ability for the human mind to apprehend motion, objects, and agents through the sensory modalities (Given, 2008). For example, inferring the motion path, the speed and direction

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of a moving object. The processes that result in action processing, on the other hand, include attending to an observed action, the neural processing of the action, recognition, evaluation, and then an action response (Goldstein, 2010). Action processing also involves considering perceptions in relation to individ-ual experiences, the environment, and contexts. The information from action processing influences an individual in evaluating and responding to an event. For example, predicting the end-goal of a person’s reach or knowing when to respond in a turn-taking exchange.

Since the early days of experimental psychology, the relation between per-ception and action has been featured prominently in many theoretical perspec-tives. For instance, developmental psychologist Jean Piaget famously ex-plored the relation between perception and action by examining how children form mental models of the world at different develop-mental stages. Under his classic developmental theory of schemas (Piaget, 1952), Piaget interpreted the failure of infants in the first two years of life to recover a hidden object as evidence that they do not keep in mind objects that are not perceptually present (Piaget, 1954).

The capacity for representation and conceptualization was presumed to emerge through developmental processes, that newborns are born with only a very simple repertoire of sensorimotor behaviors. Through development these are gradually integrated and internalized with perceptual abilities. This has been the stance of most of the classical early developmental theories (Baldwin 1906, Bruner 1973). Yet later work would go on to show that infants from an early age do in fact demonstrate a remarkable ability to represent motion and actions in relation to constraints, such as continuity and solidity, among other complex principles (Baillargeon, 1987a; Diamond, 1991; Marr, 1982; Spelke, Breinlinger, Macomber, & Jacobson, 1992; Ungerleider & Mishkin, 1982). Today, developmental perspectives largely agree that already from birth (Frankenhuis, Barrett, & Johnson, 2012) infants have the mechanisms in place to perceive motion in their environment. Within the first minutes of life, with the aid of these mechanisms, infants begin attending to complex actions and interactions in their environment. How these observations, experiences, and the environment of the child influence perception and processing throughout development is still debated (for review, see Bertenthal, 1996).

When young infants observe a social interaction, for example, they appear to interpret the actions of agents as helping or hindering one another based on the child’s own assumptions of empathetic rationality (Hamlin, Wynn, & Bloom, 2007; Kuhlmeier, Wynn, & Bloom, 2003). Some argue that this rela-tionship between action perception and processing represents an innate, core ability resulting from evolutionary pressure (Hoffman, Singh, & Prakash, 2015). Others theorists emphasize the role of active experiences shaping ac-tion processing through infancy, based on work showing a close link between visual and motor action experience and action understanding (Shapiro, 2010). The truth is probably closer to a combination of the infant’s brain, learning

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mechanisms, experiences, and context, which each interact to shape the de-veloping child.

Alva Noë writes that "Perception is not something that happens to us, or in us, it is something we do (Noë & Noë, 2004).” Indeed, infants are active agents in their own perception of actions. But the dynamic nature of perception and processing also includes various contextual factors, such as exposure to emo-tions or stress, or the environment in which the action is being perceived in. Already in the nineteenth-century, William Wundt and William James be-lieved the immediate context and the experiences of actions, and the conscious experiences related to those actions, were essential to understanding human perception (James, 1890; Wundt, 1980). Work that is more recent has contin-ued to support the notion that variations in an infants’ environmental context, such as cultural differences, shapes infants’ perception and processing actions (Lockman, 2000; Green, Lockman, & Gredebäck, 2016).

There is, unfortunately, no clear answer in the literature for what defines a context. But research examining contextual factors often include context as a broader sense of the child’s life. Therefore, the context of an infant can include prior experiences, immediate emotions, and various environments of a child. It can include the immediate short-term context, such as emotional primes in the environment, as well as a long-term context, for example life stressors. The context can even include another individual and interactions with people and objects (Walker-Andrews, 1997). Due to limited work examining these factors, particularly their joint impact on a person, there are still questions about how individuals and different contexts affect action perception and pro-cessing abilities during development. In this thesis I use the term context to broadly refer to the variability in both internal and external states of the child and the relation to environmental inputs during perception, including both short- and long-term effects.

Perception of actions When an adult observes the simple movements of someone reaching for a glass of water, they interpret the other person’s state of thirst and ultimate goal of quenching that thirst (Thorgrimsson, Fawcett, & Liszkowski, 2014). Like-wise, a skilled adult can observe the far more complex movements of a foot-ball player on the field and anticipate a run down the center or sweep to the outside, responding to and defending the action accordingly (Allard, Graham, & Paarsula, 1980; Williams, Davids, Burwitz, & Williams, 1994). In both cases, the perception of motion in action allows adults to extract information from observed movements in the occurring context. Together, with acquired knowledge and experiences, adults can then form an interpretation of inten-tions, beliefs, desires, and goals of the action (Behne, Carpenter, Call, & To-

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mosello, 2005; Liebal, Behne, Carpenter, & Tomasello, 2009). In some in-stances, the perception may directly impact the person’s own response or be-havior (Gräfenhain, Behne, Carpenter, & Tomasello, 2009). But how does this complex system emerge and develop?

The perception of action is an ability that involves multiple perceptual sys-tems that begin developing already from birth and provide a foundation for later interpretational systems, such as theory of mind (Blakemore & Decety, 2001). Infants are highly sensitive to others’ actions (Woodward & Guajardo, 2002). Recent work has begun to reveal the complexity of action perception already in infancy, such as being able to predict goals (Falck-Ytter, Gredebäck, & von Hofsten, 2006). There is an abundance of work investigat-ing the importance of the child’s experiences, such as with people and objects, for action perception. I will review research here that demonstrates the ways in which the developing infant’s perceptions of the world are shaped by the interaction between the brain, the environment, and learning mechanisms, be-ginning with simple motion perception.

The detection of motion Before reviewing the literature on infants’ development of action processing, I will first describe basic motion perception – how we detect motion and its developmental function. The detection of motion is one of the most basic and pervasive features of the visual system and is essential to perception, cogni-tion, and motor actions not only in humans, but across every species in which it has been examined (Braddick, Atkinson, & Wattam-Bell, 2003). At its most rudimentary level, visual motion detection is defined as “moving stimuli pro-ducing local changes in luminance and contrast as they pass over the ret-ina…exciting any neurons that are responsive to change” (Braddick, Atkin-son, & Wattam-Bell, 2003, p. 1769). This alone, however, does not ensure that an individual will extract motion information. To perceive motion, it also re-quires the visual system to detect speed and direction, incorporating multiple other neural systems.

Detecting motion is important in many of our visual abilities, such as depth perception (Rogers & Graham, 1979) and recognizing the dynamics and ori-entation of kinematic events (Bingham, Schmidt, & Rosenblum, 1995). It is also important for our own motor abilities, such as motor control (Milne et al., 2006). It is not surprising then that the ability to detect motion is found very early in development (Kuhlmeier, Troje, & Lee, 2010). Infants demonstrate a sensitivity to flickering lights already at birth (Vitova & Hrbek, 1970). Within the first weeks, newborn infants will also to show orientation to moving ob-jects (Volkmann & Dobson, 1976) and are highly sensitive to dynamic stimuli, such as moving people and objects (Regal, 1981; Tondel & Candy, 2007). A skill related to the detection of motion is the ability to stabilize gaze on a mov-ing target and to move gaze at the same angular speed as the moving target,

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known as smooth pursuit (von Hofsten & Rosander, 1997). The development of this ability to track moving objects has provided insight into the early func-tions and organization of the oculomotor system and the mechanisms under-lying motion detection. As early as 1-2 months of age, infants already begin to demonstrate the ability to follow moving objects (Aslin & Shea, 1981; Grönqvist, Gredebäck, & von Hofsten, 2006). Infants show considerable im-provements in following moving objects at 3 months and perform adult-like levels of smooth pursuit at 5 months (von Hofsten & Rosander, 1997).

The detection of motion is the result of a complex neural network, but there are two particular mechanisms thought to contribute to the development of motion perception: the ventral and dorsal cortical streams (Atkinson & Brad-dick, 2003; Braddick, Atkinson, & Wattam-Bell, 2003). The ventral stream (the “what pathway”) is involved with recognizing an object (Goodale & Milner, 1992). At birth, form processing appears in ventral cortical streams aiding in static orientation discrimination (e.g. detecting patterns) and by 3 weeks allowing for dynamic orientation, with improving global form coher-ence and increasing frequency sensitivity continuing through the first years of development (Atkinson, Wattam-Bell, & Braddick, 2002). The other im-portant system is the dorsal cortical stream (the “where/how pathway”), and is involved in the object’s location as well as motor control/adjustments (Goodale & Milner, 1992). This stream is responsible for motion-based seg-mentation and involves precise temporal organization of inputs to the visual cortex (Braddick, Atkinson, & Wattam-Bell, 1989). These two systems do not operate in isolation, but are rather both important for efficient visuo-cognitive and visuo-motor function. An object’s motion must be perceptually bound to the identity of the recognized object (Braddick et al., 2003). The MT+ com-plex is also activated by visual motion and is suspected to be major contributor in visual motion perception tasks including perceived motion direction and smooth pursuit (Rosander, Nyström, Gredebäck, & von Hofsten, 2007).

Motion processing: Biological and non-biological motion Motion processing involve the mechanisms allowing for infants to analyze detected motion, leading to subsequent expectations about the motion. It is clear from the literature that the human brain does not treat all motion as equal (Carter & Pelphrey, 2007; Stevens, Fonlupt, Shiffrar, & Decety, 1999). Two of the most well studied aspects of motion in which there appear to be differ-ences in motion processing are biological and non-biological motion. These two types of motion differentially contribute to the processing and interpreta-tion of more complex actions (Baldwin, Baird, Saylor, & Clark, 2001). In this thesis I define biological motion as the movements (and actions) performed by a biological agent. This is a broader definition than, for instance, Gunnar Johansson’s definition, which was primarily focused on the mechanisms of perceptual organization (Johansson, 1973). Since Johansson’s pioneering

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work on biological motion perception, much of the research has deviated away from Johansson’s original questions. Here I review some of the research on biological and non-biological motion as it relates to the development of in-fants’ processing of actions and action understanding.

Biological motion processing Detecting and interpreting the movements of others are important abilities for an infant’s survival (Miller & Saygin, 2013). This is evident by infants’ ability to detect human biological motion with even the most limited amount of visual information and when other visual cues are impoverished (Johansson, 1973). The perception of a particular pattern of motion, such as walking, was first described in 1973 by Uppsala University’s own Johansson in a series of in-genious studies using point-light displays. In these displays, motion is repre-sented through 12 dots on the mayor joints (feet, knees, hips, shoulders, el-bows, and wrists) of a moving person (e.g. Bertenthal, Proffitt, & Kramer, 1987; Blake & Shiffrar, 2007). This ability to detect point-light biological mo-tion has been observed even as young as the first days of life (Simion, Regolin, & Bulf, 2008). Such findings offer convincing evidence that newborns are predisposed to attend to motion information from a biological agent. Individ-ual differences in such motion tasks examining the perception of biological motion have been correlated with general perceptual and cognitive domains, such as tests of attention and executive function (Chandrasekaran, Turner, Bulthoff, & Thornton, 2010).

Not only do infants have a predisposition to prefer biological motion com-pared to non-biological motion, infants are born with the ability to detect cues for biological agents, such as self‐propelled motion (Luo, Kaufman, & Bail-largeon, 2009; Rakison & Yermolayeva, 2010). Self-propelled motion is a crucial visual cue that allows newborns to differentiate between self‐ and non‐self‐propelled objects. Self-propelled motion is one of the most powerful low‐level perceptual cues that convey that something is alive, or animate (Di Gior-gio, Lunghi, Simion, & Vallortigara, 2017). Infants recognize moving human bodies much earlier in development than static ones thanks in part to these cues (Christie & Slaughter, 2010). By 3 months of age this sensitivity to bio-logical motion extends to a unique preference specifically for human motion over any other kind of motion display, such as drifting dots (Bertenthal, 1993), and even other non-human biological motion (Fox & McDaniel, 1982).

Unique neural networks in the brain specialized for biological motion also point to an adaptive ability to detect life through motion. By at least 8-months there is specific activation of the right hemisphere that resembles that of adults when observing biological motion compared to scrambled motion (Hirai & Hiraki, 2005). A specific sensitivity for biological motion is further supported by evidence that there exist specialized neural mechanisms for registering bi-ological motion. When viewing point-light sequences of biological motion compared to scrambled versions, subjects show a distinct activation in the

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right-posterior-superior-temporal sulcus (STS) region (Grossman, Battelli, & Pascual-Leone, 2005). Several other neuroimaging studies have pinpointed the STS region to be uniquely activated by biological motion (Allison, Puce, & McCarthy, 2000; Grossman et al., 2000; Puce, Allison, Bentin, Gore & McCarthy, 1998). A study with 7- to 10-year-old children found increasing specificity for biological motion with age in the STS region (Carter & Palphrey, 2007).

Together, the body of work on biological motion perception points to a specialized sensitivity that is present already in early infancy and is distinct from non-biological motion. This sensitivity for biological motion is adaptive because it enhances our ability to recognize a human body performing an ac-tion (Fox & McDaniel, 1982). An efficient means to recognize human biolog-ical motion at an early age is essential for identifying a relevant agent. Once the agent is identified, an infant can then interpret the motion and understand the goals of that agent. This is seen, for example, in older children who will interpret human motion, such as reaching, differently than other similar non-human movements, such as a mechanical claw reaching (e.g., Bertenthal, 1993; Christie & Slaughter, 2010; Ellsworth, Muir, & Hains, 1993; Legerstee, Pomerleau, Malcuit, & Feider, 1987; Reid, Hoehl, Landt & Striano, 2008; Woodward, 1998). This bias for attending to biological information in the con-text of motion and actions may also be a foundation for later social cognition (Miller & Saygin, 2013). However, there is limited longitudinal research on the relationship between early understanding and later social cognition and behaviors.

Non-biological motion processing Infants appear to process objects differently than biological motion. By at least 2 months of age infants begin to reason about non-biological objects that are in motion. At this age in development, infants show the ability to form expec-tations that objects will continue to exist and move continuously in time and space (Baillargeon, 1987a, 1993; Spelke, 1990; Spelke et al., 1992). This abil-ity can be thought of in two terms. First, in terms of continuity of objects in motion; that objects move on connected, unobstructed paths. Second, in terms of solidity of objects in motion; that objects move only on unobstructed paths and that no parts of two distinct objects coincide in space and time. Much work has been done to explore the nature of “intuitive physics”, the notion that knowledge about objects, substances, and number concepts are found in rudimentary forms very early in infancy (Hespos & vanMarle, 2012). Some of the first work to demonstrate knowledge of object principles in infancy has shown that by 3 months of age infants expect that solid objects cannot pass through one-another (Spelke, 1994). Another study showed that 3.5-month-old infants looked longer at a rotating screen that appeared to pass through a solid box (Baillargeon, 1987b), indicating surprise. Infants’ knowledge at this age regarding object principles are primitive (Hespos & vanMarle, 2012); but

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over the course of infancy, development of previously mentioned neural path-ways and additional active and observational experiences expand this knowledge (Hinnius & Bekkering, 2014). For example, 8-month-old infants who had been trained to shake a novel rattle that produced a specific sound showed activation in the motor areas of the brain to just the rattle sound when tested later (Paulus, Hinnius, Van Elk, & Bekkering, 2012).

Understanding how infants perceive the trajectory, velocity, and amplitude of a moving non-biological object has been studied using an extension of Pia-get’s original occlusion work (the A not B task; Piaget, 1954) in which infants typically observe a self-propelled object passing behind an occluder (e.g., Gredebäck, von Hofsten, & Boudreau, 2002; Gredebäck & von Hoftsen, 2004). At 4 months, infants can already predict the motion of a linear trajec-tory (Gredebäck et al., 2002) and by 6 months can predict a circular trajectory (Kochukhova, & Gredebäck, 2007). Learning associated with occluded ob-jects appears to be closely linked with infants’ development of object repre-sentation, an ability that is developed around 4 months of age. After this age, however, it is clear that infants are able to represent and predict the appearance of a moving object after passing behind an occluder (Mareschal, 2000).

Together, the large body of work covered here suggests that infants can reason about the motion of non-biological objects. Infants assume that objects do not disappear from existence when passing behind an occluder (Ko-chukhova, & Gredebäck, 2007). Infants also understand that two objects can-not occupy the same space (Spelke, 1994) and that an object in motion should continue on a linear trajectory, even when being occluded (Johnson et al., 2003). One perspective is that non-biological motion perception is largely un-related to the perception of a biological agent’s motion, who are more unpre-dictable and not subject to such properties as continuous motion paths (Saylor, Baldwin, Baird, & LaBounty, 2007). Thus, the same principles for an object may not apply for a human figure. It is unquestionably true that human agents are able to overcome the immediate physical forces and act according to in-ternal processes and motivations (Poulin-Dubois, 1999). However, it can be argued that a rich understanding of the physical world is also required in order to perceive and make sense of the social world. There are indeed clear differ-ences between the perception of biological and non-biological motion. At the same time, the perceptions of both are at the center of mature, common sense conceptions, and allow infants to operate on representations of the world (Spelke & Kinzler, 2006). For instance, predicting where an object or person will reappear after disappearing behind an occluder.

There is no clear-cut border between stimuli that can be seen as biological, agent-like, and non-biological moving objects. But as an infant develops, there is a strong bias for interpreting and explaining even non-biological events in terms of agency (Johnson, 2003). In one of the more classic experiments, Hei-der and Simmel (1944) showed that older children and adults readily interpret and explain events involving moving geometric shapes randomly moving

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about as interacting agents. This initial experiment has since had many repli-cations and extensions (see Galazka & Nyström, 2016; Galazka, Roché, Nys-tröm, & Falck-Ytter 2014; Scholl & Tremoulet, 2000; Scholl & Gao, 2013). There is also work with young infants showing when infants are provided with additional or alternate information, such as learning that an object is self-pro-pelled, this can modify their expectations regarding the trajectory of that ob-ject (Kochukhova & Gredebäck, 2007; Luo, Kaufman, & Baillareon, 2009). These latter studies once again suggest the importance for how an infant per-ceives an object or figure, as well the infants’ own experiences, for shaping individual processing and expectations of events. It is therefore important to better understand the development of immediate motion perception as it re-lates to both biological and non-biological motion.

Apparent motion Even at the most basic level of perception, such as in still images, humans seem to be hard-wired to anticipate motion. Examining such low-level forms of motion perception is one way to understand the development of action pro-cessing and interpretation. When individuals view stationary stimuli, such as a still photograph of an object in the process of motion, there is nonetheless an anticipation of movement for the observer (David & Senior, 2000). For example, when shown a photograph of an athlete in the posture of throwing a ball, the brain region that is specialized for processing visual motion is acti-vated. This demonstrates a readiness to perceive motion. However, there is no activation in this area of the brain when shown a photo that has no dynamic information, such as someone sitting in a chair.

Presenting still images of a figure or object on screen in succession at a specific time interval, or frame rate, also gives rise to a powerful perception of fluid motion. This phenomenon is referred to as apparent motion, a robust visual effect that uses our dependency on timing information when interpret-ing motion. For example, when watching the rapid display of two static images involving a human actor shifting position, one experiences the phenomenon of perceiving the human in motion, despite there being no actual motion at all.

There are two types of apparent motion, which were first described by Max Wertheimer (Wertheimer, 2012), and are distinguished by the frame-rate in which they are presented. Phi movement is considered “pure movement” and is considered analogous in experience to real motion (Braddick, 1974). The second is called beta movement and can involve images being shown quickly or up to several seconds. In both cases, the static images do not actually phys-ically change but instead give rise to the appearance of motion because they alternate faster than what the eye can see (Boring, 1942). In a classic example, a group of circles is shown that switch on and off in quick sequence. In beta movement, the circles appear as if they are moving, while in phi movement the circles appear stationary and instead movement is perceived around them.

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There is evidence that apparent motion impressions, particularly phi move-ment, involve an explicit representation of movement, much like that which occurs during motor imagery. For example, one study found that the timing of apparent motion and an imagined object rotation is similarly chronometric (e.g., in similar timing; Cooper & Shepard, 1973).

In infancy, the apparent motion phenomenon has been important for con-tributing to our understanding of mechanisms underlying motion perception. Using apparent motion has allowed researchers to test the degree to which infants’ perception of motion is mediated by several possible underlying mechanisms, such as sensitivity to temporal variations in luminance, relative position of patterns, or time-locked spatial displacements (Aslin & Shea, 1990). This work has suggested that in infants as young as 6 months old, ap-parent motion perception is based either on a position-sensitive or a motion-sensitive mechanism. In another study, it was shown that apparent motion stimuli aided in 7-month-olds’ perception of visual temporal information (Brandon & Saffran, 2011).

The apparent motion phenomenon has traditionally been explained by a bias in the visual system toward selecting the simplest interpretation of the images, which is the shortest and most direct path (Stevens et al., 1999). This explanation for a very low-level phenomenon mirrors many well-supported explanations for more complex action perception - that we interpret actions in their most efficient means. For apparent motion stimuli, it is believed that mo-tion dominates the perception and the figure is dependent on the motion (Kolers, 2013).

Similar to complex motion perception studies, findings of apparent motion displays have suggested that perception operates somewhat differently when the object presented is a human figure. Shiffrar and Freyd (1990) first found that, when using apparent motion stimuli of a human hand placed behind and in front of a leg, adults will report seeing an indirect but biologically possible paths of apparent motion (the hand moving around the leg). Perceiving the shortest path would instead mean that the hand would pass through the leg, a biologically impossible action. This means that the perception of motion paths is not independent from the characteristics of the object. Adding to these find-ings, Stevens and colleagues (1999) used similar stimuli and demonstrated that the perception of biologically impossible and possible apparent human movements is mediated by distinct brain regions. They found that a perceptual shift between a possible and impossible path in adult observers was accompa-nied by a shift in activity between the ventral prefrontal cortex and motor and sensory regions in the brain. Using PET to measure changes in regional cere-bral blood flow the authors demonstrated that possible human movements showed activation in the motor and sensory regions of the brain. Impossible human movements, in contrast, was accompanied by activation in the ventral prefrontal cortex in adults. Interestingly, they failed to find this effect when

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the apparent motion stimuli consisted of a non-biological ball and a barrier. That is, participants reported a ball passing through a barrier.

Missing pieces It is well established that infants and adults make a distinction between bio-logical and non-biological motion (Legerstee et al., 1987; Legerstee et al., 1990; Ellsworth et al., 1993) and are attuned to specific biological cues, such as motion (Bertenthal, 1993). It is not fully understood, however, what differ-ence biological or non-biological stimuli makes for perceiving motion paths. For example, we do not know how if, similar to adults, infants are already applying constraints to human apparent motion. Can infants differentiate be-tween physically possible and impossible apparent motion paths performed by a human, similar to adults? This differentiation would require some degree of understanding of the constraints on human actions already in infancy. Results from adults suggest the perception of physically possible and impossible hu-man motion selectively activates motor and sensory regions of the brain (Ste-vens et al., 2000). This occurs even when no physical motion is actually ex-plicitly presented during apparent motion stimuli. But we don’t know if the same process applies to infants. Using apparent motion, researchers can ob-serve motion at its most basic level, in which a motion path is not actually observed, but rather interpreted.

Goal-directed actions With this distinction between biological and non-biological motion also comes differential expectations of actions depending on the agent (Johnson, 2003). In particular, the detection of and bias toward biological motion may be adaptive to allow infants to understand the intentions of human agents from an early age (Frith & Frith, 1999). Infants and adults alike make sense of bio-logical actions and behavior in terms of other agents’ goals. Unlike non-bio-logical motion, infants will typically interpret the actions as driven by inten-tions (Woodward, Sommerville, & Guajardo, 2001). If biological motion is detected, then goals can be perceived, allowing us to move from the detection of movements to the interpretation of action goals.

Actions serve a functional role in humans, as evidenced by how we tend to prefer to see others’ behaviors as means to ends (a goal; Csibra, Gergely, Bı́ró, Koos, & Brockbank, 1999; Csibra, 2008; Southgate, Johnson, & Csibra, 2008). Even in simple two-dimensional moving shapes (Heider & Simmel, 1944). As long as the animate object shows signs of agency – that it is self-propelled and varies suddenly in velocity and trajectory – then humans will infer complex internal states, such as goals, in that object (Csibra, 2003; Kas-sin & Baron, 1985; Opfer, 2002). A large literature has been developed since

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Heider & Simmel’s (1944) seminal work exploring the perception of animacy (e.g., Gelman, Durgin, & Kaufman, 1995), intentionality (e.g., Dasser, Ulb-aek, & Premack,, 1989; Galazka, Bakker, Gredebäck, & Nyström, 2016), goal directedness (Csibra, 2008), action understanding (Baker, Saxe, & Tenen-baum, 2009), and the perception of interactions (Galazka et al., 2014). To-gether, this body of work has shown that sensitivity to the goal of an action is important for many aspects of socio-cognitive development, such as language development (Baldwin & Moses, 2001) and problem-solving abilities (Car-penter, Call, & Tomasello, 2002).

Brain areas associated with perceiving intentionality are activated when observing an agent performing a goal-directed action (Castelli, Happé, Frith, & Frith, 2000). Specifically, the activation of the superior temporal sulcus and superior temporal gyrus suggest that these brain regions are important for the identification and processing of intentional agents and intentional actions (Saxe, Xiao, Kovacs, Perrett, & Kanwisher, 2004). Observations of an agent engaging in motion that appears to be goal-directed, whether it is a human or an object (such as the previously mentioned abstract geometric shapes), results in increased activation in STS. Activation in this area further increases when this action is understood to be intentional (Castelli et al., 2000). When subjects observe two agents engaging in chasing behavior, they show activation in the STS as well as the superior temporal gyrus. But this activation only occurs when they understand that the behavior of the agent is intentional, such as ‘chasing’ instead of simply ‘following’ (Schultz, Imamizu, Kawato, & Frith, 2004).

Development of goal-directed action perception The ability to infer goal-directedness has been proposed to be “one of the first elements present in children's reasoning about human behavior, preceding an understanding of beliefs and other cognitive processes” (Woodward, 1998, p. 3; Poulin-Dubois & Schultz, 1988; Premack, 1990; Wellman, 1992). As early as 3 months of age, infants demonstrate the understanding that a human grasp-ing an object will support that object, but expect the object to fall when the hand releases the object (Needham & Baillargeon, 1993; Baillargeon, 1995). At 5 months of age it has been shown that infants can understand the effects that constraints, such as physical barriers, have on a human reaching for ob-jects (Baillargeon, Graber, Devos, & Black, 1990). By 6 months it has been shown that infants can encode the actions of a human in ways that are con-sistent with goal-directed actions. Infants were habituated to an actor reaching for one of two toys and then showed surprise when the actor reached for a different toy during test, but not when the actor simply changed motion paths for the toy. Interestingly, infants did not show this pattern of looking when it was a mechanical arm reaching for the toy (Woodward, 1998).

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Other studies have shown that by 6 months, infants will encode the goal of a human action by fixating on that goal before the hand arrives at the target (Cannon, Woodward, Gredeback, von Hofsten, & Turek, 2012; Kanakogi & Itakura, 2011). For example, in Falck-Ytter and colleagues’ (2006) study, in-fants from 12 months predicted the action goal of an actor moving a ball to a bucket, but failed to do so when the balls appeared to be self-propelled and moved on their own. Infants at this age also can predict a hand reaching for a target (Ambrosini et al., 2013; Kanakogi & Itakura, 2011). Together, there is sufficient evidence to suggest that sometime between 6- to 9-months most in-fants have developed the ability to identify an agent’s goal and analyze its actions causally in relation to it (Gergely, Csibra, Biró, & Koós, 1994; Gergely. Nadasdy, Csibra, & Biró, 1995).

Work with older infants shows a pattern of behavior that supports the au-tomatic inference of goal-directed actions when observing human actors. As children develop, so does their ability to predict a wider range of human ac-tions, such as predicting interactions involving putting objects in containers (Falck-Ytter et al., 2006; Rosander & von Hofsten, 2011) and solving puzzles (Gredeback & Kochukhova, 2010). By 10 months infants can also interpret human actions as goal directed, even when observing an action that is never completed (Brandone & Wellman, 2009). Infants can also interpret goal-di-rectedness in action events where the goal is unseen (Csibra, Bíró, Koós, & Gergely, 2003). One of the first studies on goal-directed actions demonstrated that 12-month-olds were sensitive to goal-directed actions of an agent. In this study, infants were first habituated to an event in which two balls moved in alternation as if talking to each other, followed by the smaller ball (the agent) traversing the distance that separated the two and jumped over a barrier to reach the goal. During subsequent test trials, when the barrier was removed, infants were surprised when the agent moved to its goal using the same trajec-tory of motion as before (the agent jumped even though the barrier was ab-sent), rather than taking a straight linear path (Gergely et al., 1995).

In another series of studies, Meltzoff (1995) showed that 18-month-olds would infer the goal of a failed action and imitate the completed action, rather than the observed failed behavior. This has been suggested to be evidence that older infants show spontaneous imitation related to their understanding of in-ferred goals. Together, these studies suggest that infants selectively attend to the intended goals of action events over other more salient properties (such as movement or features; Woodward, 1998). When a person identifies an agent, they see the actions of that agent not as raw physical movements, but rather as structured by intentions (Woodward & Gerson, 2014).

Social interactions and theory of mind In adulthood, conceptions of intentional actions are complex; they involve conceiving of others’ actions in terms of an intentional relation to a real or

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abstract goal and allow for explaining and predicting own and others’ behav-iors (Baldwin & Baird, 2001). Theory of mind is the ability to attribute and understand the mental states of another person, including beliefs, desires, emotions, and knowledge (Premack & Woodruff, 1978). This is important for making sense of other people’s actions and engaging in appropriate social in-teractions. Theory of mind has received a considerable amount of research attention (Leslie, 1987; Leslie, Friedman, & German, 2004; Wellman, 1985; Wellman, Lopez-Duran, LaBounty, & Hamilton, 2008).

When actions occur in a social context, with two or more individuals shar-ing intentions and goals, we can say that a social interaction is taking place. Theory of mind underlies social interactions, the processes in which we act and react to other people around us (Carrington & Bailey, 2008). It involves a person performing actions toward another with shared goals and understand-ing the other person’s response to their actions. Between 2 to 5 years of age, it is proposed that children go through a major conceptual change in how they view the mental states of others (Wellman, Cross, & Watson, 2001). At 2-3 years of age children begin showing that ability to discriminate between an experimenter’s intentional vs. accidental behaviors (Call & Tomasello, 1998). Also at around this age there is evidence that child begin to understand that other individuals can have false beliefs, those beliefs that contradict reality (Wellman, et al., 2001). It is largely agreed upon that at least by 4 years of age, children are able to explain and predict others’ actions by attributing causal intentional mental states to them, such as intentions (Flavell, 2004; Henry, Phillips, Ruffman, & Bailey, 2013).

There is growing evidence that children already at 13 and 15 months of age may be sensitive to the beliefs of others (Onishi & Baillargeon, 2005; Surian, Caldi, & Sperber, 2007). Using an anticipatory looking paradigm, Southgate, Senju, and Csibra (2007) additionally showed that by 25-months children could predict a behavior in line with a false belief. Not without controversy, this body of work suggests that infants may have an early-developing system that operates on an implicit level. This would mean that children’s difficulties on other theory of mind tasks may instead be a result of performance limita-tions due to explicit developmental constraints.

One recent large-scale study, however, attempted to replicate the findings from influential anticipatory-looking implicit theory-of-mind tasks using orig-inal stimuli and procedures (Kulke, von Duhn, Schneider, & Rakoczy, 2018). Results revealed that very few of the original studies replicated and the corre-lations among the studies were very poor. Another attempt similarly failed to replicate some of these findings (Schuwerk, Priewasser, Sodian, & Perner, 2018). Continuing the debate, Baillargeon, Buttelmann, and Southgate (2018) responded to failed replication attempts pointing out study procedures and participant motivation and attention differences that could explain the lack of replication. Therefore, it remains unclear to what extend infants between 6 and

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36 months demonstrate a false-belief understanding. Nonetheless, investigat-ing how and when theory of mind emerges has been important for understand-ing a wide variety of concepts related to a child’s development, including emotions, beliefs and false-beliefs, desires, and reasoning, among others (e.g., Astington, 1993; Wimmer & Perner, 1983).

A large literature of neuro-imaging studies shows that there are multiple regions in the brain and in both hemispheres that form a network for theory of mind processes (Carrington & Bailey, 2008; Grezes, Frith & Passingham, 2004a,b). This network includes parts of the prefrontal cortex, STS, and the amygdala (Brothers, 1990). There is also evidence that different mental states, such as false belief and the detection of deceit, show activation in distinct brain regions (Grezes, et al., 2004a,b).

Longitudinal links to theory of mind A particular area of research where there are specific predictions regarding long-term consequences is the association between early action understanding and later theory of mind, including desires and beliefs (Aschersleben, Hofer & Jovanovic, 2008). Many theorists believe that early understanding of inten-tional action is an important foundation and precursor for later theory of mind abilities (e.g. Blakemore & Decety, 2001). There is strong evidence to show a connection between infants' understanding and abilities in the first two years of life and the understanding of mental states during childhood (Charman et al., 2000; Olineck & Poulin‐Dubois, 2005; Wellman, Phillips, Dunphy‐Lelii, & LaLonde, 2004). In a comprehensive longitudinal study examining gaze following, language, and theory of mind, Brooks & Meltzoff (2015) found cascading predictive effects, such that gaze following ability at 10.5 months predicted use of mental-state terms at 2.5 years, and that the use of mental-state terms at 2.5 years predicted theory of mind at 4.5 years.

As previously reviewed, infants show a developing ability to recognize agents and understanding intentional actions by agents (e.g., Gergely, et al., 1995; Woodward 1998). For example, 6-month-old infants’ action under-standing was shown to be related to theory of mind at 2 years (Aschersleben, et al., 2008). In a study by Wellman et al. (2004), they found that 14-month-olds’ habituation to human intentional actions significantly predicted theory of mind at 4 years of age. In a more recent study, Wellman et al., showed not only a longitudinal link between infant attention to human action and later theory of mind, but also that it holds across multiple age groups and a variety of tasks. Furthermore, the robustness of the association (r = .40 to .50) re-mained even when controlling for other general information processing abili-ties (IQ, verbal competence, and executive function).

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The gap between early understanding and later Theory of Mind In the next section, I will focus on several prominent theoretical perspectives that account for how infants learn to reason about goal-directed actions. Im-portantly, these theories also attempt to explain the gap between action under-standing in early infancy and later theory of mind abilities in childhood.

Teleological Stance Some theorists have suggested that physical principles of action perception serve as precursors to understanding that other people have mental states, de-sires, and beliefs. This theory is referred to as the teleological stance (Aschersleben et al., 2008). This theory proposes a principle of rationality that underlies action perception – that infants expect an agent to perform an action in the most economical manner possible (Csibra & Gergely, 2007). When an observed behavior is judged to be the most efficient action toward a goal given environmental constraints, it is said that a teleological representation of the action is created (Csibra & Gergely, 2003). This means that infants are able to represent goal-directed actions in a way that allows them to draw inferences toward unobserved states, and to rationally interpret an action based on the shortest or most efficient path (Csibra et al., 2003; Gergely, 2003; Paulus & Sodian, 2015).

Teleological interpretations are proposed not to be an explicit inferential system, but a tendency to construe events in accordance with a certain formal structure (Csibra, et al., 2003). The structure consists of three aspects of reality that infants use to represent actions. The (1) goal and the physical constraints allow infants to infer new actions; (2) knowing the constraints and observing the action allow infants to infer goals; and (3) knowing the actions and the goal, infants can infer the constraints. Csibra and colleagues (2003) propose that teleological interpretations involving this structure occur through the pro-cesses of hypothesis formation and verification, with the principle of rational-ity serving two important functions. First, as the criteria for action interpreta-tions, and second, as an inferential principle guiding and constraining the con-struction of such action interpretations.

Statistical Learning There are several other prominent perspectives that account for the findings in infancy that have shown rather complex abilities and understanding within the first year, prior to the emergence of theory of mind. One theoretical per-spective proposes that infants are endowed with a more general-purpose mechanism that allows for statistical learning (Brandone, Horowitz, Aslin, & Wellman, 2014; Cicchino, Aslin, & Rakison, 2011; Henrichs, Elsner, Elsner,

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Wilkinson, & Gredebäck, 2014). It is thought that this broad and flexible mechanism allows infants to compute elements from environmental input and to extract patterns for learning (Saffran & Kirkham, 2017).

Indeed, studies have shown that that infants’ understanding of an agent’s motion is related to how frequent they are visually exposed to such events in their environment (Cicchinoet al., 2011; Henrichs et al., 2014). Therefore, this perspective would suggest that infants’ ability to understand goal-related ac-tions and the ability predict others’ actions is a result of increasing experiences observing such goal-directed actions (Aslin & Newport, 2012; Perruchet & Pacton, 2006). Statistical learning mechanisms are integrated with other cog-nitive systems and occurs implicitly and quickly (Turk-Browne, Scholl, Chun, & Johnson, 2009); this makes sense in early infancy, as a statistical learning mechanism would be rather ineffective should it continuously require con-scious and deliberate processing.

Embodied Account Another dominant model is the embodied account, in which active experi-ences directly contribute to infants’ ability to view action as goal-centered (Flanagan & Johansson, 2003). In particular, infants’ first-hand experience as active agents acting on the world serves a role of supporting their ability to construct goal-centered action representations (Sommerville, Woodward, & Needham, 2005). Over the first year of life, infants’ increase their experiences of goal-directed actions and this increase in self-experience contributes to the construction of goal representations of actions (Sommerville et al., 2005). Ac-tions become more precise and refined through experience, and the infor-mation from these experiences provides a foundation for understanding other’s actions and interpreting a wide range of actions in relation to goals. Needham, Barrett, and Peterman (2002) demonstrated an embodied account for infant learning by applied a sticky-mitten paradigm with 3-month-olds. In this paradigm, infants’ hands are dressed in a Velcro covered mitten that allow infants to “reach for and grasp” objects prior to the onset of natural grasping (also see Bakker, Sommerville, & Gredebäck, 2015; Libertus, Joh, & Need-ham, 2016) This provides young infants with ‘enriched’ reaching experience. Those infants that had taken part of the sticky mittens training exhibited changes in their object-directed activity, performing more intentional reaches. It appeared that the additional experience of interacting with objects while wearing the mittens facilitated the development of infants’ object-directed ac-tivity, regardless of whether the mittens were on or off (Needham, et al., 2002).

Mirror neurons are proposed to mediate our capacity to understand the meaning of actions, intentions, feelings, and beliefs of others. Mirror neurons are one of the neurological underpinnings for an embodied perspective that have received significant attention (Gallese, Fadiga, Fogassi, & Rizzolatti,

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1996; Rizzolatti, Fadiga, Gallese, & Fogassi, 1996; Woodward, & Gerson, 2014). There are a wide range of hypotheses about mirror neurons (Wood-ward, & Gerson, 2014), but they can be described simply as “premotor neu-rons that fire both when an action is executed and when it is observed being performed by someone else” (Gallese, 2009, p.520). It is thought that when observing an action, there is an automatic activation of the same neural mech-anisms in the observer, providing a direct mapping between visually observing an action and executing the same action. This mapping allows an observer to extract meaning from what is being observed based on the motor experiences of the observer. In the first study to demonstrate mirror neurons, a subset of premotor mirror neurons in macaque monkeys would fire during the observa-tion of partially hidden actions. This meant that the mirror neurons were acti-vated not only in response to what was visually being observed, but also acti-vated from a stored motor “representation” in the premotor cortex to the un-observed prediction of the action’s end goal-state (Umilta et al., 2001). Through an embodied perspective, mirror neurons are a neural mechanism that allows a direct form of action understanding (Gallese et al., 2005; 2009; Rizzolatti et al., 1996). Mirror neurons alone are not sufficient to explain all action perception, but such a neural mechanism could aid in the coding of actions and their goals by mapping observed or implied motor acts onto motor neural substrates in the observer’s motor system (Woodward, & Gerson, 2014).

Integrative perspectives The statistical learning, teleological stance, and embodied account may seem at first to be in conflict with one another. On one hand, teleological processes emphasize a system built on a potentially hard-wired rationality principle that guides action interpretation, while both statistical learning and an embodied account speaks more toward an experience-dependent approach. However, several frameworks demonstrate that these perspectives may explain different components of the development of action understanding and are actually quite compatible with one another. For example, in a recent review it was proposed that the embodied account might actually be a parsimonious explanation for several of the existing gaps in the teleological model, such as the origins and learning mechanisms involved in the teleological system (Juvrud & Gredebäck, in press).

Neoconstructivist Perspective Another larger theoretical framework, neoconstructivism, also offers a devel-opmental theory that expands on many of Piaget’s initial ideas of constructiv-ism: building concepts from simpler perceptual and cognitive precursors. In particular, Piaget emphasized an interaction between the child and his/her en-vironment (Newcombe, 2011). Neoconstructivism is rooted in this emphasis

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on developmental mechanisms and explains how new knowledge emerges, as well as how new knowledge builds on existing knowledge in the developing infant (Mareschal, 2011). This model argues that “a biologically prepared mind interacts in biologically evolved ways with an expectable environment that nevertheless includes significant variation” (Newcombe, 2011, p. vi). It also makes the assumption that the world is richly structured in such a way to create perceptual redundancies that support experience-expectant learning (Newcombe, 2011). This is the process of mechanisms being shaped by expo-sure to typical aspects in the environment. For example, it is thought that in-fants’ emergence of emotion processing is shaped by a sensitivity and expo-sure to species-typical aspects of emotional expressions (Leppänen & Nelson, 2009). As with teleological explanations, neoconstructivism acknowledges that infants might be equipped with specific content, such as computing capa-bilities (i.e., statistical learning; Saffran, Aslin, & Newport, 1996). These abil-ities aid in and form a basis for learning. However, similar to the embodied account, the model also emphasizes the roles of action experiences, which are rich with information for learning.

Evidence across multiple studies (Csibra, 2008; Gergely, 2003; Hunnius & Bekkering, 2010; Paulus & Sodian, 2015; Skerry, Carey, & Spelke, 2013) point to the importance of constraints within both the immediate and prior environment for perceiving goal-directed behavior that allows for the inter-pretation of an action. A particularly important contribution of Neoconstruc-tivism is the emphasis on context-dependent development. It includes the im-portance of collective agency, for example, which includes other people in the environment who provide information (van Geert, 2017). Neoconstructivism acknowledges the complexity of the “system” of the child – the ways in which the brain, biology, environment, and learning mechanisms continuously inter-act (Mareschal, 2011).

The role of context The growth of the system of the child is context bound, with new representa-tions increasingly dependent on the prior ones (Mareschal, 2011). The infants’ action, specifically, is a product of the constant and dynamic interaction be-tween the infant and the context (Fogel, 1990). It is not the action alone that is important for perception; it is also the context that constrains the action. Given differential environmental contexts and experiences, it is very likely there will be differences in how individuals perceive, interpret, and respond to actions (Fischer & Hencke, 1996). In two experiments using an eye-track-ing paradigm, Gredebäck and Melinder (2010) showed that, as early as 6 months of age, infants were sensitive to the action goals and the contextual constraints of humans in a social interaction context. The first experiment showed a sensitivity for goal-directed actions. Not only did infants anticipate

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a spoon arriving at the mouth during a feeding action between two people, but infants also showed increased pupil dilation when one individual moved the spoonful of food to the other person’s hand instead of mouth. The action of putting the food on the person’s hand was an inefficient means to the goal and infants reacted with surprise to this particular action. It is very likely that the experiences of the child played an integral role in shaping the expectations and interpretations of such feeding actions, as feeding actions are particularly relevant at this age. Infants also took the contextual constraints into account during the second experiment when a barrier was introduced. The findings showed that infants’ pupils no longer dilated when the food was passed to the actors hand when there was a barrier that blocked the direct path to the per-son’s mouth. Passing the food to the hand was now the most efficient means to reach the goal, as the barrier constrained the agent’s actions.

In another study, it was shown that infants could construe an ambiguous action (touching the lid of a box) as goal-directed (retrieving a toy within the box), but only when they first observed the action within a particular context (Woodward & Sommerville, 2000). Infants' would only interpret the ambigu-ous action as being directed at the toy within the box if infants first saw the ambiguous action embedded in a sequence. The sequence culminated with an action infants readily construe as goal-directed - grasping a toy inside the box. Additional work has also shown that infants can learn information from one context of an own-action event and transfer that knowledge to another visually observed event (Sommerville et al., 2005). In the study by Sommerville and colleagues, infants’ own-action experience with reaching allowed them to form a goal-based representation of a separately observed reaching action.

Together, this body of work points to the functional role of context in learn-ing and interpreting goal-directed action events. The components of the con-text may even be inseparable from understanding or performing an action for the child (Woodward & Sommervill, 2000). The context of an infant can also include a wide range of components, such as both prior and immediate states, whether it is a social context, and the internal and external states of the child.

Cultural context Studies that have examined goal-directed actions have suggested that this abil-ity is constrained by the cultural context of a child and is not a universal con-struct. While there is little cross-cultural research on the early development action understanding, one study showed cultural differences in early socio-cognitive abilities between 1 and 2 years of age (Callaghan et al., 2011). These abilities included gaze following, imitation, helping, and communicative pointing. Another study with even younger infants demonstrated that Swedish and Chinese 8-month-olds showed different patterns of action prediction for feeding behaviors (Greenet al., 2016). Swedish infants would fixate at the

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mouth when food was handled with a Western-style spoon, while Chinese in-fants failed to make such a prediction to the same goal. The opposite pattern was true when the feeding utensil was chopsticks; Chinese infants would fix-ate to the mouth, while Swedish infants no longer predicted the feeding action. This work provides evidence that the surrounding cultural context has an in-fluence on goal prediction during action observation. Importantly, it also demonstrates that not all individuals perceive similar actions in the same way.

Emotional context Another very salient context for infants is the emotional context that is pro-vided by a nearby adult. A child can learn associations between emotional contexts and their environment, resulting in individual differences in percep-tual mechanisms becoming attuned to the contingencies of affective signals in the environment (Pollak & Sinha, 2002). Previous work has demonstrated, for example, the effects of experience on the formation of perceptual representa-tions of basic emotions in children (Pollak & Kistler, 2002). Emotion recog-nition of faces is a component in the development of social cognition that is important in regulating a wide range of behaviors and is critical for infants’ understanding of social phenomena (Adolphs, 2002). For developing infants, facial expressions help convey meaning and intention. (Trautmann, Fehr, & Herrmann, 2009). Facial expressions also influence distribution of visual at-tention (Vuilleumier & Schwartz, 2001). Numerous studies have examined the development of emotion processing in social cognition (for review, see Batty & Taylor, 2006). An individual’s decision to fight, flee, mate, cooperate, or seek protection often depends on the ability to recognize the emotional ex-pression of others’ faces (Plutchik, 2001).

Studies have demonstrated a clear tendency for newborn infants to orient towards faces and discriminate face-like patterns (see Johnson, 2005 for re-view). Emotional discrimination of faces is observed in infants as early as 3 months of age. By 4 months, infants’ brain activation in response to faces var-ies as a function facial stimuli eye gaze direction. It will also vary depending on whether the facial stimuli are of fearful or angry faces (Hoehl & Striano, 2008). More specifically, an attunement to threat-cuing facial expressions ex-ists by 7 months of age. By this age, infants can not only differentiate between emotional expressions, but also attend more to negative emotions (Balconi & Pozzoli, 2003; de Haan, Belsky, Reid, Volein & Johnson, 2004; Kotsoni, De Haan & Johnson, 2001; Nelson & Dolgin, 1985; Peltola, Leppänen, Mäki & Hietanen, 2009). Infants show this differentiation and bias for emotional faces regardless of the specific individual (Nelson & Dolgin, 1985). By 10-months of age, infants are sensitive to congruent emotion expressions and actions (Hepach & Westermann, 2013). By 14-months infants are able to use the emo-tions of others to gain information about objects or events, such as inferring desires in others (Repacholi & Gopnik, 1997). Infants will also actively search

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a person’s face for emotional information and then subsequently use that emo-tional information to help appraise an uncertain situation (Klinnert et al., 1983). Also by this age, infants are rapidly beginning to use emotion expres-sion to gain new information, to understand social interactions, congruent be-haviors, and desirability (Nelson & Dolgin, 1985). This research demonstrates how early emotional face perception form the foundation for more complex social processing, including an attunement to environmental and social cues.

Threat bias By 7 months infants show a very specific face emotion attunement – a bias for threat-cuing facial expressions, particularly in uncertain situations (LoBue & DeLoache, 2010). This bias is often referred to as a threat bias and involves a sensitivity to negative emotions, such as fear and anger. This proposed adap-tive system makes sense in the context of aiding survival and a threat detection system. Social stimuli, faces in particular, provide useful cues to identify threats in the environment. Negatively valenced facial expressions aid in threat identification by actually facilitating fast reallocating attention to scan-ning the environment in search of immediate threat (Becker, 2009).

There is limited work examining a fear bias in infants, but at least one study has shown that 7-month-old infants’ attention is increased by a fearful expres-sion compared with a neutral face looking at an unfamiliar object (Hoehl, Pa-lumbo, Heinisch, & Striano, 2008). Another more recent study showed that exposure to mothers’ negative emotions play a role in the development of in-fants’ attention to facial expressions in typical development (Aktar et al., 2018).

There is evidence that the amygdala is the brain area that is associated with responses to facial emotions and early negative experiences, showing en-hanced activation (Adolphs, 2002). The amygdala is one of the critical regions responsible for the early detection of facial emotions (Compton, 2003) and is the brain region that supports reactivity to emotional stimuli and emotional learning (Tottenham et al., 2011). It is also closely associated with the pro-cessing emotional experiences (Gallagher, & Chiba, 1996). For example, cu-mulative early life stressors have been shown to be associated with smaller amygdala volumes (Hanson et al., 2015). Enhanced amygdala reactivity is also seen in individuals with early childhood maltreatment (van Harmelen et al., 2012) and early rearing environment and subsequent eye‐contact is medi-ated by amygdala activity (Tottenham et al., 2011). The activation of the amygdala is largely dependent on the type and intensity of an observed emo-tion, with particularly more activation to fearful expressions (Gläscher, Tü-scher, Weiller, & Büchel, 2004). Arousal triggered by an emotionally fearful face also shows a significantly longer decay than other emotional expressions (Luo, Ainslie, & Monterosso, 2012), suggesting an overall increase in arousal.

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Face Experience The caregiver of the child is another important context and a particularly im-portant source of information for children. The kinds of visual experiences that infants have with their caregivers, such as the distribution of caregiving between the mother and father, can have an impact on various attentional mechanisms like facial emotion perception (Gredebäck, Eriksson, Schmitow, Laeng, & Stenberg, 2012), the perceptual narrowing of face categories (Ren-nels, Juvrud, Kayl, Asperholm, Gredebäck, & Herlitz, 2017), and expertise for particular faces (Quinn, Yahr, Kuhn, Slater, & Pascalis, 2002; Ramsey, Langlois, & Marti, 2005). For example, a recent study showed that caregiving experience with a male or female caregiver between 10- and 16-months influ-enced face processing abilities and the pathways by which infants developed an expertise for faces (Juvrud, Rennels, Kayl, Gredebäck, & Herlitz, 2019). Therefore, the actual faces that infants are exposed to constitute an important environmental context in shaping social perception.

Long-term consequences related to actions and interactions General cognitive-development In cognitive-developmental theory, it is believed that early infant understand-ing is continuous with later understanding (Spelke et al., 1992; Wellman et al., 2008). More specifically, it has been proposed that cognition in part develops in infancy by “internalizing accommodations to constraints on perception and action” (Spelke, 1992). In other words, as infants’ capacities to represent and reason about the world develop in tandem with capacitates for actions in the world, early knowledge is actively shaped through experiences that guide early reasoning. Infants’ early reasoning should thus be related to later more mature knowledge concepts in childhood and adulthood.

There are individual differences across in development and experiences, which may manifest in individual differences in the long-term stability of cog-nitive and behavioral constructs related to action understanding. I previously discussed the long-term relation to theory of mind, but there is limited work that has examined individual differences in early action understanding and later consequences for cognition and behavior. Research exploring long-term consequences have largely been focused on other areas of social cognition, including gaze-following, joint attention, joint action, and the development of prosocial behaviors. These are also important constructs for socio-cognitive development, but the relationship between early action understanding and these processes is still lacking.

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Gaze-following There is a large body of longitudinal work that shows links between early un-derstanding in infants and later understanding and behavior. The ability to fol-low another’s gaze (referred to as gaze-following) for example, is associated with later language skills (Brooks & Meltzoff, 2008). Several studies involv-ing pointing have also shown that an infant’s ability to share attention with others is predictive of later social competence and language development (Mundy & Gnomes, 1998; Tomasello & Farrar, 1986). Other studies examin-ing infants’ attention to perceptual-object displays have shown that both infant habituation and dishabituation-novelty responses predict later IQ at preschool (Bornstein & Sigman 1986). In a study by Sommerville and Woodward (2005), individual differences in 10-month-olds’ understanding of a causal structure of an action were predictive of their own performance on a related task. These findings, as well as other prior work (Piaget, 1952; Uzgiris & Hunt, 1975), suggests that understanding the differentiation of means and ends may be an important associated component for the production of goal-directed actions. There is, however, a very large gap in our understanding about what the actual long-term consequences of differences in early action understanding may be. But thus far, the evidence in cognitive-development does suggest that many particular areas in early infant understanding serve as a precursor to later behavior and understanding.

Joint attention Join attention develops around one year of age and is the ability for infants to follow or direct the visual attention of other adults to outside entities (To-masello, 1995). One of the first longitudinal studies suggesting the importance of joint attention in the development of understanding of mental states (Char-man et al., 2000) found an association between early joint‐attention abilities and later theory‐of‐mind understanding. In another study linking understand-ing of intentional actions and imperative and declarative pointing gestures (such as finger-pointing), Camaioni, Perucchini, Bellagamba, & Colonnesi (2004) found that 12-month-old infants with high intention understanding pro-duced more declarative pointing than children with low intention understand-ing. Furthermore, infants’ understanding of adults’ use of pointing gestures at 12 months predicted later theory of mind abilities (Colonnesi, Stams, Koster, & Noom, 2010). Similar to the ways in which human language can communi-cate meaning, so can the use of hand gestures. A gesture, such as hand point-ing, can be used to communicate meaning in a wide variety of ways, such as for communicating intentions and goals, directing attention, and communi-cating thoughts and feelings (Krauss, Chen, & Chawla, 1996). The more a child is able to interpret an agent’s actions as being intentional and goal-di-rected, the more likely they are to also demonstrate certain abilities, such as

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efforts to share information and influence other people's attentional states to-wards the world (Calonnesi, Rieffe, Koops, & Peruchini, 2010; Camaioni et al., 2004). Longitudinal work suggests that early forms of gesture use and un-derstanding are based on similar structures for far more complex and diverse interactions that emerge later in development (Carpendale & Carpendale, 2010). Other foundational components suggested to emerge already in infancy include retrospective evaluations of rational behaviors, joint action behaviors, and giving and helping behaviors (Colonessi et al., 2008).

Joint Action Interacting with another individual requires not only communication but also coordination of actions. The ability for two individuals to coordinate their be-havior in time and space is referred to as joint action (Sebanz, Bekkering, & Knoblich, 2006). This ability is highly adaptive and therefore found in most species, such as migrating geese and schooling fish, and can take many differ-ent forms and involve different degrees of complexity (Brownell, 2011). For example, joint action in the developing infant is important for when an infant grasps an object passed to him/her, or when a child takes turns during joint toy play.

The ability to engage in joint action starts sometime during late infancy at a less complex and lower level than what adults are capable of. Some of the more low-level joint action abilities are outside of the child’s conscious aware-ness (Sebanz & Knoblich, 2009). A challenge for researchers has been deter-mining precisely when and how this low-level joint action develops into more cognitively sophisticated forms, such as those actions involving goal repre-sentations, complex reasoning, and theory of mind (Brownell, 2011). It is clear that by 12 months of age infants themselves can participate in reciprocal social games with adults (e.g., peek-a-boo) and both vocally and verbally communi-cate with adults in coordinated ways (Ross, & Lollis, 1987). However, at even younger ages infants have been shown to be sensitive to violations of expected joint actions (Sebanz et al., 2006), suggesting that joint action understanding may precede and perhaps even form the basis of own joint action ability.

A more complex form of joint action is sharing a cooperative goal with a partner, such as helping behavior. At 12 months, while infants are sensitive to another individual failing a shared goal, their own behavior during an interac-tion is only minimally coordinated with a partner and instead focused primar-ily on own goals (Behne, Carpenter, Call, & Tomasello, 2005). It is not until around two years of age that children are able to coordinate their behavior to a shared goal within an interaction with a peer or adult (Brownell, 2011). At any given age, infants appear able to understand and engage in a shared goal during a joint action, but then unable to do so on other very closely related criteria (Carpenter, 2009). It is therefore likely that there is a large degree of individual variability in the developmental mechanisms for joint action as it

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relates to cooperative goals. Furthermore, it may be that some joint actions are more import for the development of socio-cognitive processes than others.

Following social interactions requires understanding the actions and goals of the members involved and often include the support of low-level cognitive processes, such as rational evaluation. Indeed, the ability to follow a turn-tak-ing social exchange based on understanding the use of social gestures has been shown to be closely tied to early socially-based cognitive processes (Elsner, Bakker, Rolfhing, Gredebäck, 2014; Thorgrimsson et al., 2014). For example, previous research has argued for the importance of understanding such turn-taking social events for later verbal communication (Messinger & Fogel, 1998; Iverson & Goldin-Meadow, 2005).

Prosocial behavior Infants’ action understanding and own action performance is an emergent product of the dynamic interaction between the infant and other individuals (Fogel, 1990). To better understand the long-term consequences of early so-cial cognition as it related to actions and interactions, researchers have often explored the long-term consequences of early socio-communicative abilities, such as prosocial behavior. Prosocial behavior is broadly defined as actions intended to benefit another (Eisenberg, 2007) and is suggested to be important for the well-being of social groups and fostering positive traits in individuals (Hopkins et al., 2007). Understanding the emotions and mental states of others are important social-emotional competencies that are part of more broad pro-social development (Eggum et al., 2010; Eisenberg et al., 2006). Children begin to clearly show prosocial behaviors, such as helping, sharing, giving, and comforting between 1 to 2 years of age (Zahn‐Waxler, Radke‐Yarrow, Wagner, & Chapman, 1992; Warneken & Tomasello, 2006). Joint action is one important early predictor of prosocial behavior.

Some explanations of prosocial behavior include emotional contagion, the phenomenon of one person’s emotions triggering similar emotions in other people, and empathic concern, emotions toward others elicited by the per-ceived welfare of someone in need. Other explanations include social learning accounts and the ability for one to align goals with another (for review see Paulus, 2014). Importantly, the findings thus far suggest that different in-stances of prosocial behavior are explained through different mechanisms with different developmental trajectories and outcomes (Yarrow et al., 1976). Few studies have examined the mechanisms for the emergence and develop-ment of prosocial behavior beginning in infancy. One study that used eye-tracking and behavioral data at 3 to 5 years of age showed distinct neural pat-terns when viewing helping and harming scenes. Furthermore, later cogni-tively controlled neural response patterns predicted actual behavioral gener-osity (Cowell & Decety, 2015).

While there is evidence that early understanding is important for later ac-tion ability, rather than early action-experience driving the understanding of

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others’ actions (Gredebäck & Falck-Ytter, 2015; Gottwald, Achmermann, Marciszko, Lindskog, & Gredebäck, 2016), there is limited work examining how early understanding of actions is associated with later prosocial behav-iors, such as helping. There is also no work specifically examining individual differences in understanding and how individual differences might differen-tially predict later behavior.

Missing links There is not a lot of working examining the long-term consequences of indi-vidual variations in perception and processing. As demonstrated in several ar-eas of psychology, examining the long-term consequences of early under-standing and early abilities has implications for individual differences in de-velopment, particularly children that might be at elevated risk. For example, children with developmental disorders may be exposed to down-stream defi-cits (i.e., consequences that will emerge later) beginning already early in in-fancy. Other normally developing children with early exposure to increased stress of impoverished environments are also at risk for down-stream conse-quences, such as altering cognitive development to adapt to their environ-ments (Frankenhuis & de Weerth, 2013). Studies additionally have shown that children reared in contexts with impoverished affective expression infor-mation, such as children with clinically depressed parents (Dawson, 1994), are influenced in their emotion processing later in other contexts (Cummins, 1995). Efforts to understand the long term-consequences and intervene at an early age could alter outcomes later in development by designing more spe-cific and effective interventions (Meltzoff, Kuhl, Movellan, & Sejnowski, 2009).

Remaining questions On the level of motion perception, research has shown that from birth infants are attuned to specific motion cues (Bertenthal, 1993). But the importance for perceiving something as biological or non-biological for action processing is still not understood. Furthermore, do infants, like adults, use the context and constraints of motion during perception in order to interpret motion paths and subsequent actions? For example, are infants sensitive to different actions per-formed by a human agent, and do their interpretations of motion change based on their understanding of constraints? It could be that what is considered an efficient action, or even what is a possible action, may differ depending on the basic perception of biological versus non-biological motion. Using apparent motion, research with adults has shown differential processing of motion paths when it is biological or non-biological (Stevens et al., 1999). To what extent this is true for infants is still unknown.

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A large body of work suggests that early knowledge is actively shaped through experiences that guide early reasoning (Hunnius et al., 2014; Skerry et al., 2013; Sommerville et al., 2005). It could be that own experiences with human movement may result in different perceptions of human motion com-pared to an unfamiliar object. It is also reasonable to expect that infants’ early understanding should be related to later more mature knowledge concepts in childhood and adulthood. But unfortunately, research examining the relation-ship between early action understanding and later processes and behaviors is still lacking.

Finally, there is the question about a child’s larger context. As reviewed here, there is no shortage of work suggesting the importance of a child’s own and observed experiences, environments, and interactions for shaping the de-velopment of action perception and processing. For instance, research explor-ing culture and stress (Pollak, 2008). But there is still not a lot of studies spe-cifically examining the short- and long-term consequences of individual vari-ations in processes and context for development. The majority of studies on action understanding ignore contextual factors or do not account for them. There is also remaining questions about the importance of contextual factors for action understanding and perception. Consequently, there is a need for more research exploring the capacity for various contexts to actively influence how the child perceives the world.

Eye-Tracking The current thesis incorporates eye-tracking techniques in each included study. While other measures were also used in Study II and III, eye-tracking methodology was the primary tool across all studies. The analyses and inter-pretation of the results is heavily influenced by the eye-tracking literature. Therefore, a brief review of the eye-tracking literature is presented here.

Traditionally, infant studies have incorporated very rough habituation methods that rely on cumulative looking times (Fantz, 1964), a method which reduces information processing in infants to a discrete value that represents how long an infant has looked at a new display after an alternative display (Sirois & Jackson, 2012). Another technique used with infants, corneal reflec-tion eye-tracking, provide a much more rich and detailed look into the cogni-tive processing of infants (see Gredebäck, Johnson, & von Hofsten 2009). Corneal reflection eye-tracking is a measure of where an individual is looking (point of gaze). This is achieved through a near-infrared light that is directed towards the pupil of the eyes. This creates visible reflections in the cornea. Infrared camera then tracks the vector between the cornea and the pupil.

Due to the incredible importance of looking behaviors in infancy, high spa-tial and temporal accuracy of eye-tracking techniques allows researchers to assess looking data over time and time-lock data to specific events (Gredebäck

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et al., 2009). It also allows for a comparable measure across different groups, such as ages or contexts (Juvrud et al., 2018). Eye-tracking is therefore a use-ful tool for understanding underlying processes behind behavior. Eye-tracking techniques allows for several dependent variables, including location of gaze at any given time (fixations and saccades), looking time to particular areas, speed or latency to look at a target, and the measurement of the dilation of the pupils (Karatekin, 2007; Luna, Velanova, & Geier, 2008; Trillenberg, Lencer & Wolfgang, 2004).

Pupil dilation Measurements of pupil change independent of luminance have been demon-strated to be a unique window into the mind, providing momentary, involun-tary, and unbiased measures of arousal, attention, and cognitive load (e.g., Fitzgerald, 1968; Gredebäck, Johnson & Hofsten, 2010; Nassar et al., 2012; Sirois & Jackson, 2012). In the human eye two muscles, the dilator and the constrictor, control the size of the pupil as a result of either a stimulation of the dilator or inhibition of the constrictor (Laeng, Sirois, & Gredeback, 2012). The resulting changes in the pupil size are an involuntary response that occur outside of an individual’s consciousness and have been closely linked with activity in the locus coeruleus (LC). The LC is a subcortical structure in the brain that has been demonstrated to be involved in high-attention situations, stress and anxiety, and the process of memory retrieval (Sara, 2009). Located within the LC is the only source of norepinephrine, a neuro-transmitter re-sponsible for moderating activity of the prefrontal cortex. This noradrenergic system in the LC has been suggested as being responsible for the functional integration of the whole attentional brain system (Laeng et al., 2012) and a key component of the neural circuitry that controls muscles in the eye. There-fore, external changes to the size of the pupil are likely to reflect internal ac-tivity in the LC. The tight link between pupil change and activity in the LC has provided unique opportunity to examine changes in attention and arousal through noninvasive means, and has become a rapidly growing measurement in research, particularly in developmental research for its usefulness in non-verbal infants. Increased pupillary change has been shown to be a response to arousal, comparable with other measures in the peripheral nervous system, such as skin conductance response (Bradley, Miccoli, Escrig, & Lang, 2008); Juvrud et al., 2018; Wieser, Pauli, Alpers, & Mühlberger, 2009). It has also been associated with blood-oxygen level dependent imaging (BOLD re-sponse) in the amygdala (Nassar et al., 2012).

Since the early work of Hess and Polt (1960) pupil dilation measurements have been used as indicators of cognitive and arousal states in adults and in-fants (Loewenfeld, 1993). The benefits of using the pupil diameter in the measurement of infant cognition is that it provides an involuntary and unbi-ased change that reflects cognitive effort exerted at any given time (Nassar et

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al., 2012; Sirois & Jackson, 2012). Previous studies demonstrated the useful-ness of pupil dilation as a methodology for use with infants, such as assessing differences in face processing (Gredeback et al., 2012), emotional expression (Granholm & Steinhauer, 2004), perceived change (Gilzenrat, Nieuwenhuis, Jepma, & Cohen, 2010), violations of expectations (Jackson & Sirois, 2009; Sirois & Jackson, 2012), and irrational social interactions (Gredeback & Melinder, 2010).

Current study In Study I and II, pupil dilation measurements are used to examine a state of arousal or surprise to observed events. In Study III, the latency to locate a target is used as the dependent variable. Study III additionally incorporates a gaze-contingent eye-tracking paradigm. This means that the infants’ real-time gaze is actively used to control the progression of the trials; if the infant looks at the correct target, then the trial will progress.

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Aims

The present thesis took a developmental and individual differences approach to understanding action perception and processing. The overarching aim of this thesis was to understand the development of action perception, and how individual differences contribute to the perception and processing of actions. To this aim, it was important to understand the capacity in which variations in a child’s context can affect the development of action perception.

The research reported here therefore addresses the following questions:

I How do infants process simple human apparent motion? II How do infants process the intentional actions of two humans in a

social interaction and what long-term consequences do individual differences in this type of early social perception have for later be-havior?

III In what ways are infants’ perception of the world influenced by dif-ferences in their environmental and internal contexts?

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Method

Overview of the studies The three studies in the present thesis examine the development of individual differences in action perception in three stages. In study I, we first examined action perception in 12-month-olds at one of the most basic levels using static images and the apparent motion phenomenon. In this study, it is important to understand that the context here includes who/what is performing that action. Whether it is a biological or non-biological apparent motion stimulus may re-sult in different perceptions of motion paths. Understanding how infants and adults differentially process motion paths at a low-level provides insights into the constraints and principles that serve as building blocks for more complex actions. As such, in study II we therefore move to the level of more complex goal-directed actions. We have two contexts that are examined. First, in the context of a human interaction incorporating a communicative gesture, we ex-amine individual differences in action processing at 9 months and to what ex-tent the individual differences in infancy predict later behavior at 2 years of age. The second context is thus the long-term effects of individual differences in giving perception on later behavior. Finally, study III focuses on the relation between infant’s selective attention and specific internal and external contexts of a child: maternal negative affect, infant temperament, and the immediate emotional context. These contexts have independently been shown to affect infants’ cognitive processes (Bhatt & Quinn, 2011; Newcombe, 2011). How-ever, here we look at the integrative effects of these contexts and their joint capacity to affect how infants perceive the world. These contexts are both in the short-term (i.e., the result of an emotional prime) and the long-term (i.e., the result of the emotional affect of the mother). All three studies utilize eye-tracking methods along with multiple other complimentary measures.

Participants Demographic statistics for the three studies are presented in Table 1. All in-fants in Studies I, II and III were initially recruited using an existing laboratory list of parents who had previously responded to an invitation letter sent to all families with children of appropriate age living in Uppsala, a medium-size Swedish city. Therefore, participants were mostly ethnically Swedish and had

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mixed socioeconomic backgrounds biased towards the upper and middle clas-ses. For the second experiment in Study II, participants were returning fami-lies that had participated in Experiment 1. The number of infants excluded from each study are presented in Table 1. Exclusion criteria for the participa-tion of infants in all three studies included being born pre-term (<37 weeks) and having indications of neurological abnormalities or atypical development. Participant exclusion rates across all three studies are presented in Table 1. All adult participants for Study I were students at Uppsala University and were recruited through posted advertisements. In study II, data from a previously published electroencephalogram (EEG; Bakker et al., 2015) study are in-cluded for the correlation analyses. In the EEG task, twenty-nine 9-month-olds participated (15 girls, mean age 8 months and 28 days). An additional 30 infants (16 girls) participated but were excluded due to fussiness (less than 10 artifact-free trials, n = 25) or technical problems (n = 5). This high attrition rate is common for infant EEG studies (Bell & Cuevas, 2012).

In all three studies, a researcher explained the purpose of the study and adult participants and parents of infants provided informed consent prior to participation. Each participating adult and family received 100 SEK gift voucher for participating. All studies were conducted in accordance with the standards specified in the 1964 Declaration of Helsinki and approved by the local ethics committee.

Table 1. Demographic statistics for Study I, II, and III Study I Study II Study III

Experiment 1 Experiment 2 9 Months 2 Years Demographic Statistics

Participants NR Infants 47 Adults 29

46 59 42 70

Age M Infants 24.43 Adults 11.29

11.28 8.7 25.1 8.27

Sex Female NR (%)

Infants 18 (38) Adults 22 (47)

24 (52) 30 (51) 23 (55) 40 (57)

Excluded Participants NR*

Infants 14 Adults 0

3 7 0 10

*Exclusion criteria for all studies included: fussiness, parental interference, technical prob-lems, and experimenter error.

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Measures Perception of biological and non-biological apparent motion Possible/Impossible apparent motion task (Study I) In both experiments of Study I, infants and adults viewed apparent motion stimuli with the aim of assessing basic perception of biological motion (ap-parent motion with human figures; Experiment 1) and non-biological motion (apparent motion with objects; Experiment 2). The stimuli for both experi-ments consisted of two sets of image pairs - physically possible condition and physically impossible condition - that were alternately presented to create ap-parent motion. The images presented to infants and adults in both experiments were identical and presented at equal timing rates (beta movement) across conditions and within subjects (1250ms per image, 50ms onset/offset between images). This was done in contrast to Stevens et al. (1999) to avoid any changes in the pupil as a result of timing differences. Images for both experi-ments were created to be similar to those used by Shiffrar and Freyd (1990) and Stevens et al. Luminance between all images was matched using a lumi-nance meter.

In Experiment 1, in the two conditions (possible and impossible) an arm and hand moved from behind a raised leg to in front of the raised leg (see Figure 1). In the physically possible condition, the hand traveled a short dis-tance from the first image position to the second image position. In the phys-ically impossible condition, the hand had to travel a long distance from the first image position to the second image position, a distance that was physi-cally impossible given the time constraint (see Figure 2).

In Experiment 2, the two conditions were instead illustrations of a round object moving from one side of a barrier to a position on the other side. Similar to in Experiment 1, the manipulation in the two conditions was the distance the object would need to travel to go around the barrier. The images were therefore visually similar to Stevens et al., (1999), as well as to the possible and impossible conditions in Experiment 1.

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Figure 1. Time series illustrating the stimuli presentation sequence and correspond-ing baseline and analysis period for the pupil. A center fixation cross was first pre-sented for 1000 ms, followed by presentations of stimuli images for 1250 ms each with a 50 ms onset/offset interval.

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Figure 2. Test stimuli from Study 1 (apparent biological motion) and Study 2 (ap-parent non-biological motion) in which the only difference between the two pictures was the placement of the right limb or circle object. To avoid the possibility of a dif-ference in pupil dilation being caused by differences in beta and phi movement, in both Study 1 and Study 2 we maintain the temporal structure of the stimuli between conditions but change the configuration of the arm/leg and shape object/barrier in order to create similar movements. The change in distance makes the motion path ei-ther possible or impossible. The (a) possible condition had a shorter path from the placement of the right limb in Study 1 from the first to the second image and a visu-ally similar configuration in Study 2, while the (b) impossible condition had a longer path, making it appear that the hand passes through the leg, with a visually similar configuration in Study 2. Image set (c) shows the original stimuli from Stevens et al. (1999).

Give-me gesture understanding at 9-months Evaluation of give-me gesture interactions (Study II) This eye-tracking task assessed infants’ sensitivity to an inappropriate re-sponse during a social giving interaction incorporating the give-me gesture. A total of four appropriate and four inappropriate trials were presented. Each trial was a 40s interaction that had a baseline period, a familiarization phase (establishing a giving interaction), and the test phase (giving action and re-sponse to give-me gestures; see Figure 3). The baseline period (1s) occurred at the start of the trial before the interaction. The familiarization phase (13s total) included the first actor moving a block placed at one end of the table to a bowl at the other end of the table; this action was repeated three times. The test phase (26s total) began with the second actor non-verbally requesting a block with a give-me gesture, followed by the first actor passing one block from the bowl to either the receiver’s outstretched hand (appropriate response condition) or the top of the receiver’s head (inappropriate response condition).

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This sequence of a give-me gesture followed by appropriate or inappropriate giving was repeated three times, once for each block.

Figure 3. Time series of one evaluation of give-me gesture interaction trial, illustrat-ing the sequence of actions in the video stimuli with two conditions: an appropriate response to the give-me gesture (object being passed to the hand), or an inappropri-ate response to the give-me gesture (object being passed to the head).

Gaze-following Gaze-following task (Study II) This task assessed whether infants can reliably follow an actor’s gaze to an attended object. In each trial, an actor seated at a table first greeted the infant with a “hi” with her gaze directed forward. She then moved her gaze, head, and eyes (approximately 60°) to fixate on one of two toys placed on either side in front of her and kept a static gaze for 5 seconds. The properties of the toys (e.g., color, shape) were randomized across trials.

Neural correlates of the give-me gesture Give-me gesture EEG task (Study II) The aim of this measure was to assess infants’ sensitivity to the give-me ges-ture. Infants viewed both a give-me gesture (experimental condition) and a non-communicative hand configuration (control condition). In both condi-tions, the stimuli included a hand (palm facing upward in the experimental

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condition and the same hand rotated 90 degrees in the control condition) pre-sented in the center of a gray background (see Figure 4). The size of the hand in both conditions was 5 horizontal and 16 vertical visual degrees. The results of this task have previously been published elsewhere (see Bakker, Kaduk, Elsner, Juvrud, & Gredebäck, 2015).

Figure 4. Stimulus for the give-me gesture condition on the left and a control hand on the right.

Giving behavior at 9 months Respond to Behavioral Request Procedure (Study II) The researcher assessed infants’ behavioral response to a give-me gesture us-ing the Respond to Behavioral Request procedure from the Early Social Com-munication Scales (Mundy et al., 2003). In this measure, the experimenter fa-miliarized the infant with three rubber toys (5 x 5 cm) and then placed the toys in front of the infant, waiting 3 seconds for the infant to give the toy back spontaneously. If the infant did not pass a toy after 3 seconds, the experimenter verbally requested the toys with the phrase “give it to me”. If after an addi-tional 3 seconds the infant did not respond to the verbal request, the experi-menter used a combination of verbal request together with the give-me ges-ture. The experimenter’s gesture stopped within reach of the infant. The total duration of this grasping test did not exceed 5 min.

Giving and non-giving helping at 2 years Give-me gesture behavior task (Study II) This was a novel giving task (similar to previous helping tasks, e.g. Warneken & Tomasello, 2006) that measured the production of giving behavior in social interactions at two years of age with the aim of relating giving behavior with prior giving understanding at 9 months of age. The task began with a warm-up phase that was a game of knocking down four bowling pins with a ball. After the game, the researcher sat across from the child on the floor with a box placed in the corner of the room. The test phase began with the researcher placing a pin into the box while crouched next it, followed immediately by

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extending her hand towards the participant and producing a still give-me ges-ture. The researcher remained in this position silently until the child placed the remaining four toys (three pins and a ball, four trials) one at a time either in her hand or in the box. If the child passed the toy to the researcher’s hand, the researcher placed the item in the box herself and once again made a give-me gesture. If there was no response by the child after 30 seconds, the re-searcher fetched one of the toys and placed it in the box. The researcher then resumed crouching with a give-me gesture.

Language Ability The Swedish Early Communicative Development Inventory (Study II) The Swedish Early Communicative Development Inventory (SECDI; Eriks-son, Westerlund, & Berglund, 2002) is a Swedish version of the MacArthur Communicative Development Inventories (Fenson, et al., 1993). The SECDI involves parental reports consisting of two inventories. The inventory used at 9 months concerns “words and gestures” and is designed for children 8 to 16 months of age. The inventory used at 18 months and two years concerns “words and sentences” and is designed for children 16 to 30 months of age. The inventory was completed at the laboratory following testing at 9 and 25 months, and at home at 18 months.

Selective visual attention Visual search task (Study III) The visual search task, a measure of selective visual attention, was adapted for infants based on the procedure from Haas, Amso, & Fox (2016). Infants were first primed with an emotional facial expression followed by a non-emo-tionally valenced visual search task.

Emotional face prime The emotional facial expressions were color images of adult female faces (~11 x 15 cm [~4.2 x 6.5 visual degrees]) including the emotional expressions happy, fearful, and angry, as well as an additional blurred neutral face (see Figure 5). A blurred face was used instead of a simple neutral face because prior work has suggested that neutral expressions are in fact not neutral at all, but may be perceived as more negative in nature (e.g., Cooney, Atlas, Joor-mann, Eugène, & Gotlib, 2006; Wieser et al., 2014). The faces were either the infant’s mother or a stranger’s face and included the hair and neck with a white sheet covering the shoulders. Prior to testing, a researcher photographed the mother in a room with standardized lighting and mothers used a mirror and example photos (from The Karolinska Directory of Emotional Faces; Lundqvist, Escape & Öhman, 1998) to standardize emotional expressions. The stimulus faces were standardized for brightness, contrast, color, and size using Adobe Photoshop CS5. A gaussian blur filter was applied to the neutral

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face (22 pixels) so that no features of the face were visible (Gilad-Gutnick, Yovel, & Sinha, 2012).

Figure 5. Emotion conditions from left to right: happy, fear, angry, and blurred

Non-emotionally valenced visual search Infants first saw a blank white screen (600 ms) followed by the visual search task (see Figure 6). The visual search task consisted of a target and either zero or 4 distractors (set size 1 and set size 5, respectively). Visual search targets were color cartoon objects (hat, butterfly, apple, cheese, pear, and sweater; ~3 x 2 cm; 1.2 x 1.0 visual degrees). Distractor pairings included: hat & banana, butterfly & basketball, apple & car, cheese & flower, pear & bow and sweater & planet (~3 x 2 cm; 1.2 x 1.0 visual degrees). The location of the target during visual search and the number and location of distractors were randomized and counterbalanced across trials. The targets were equally distanced from the center of the screen (10 cm) and did not overlap with the location of the pre-viously presented face stimulus. The trials were gaze-contingent, meaning that a short reward animation automatically played (2000 ms) when the infant fix-ated anywhere within the target image (100 ms; Frank et al., 2013). The re-ward was a short animation of the target moving up and down accompanied by a chime sound.

Figure 6. Trial progression for the visual search task

Maternal negative affect The Positive and Negative Affect Schedule (Study III) The Positive and Negative Affect Schedule (PANAS; Watson, Clark & Tel-legen, 1988) self-assessed positive and negative affect of mothers. To com-plete the PANAS, the mother responded to 20 emotional descriptions, includ-ing 10 describing positive states (committed, enthusiastic, proud, on alert,

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inspired, determined, attentive, active, keen, strong; PA scale) and 10 describ-ing negative states (desperate, upset, feeling guilty, afraid, hostile, legal, shameful, nervous, angry / fearful, frightened; NA scale), on a likert-scale from 1 to 5, to the extent to which the respondent agrees that it applies. Cronbach's α is .86 - .90 for the PA scale and .84 -. 87 for the NA scale (Wat-son et al., 1988; Crawford & Henry, 2004). The PANAS has been associated with the tripartite model of anxiety and depression (Clark & Watson, 1991). Although the measure asks subjects to report within the past week, this is a common practice among survey design to avoid telescoping (Lavrakas, 2008) and the measured components are presumed to be stable and representative over longer periods (Crawford & Henry, 2010). The scale was translated to and administered in Swedish.

Infant negative affect The Infant Behavior Questionnaire-Revised (Study III) The very short version of The Infant Behavior Questionnaire—Revised (IBQ-R; Gartstein & Rothbart, 2003) is a parent-report measure of infant tempera-ment consisting of 36 items with 3 broad scales (surgency, negative affectiv-ity, regulatory capacity). Reliability and validity of the IBQ-R has been re-ported for samples from multiple cultures, with Cronbach's α ranging from 0.77 to 0.96 (Gartstein & Rothbart, 2003; Gartstein et al., 2003). The Swedish language version was used for the current study.

Apparatus Eye-tracking (Studies I, II, & III) For each eye-tracking task we used E-Prime Professional 2.0 with Tobii Ex-tensions to present stimuli on a 33.7 x 27 cm (1280 x 1024 resolution) monitor and a Tobii T120 near infrared eye tracker (0.15° precision, 0.4° accuracy, and 60 Hz sampling rate) to measure infants’ eye movements. Infants sat on their parent’s’ lap ~60 cm from the monitor (0.022 x 0.023 visual degrees per pixel). A curtain separated the infant and parent from the researcher. Calibra-tion for each study used a standard 5-point procedure within Tobii Studio soft-ware (Tobii Technology). This procedure consists of drawing the infants' at-tention to the calibration stimuli at five 3 x 3 points on the screen, in which circles were presented expanding and contracting together with sound. The experiment was started when all five points were successfully calibrated. The calibration procedure was repeated for the missing calibration points (for more information about the calibration procedure see Gredebäck, Johnsson, & von Hofsten, 2009). Data from the Tobii T120 eye tracker were exported from Tobii Studio, with the TobiiFixation Filter used for gaze data, and were pro-cessed using TimeStudio (Nyström, Falck-Ytter, & Gre-deback, 2016) an

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open source analysis tool based on MATLAB (TimeStudio version3.17; MATLAB version R2014b).

EEG (Study II) We used 128-channel HydroCel Geodesic Sensor Nets to record infants’ EEG. The recorded signal (250 Hz, vertex referenced) was amplified by an EGI Net Amps 300 amplifier (Electric Geodesic, Eugene, OR) and stored for off-line analysis. Stimuli were presented using E-Prime 2.0, E-Studio software (Psy-chology Software Tools, Inc., Pittsburgh, PA, USA). Infants sat on their par-ent’s lap and viewed the stimuli on a 17-inch monitor at a viewing distance of 60 cm (20.7 x 16.5 visual degrees). The experimenter sat at a control computer separated from the parent and infant by a curtain and monitored the infant’s behavior via a live camera.

Summary of Studies Included in the Thesis Study I Background Stevens and colleagues (1999) have previously demonstrated that the percep-tion of impossible and possible apparent human movements is mediated by distinct brain regions. The apparent motion phenomenon, perceiving motion in alternating static images despite their being no actual motion at all, allows us to examine the low-level foundations of motion perception. In Stevens et al.’s findings, adults were sensitive to impossible motion paths when the ap-parent motion was performed by a human body, as opposed to apparent non-biological motion. It appeared that the adults in this study took the constraints of the human body into account, such as the context of a human action space and degrees of freedom, the constraints of solidity, and the timing of a human action. It is likely that adult observers use this knowledge to make sense of the world around them, and we hypothesize that this knowledge is also available to infants from an early age.

The aim of Study I was therefore to replicate the findings of Stevens et al.’s (1999) work investigating apparent motion perception in adults, and to bring this work into the infant field. It has traditionally been thought that the appar-ent motion phenomenon is a result of a bias in the visual system to interpret the simplest of actions – a straight path between two points (Stevens et al., 1999). However, that the interpretation of an apparent motion path may actu-ally be dependent on whether it is an object or a human (Shiffrar & Freyd, 1990). Replicating previous findings of low-level apparent motion perception in infancy can provide insight into how infants differentially process simple motion performed by human versus non-human figures.

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Informed by prior work, we aimed to replicate Steven’s et al. (1999) find-ings in a sample of 12-month-old infants using measurements of pupil dila-tion. In the study by Stevens and colleagues, to differentiate the possible from impossible motion paths, the authors manipulated the timing (or frame-rate) of the presented images. In the impossible condition, the images alternated much faster (250ms) compared to the possible condition (750ms). It could be that this manipulation changed subject’s perception of the movement from beta movement (possible condition) to phi movement (impossible condition), affecting the interpretation of their results. Therefore, in order to avoid differ-ences in pupil dilation as a result of differences in beta and phi movement, we kept the alternating speed constant across conditions and instead changed the position of the arm (see Figure 1).

We predicted that 12-month-old infants, like adults, will take the type of event of an action (apparent biological versus apparent non-biological) into account when perceiving the motion paths. More specifically, replicating the findings by Stevens et al. (1999), we hypothesized that infants and adults are sensitive to possible and impossible apparent human movements and that their pupils will dilate when perceiving impossible human motion, but not when perceiving possible motion. Also based on Stevens et al., we predicted that there will be no differential pupil dilation in the apparent object motion, as there is not sufficient information and not the same expectations for the motion path as there is for an apparent motion with human figures.

Procedure In both Experiment 1 (apparent biological motion) and Experiment 2 (appar-ent non-biological motion), an experimenter explained the study to the adult participant or the parent(s) of the infant and obtained informed consent. Adult participants were seated on a chair and infants sat on their parent’s lap facing the stimulus monitor. Testing began with a trial randomly selected from one of the two conditions (possible or impossible; within-subjects design). The condition order was counterbalanced across participants. Trials from each condition began with a grey screen with a center fixation-cross presented for 1 s immediately followed by the test stimuli. The number and length of trials differed between infant and adult sessions (adults saw a 14 images per trial across 3 trials, infants saw 6 images across 5 trials) due to limitations in in-fants’ attention. The entire testing lasted approximately 8 minutes for both the adult and infant procedures.

Pilot data (n = 10) confirmed that all ten adults, in a similar manner as participants in Stevens (1999), perceived apparent motion phenomenon in both the possible and impossible condition. Nine of the 10 adults also reported perceiving the hand pass through the leg in the impossible condition and all 10 adults reported seeing the hand pass around the leg in the possible condi-tion.

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Data analysis Data analyses were the same for both Experiment 1 and 2. The dependent variable for each trial was the difference between mean pupil size during the test period and mean pupil size during the baseline period. The baseline period occurred during a pre-stimulus image of a grey screen with a center fixation cross prior to the onset of the test stimuli (250 ms). We performed a moving gap interpolation (max gap jump of 15 samples) and average filter (window length of 15 samples) across all data. For analyses, we conducted a Linear Mixed Model (LMM). A LMM is an extension of the generalized linear model, in which the linear predictor contains random effects in addition to the usual fixed effects. A LMM incorporates individual differences and experi-mental manipulations into the analyses, accounts for random effects due to participants, and is a more robust model compared to repeated-measures anal-yses of variance. Our dependent measure included in the model was the base-line-corrected pupil size during the analysis period in each trial, participant and trial number were our random factors, and age (12-month-olds and adults), sex, and condition were fixed effects. Each LMM was computed using SPSS 20.0, utilized the diagonal covariance structure for the repeated fixed-effects and the random effects, and was fitted using the restricted maximum likelihood criterion. Following the LMM analyses, we conducted a two-sam-ple planned comparison t-test to examine the extent to which infants’ and adults’ pupils dilated more in one of the two conditions.

Results Experiment 1 All data met the assumption of collinearity (Age, Tolerance = .91, VIF = 1.03; Sex, Tolerance = .99, VIF = 1.02; Trial, Tolerance = .91, VIF = 1.10; Condi-tion, Tolerance = 1.0, VIF = 1.0). There was a significant main effect for con-dition, F(1, 558.15) = 7.01, p = .008, with greater pupil dilation in the impos-sible condition. There were no other main effects, or interactions, indicating that the pupil did not change as a function of sex, F(1, 111.02) = .035, p > .05, or age, F(1,30.15) = 1.38, p > .05.

We conducted a t-test to examine the difference between pupil dilation dur-ing observation of impossible and possible conditions in 12-month-olds and adults. For the 12-month-olds, there was a significant difference in pupil dila-tion between the two conditions, t(46) = 6.49, p = 0.01, d = 1.91, with greater dilation in the impossible condition (M = .259, SD = .162) than the possible (M = .211, SD = .176). Similarly for the adults, results indicated a significant difference in pupil diameter change between the two conditions, t(28) = 2.69, p = 0.01, d = 1.28, with greater pupil dilation in the impossible condition (M = .456, SD = .198) than the possible (M = .376, SD = .211). See figure 7.

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Figure 7. Results from pupil change for 12-month-old infants and adults during ob-servations for the two action conditions: possible and impossible. The left image shows the baseline-corrected pupil diameter timelines across time for the two condi-tions for 12-month-old infants and the right image is for adults.

Experiment 2 All data met the assumption of collinearity (Sex, Tolerance = .99, VIF = 1.0; Trial, Tolerance = .99, VIF = 1.0; Condition, Tolerance = 1.0, VIF = 1.0). Results for the 12-month-olds revealed no main effects or interactions, indi-cating that the pupil did not change as a function of condition, F(1, 353.46) = .357, p > .05, or sex, F(1, 153.85) = 1.23, p > .05.

Comparing pupil size in Experiment 1 and Experiment 2 To examine whether overall pupil size was different in Experiment 2 com-pared with Experiment 1, we conducted a t-test for the possible and impossible conditions from each study. Results revealed no significant differences in pu-pil dilation between Study 1 and Study 2 for the possible condition, p > .05, or for the impossible condition, p > .05. Therefore, there was not an overall greater pupil dilation for one set of stimuli (human or object) than for the other.

Conclusions When an apparent motion stimulus is that of a human figure (Experiment I), 12 month old infants can differentiate between physically possible and impos-sible motion paths, relying on the constraint that objects rarely pass through other objects . Both 12-month-old infants and adults showed greater pupil di-lation to an impossible apparent motion stimulus – which we here suggest to be indications of surprise. When the apparent motion was an object (Experi-ment II), no such differential interpretation appeared to occur. The similar findings in adults replicate and extend the findings from Stevens et al. (1999), suggesting that a perceptual shift between possible and impossible apparent motion paths can be measured via pupil dilations.

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Study II Background The aims of Study II were to examine how infants process the intentional ac-tions of two humans in a social interaction and what the long-term conse-quences of individual variability in early perception are for later behavior. We chose a social interaction involving giving behavior utilizing the give-me ges-ture. This was chosen due to the suggested importance of early gesture com-munication and the particular importance of, and the limited research investi-gating, the give-me gesture in socio-cognitive development. The background provided here will therefore include additional information that is needed to place the current study in an appropriate scientific framework.

A context in which complex goal-directed actions commonly occur is within social interactions, particularly during cooperative interactions be-tween two people. Cooperative social interactions, such as communicating, turn taking, and giving and receiving, constitute an important part of a child’s early socio-cognitive development (Meltzoff et al., 2009). It is likely that there is a large degree of individual variability in the developmental mechanisms for understanding social interactions as it relates to cooperative goals (Fogel, 1990). Furthermore, it may be that some kinds of social actions are more im-port for the development of socio-cognitive processes than others are. One particular social action that may be especially important for humans is giving and sharing, or the transfer and exchange of goods. Infants’ understanding and production of giving related behaviors offers a rich source of information on infants’ early socio-cognitive development.

The focus in Study II is on young children’s understanding and production of the act of giving itself. We define giving as the deliberate transfer of object possession by one agent to another (Tatone, Geraci, & Csibra, 2015), and the broader class of giving-related behavior to include giving and gestural re-quests for giving. Understanding the actions and intentions of others in a social interaction also involve the production and understanding of social gestures commonly used for communication (Allison & Puce, 2000). Early forms of gesture use and understanding are based on similar structures for far more complex and diverse interactions that emerge later in development (Carpen-dale & Carpendale, 2010). The ability to understand particular social gestures commonly used for communication within complex and dynamic social inter-actions allows one to better make sense of and follow social exchanges (Alli-son & Puce, 2000). While pointing is a gesture with several versatile forms of function and meaning (Butterworth, 2003; Liebal et al., 2009), a gesture that is often associated with the very specific social interaction of giving and shar-ing is the give-me gesture, in which the upraised palm is outstretched. The gesture can function as a symbol of receiving or giving, depending on whether the hand is empty or contains an object. Findings with 12-month-old infants have revealed that by this age they have clear expectations concerning object

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transfer actions in a social exchange between two agents incorporating the give-me (Elsner et al., 2014; Thorgrimsson et al., 2014).

Procedure Study II included a visit at 9 months of age and 2 years of age.

9-month visit At the 9-month visit, a researcher first explained the study, obtained informed consent, and the parent completed the SECDI. The testing session included the tasks in the following order: EEG task, behavioral giving task, and the eye-tracking tasks.

During the EEG task, stimuli were presented at random (with the constrains of maximum three repetitions of the same stimulus) and presented in the mid-dle of a gray background for 1000 ms. Between each experimental stimulus; a fixation cross was presented for 100–300 ms. The researcher paused the ex-periment if the infant became inattentive and fussy and the stimulus monitor remained black for the duration of the pause. The experimenter terminated the study when the infant was no longer interested in the stimuli. The task lasted on average 10 minutes.

Infants participated in the behavioral giving task in another room approxi-mately 5 minutes after the EEG task. The researcher first inquired from the parent(s) as to whether they have observed their child producing or responding to a give-me gesture outside of the laboratory and then administered the ESCS with the child (see ‘Measures’ for description of the task). Infants’ behavior was video recorded and later coded for the number of times the infant gave a toy to the experimenter in response to a request (verbal or verbal in combina-tion with the give-me gesture). The total duration of this grasping test did not exceed 5 minutes.

Infants participated in the eye-tracker tasks (evaluation of give-me gesture interactions and gaze following) approximately 5 minutes following the be-havioral giving task. Testing began with an attention-grabber attract the in-fants’ attention to the monitor, followed by the evaluation of give-me gesture interactions and gaze following trials interleaved. The evaluation of give-me gesture interaction trials included four presentations from each condition for a total of eight trials. For the gaze-following task there were also eight trials. Orders of trials for both tasks were counterbalanced. The two eye-tracking tasks lasted approximately eight minutes in total.

At 18 months of age, parents completed the SECDI once again via mail.

2-year visit Infants returned to the lab at 2 years of age, in which they completed the SECDI one last time and participated in the behavioral giving task. The pro-cedure for this task is described in the ‘Measures’ section. A video camera recorded each session and a researcher later coded the children’s behavior.

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Giving was coded if the toy was passed to the researcher’s hand and non-giving helping was coded if the child placed to toy instead in the box. No action was coded if the child did not respond after 30 seconds. Other behavior (i.e., throwing or kicking the toy) was coded but not analyzed, as the behavior was ambiguous. Parents wore opaque glasses during the test. In this task, the experimenter is intended to be ambiguously inviting two responses that both help with tidying but are incompatible: copying the placement in the box, or complying with the give-me gesture.

Data analysis Evaluation of give-me gesture task Preliminary analyses found no effects of presentation order of trials. One-way repeated measures analysis of variance (ANOVAs) revealed no significant ef-fect for appropriate, F(2.24, 67.19) = .1.47, p > .05, or inappropriate trials, F(2.43, 72.8) = .420, p > .05, Greenhouse-Geisser correction applied. We collapsed trials within the two conditions for all subsequent analyses as there were no effects of trial for each condition. For correlation analysis we calcu-late a difference score for each individual as the extent to which the pupil dilated more in the inappropriate than appropriate condition. The dependent variable for each trial was the difference between the mean pupil size during the baseline period and the mean pupil size during the test phase.

Gaze-following task Three areas of interest were defined, one elliptical area for the head (12 visual horizontal degrees) and two rectangular areas, one for each target (six visual horizontal degrees). A standard difference score (Moore & Corkum, 1998; Gredebäck et al., 2008) was measured from the onset of the actor’s head turn to the end of the gaze phase and calculated for each infant as the number of times the first face to toy fixation shift went to the attended toy minus the number of times to the ignored toy.

EEG task Note that the independent results of the EEG task have previously been pub-lished elsewhere (see Bakker et al., 2015). The focus in Study II and in the current thesis is on the relation between this task and the other measures.

As reported in Bakker et al., only trials in which infants paid attention were further processed. The data was manually edited for artifacts (standard proce-dure for infant event related potential (ERP) studies, see Hoehl & Wahl, 2012). We rejected trails with excessive noise levels due to movement arti-facts. Channels with moderate noise levels were reconstructed from an inter-polation of surrounding electrodes. The inclusion criterion for the final analy-sis was at least 10 artifact free trials per condition. On average, an infant saw

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90 trials across both conditions, with 44 trials for the give-me gesture condi-tion and 46 for the control hand. After visual data inspection and manual data editing, a mean of 15 artifact free trials remained (range: 10–31) for the give-me gesture condition and a mean of 17 trails (range: 10–32) for the control hand. Finally, we baseline corrected and averaged all artifact free trials, as well as re-referenced to the average in order to create individual averages for each participant, as well as calculated grand averages from individual aver-ages. We selected 11 channels in the posterior area (62, 67, 70 71, 72, 74, 75, 76, 77, 82, 83) for statistical analyses. We captured three components in the ERP wave morphology and performed the analysis in three time windows: P1 (80–140 ms), N200 (150–250 ms) and P400 (300–600 ms).

We conducted ANOVAs to compare the mean amplitudes between condi-tions (the give-me gesture and control) in all ERP components (P1, N200, P400). There are only two prior studies that have examined the neural corre-lated of gesture perception in early infancy (Bakker, Daum, Handl, & Gredebäck, 2014; Gredebäck, Melinder, & Daum, 2010), and in both of those studies the ERP component P400 was shown to be sensitive to congruent ges-tures. It is suspected that the P400 indexes a wide range of gestures and was therefore the focus of the current study.

Behavioral task at 9 months The infant’s behavior was video recorded and later assessed for the frequency of appropriate responses, that is, the number of times the child gave a toy to the experimenter at the request (verbal or verbal in combination with the give-me gesture).

Behavioral task at 2 years For each participant, we calculated the proportion of trials containing the be-havior, and the latency from the start of the first trial to the first occurrence of the behavior in any trial for (a) giving and (b) non-giving helping. To avoid excluding participants from correlation analyses of latency, we set latencies to perform actions that were never performed to a high value of 100 s, invalidat-ing the assumptions of parametric tests. Proportion data also violated paramet-ric assumptions due to skewness (>1) as a result of many zero values. For both latency and proportion data, correlations are therefore calculated using Spear-man’s non-parametric method.

Note on sex differences It should be noted that in Bakker et al., 2015, significant sex differences were found. In that study, they found a significant amplitude difference between boys and girls, t(27) = 2.320, p = 0.028, with the interaction between sex and condition driven by the size of the difference between the conditions that is larger for girls (girls: M = 8 μV, SD = 6 μV; boys: M = 10 μV, SD = 8 μV). We did not, however, find any sex differences on any of the other tasks at 9

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months or 2 years of age; subjects’ sex is therefore not discussed further in the current thesis.

Results Evaluation of give-me gesture task There was a significant difference in pupil dilation between the appropriate and inappropriate conditions, paired samples t(52) = 3.855, p < .001, d = .56, with greater pupil dilation in the inappropriate gesture response conditions (M = .544, SD = .267) than the appropriate gesture response (M = .396, SD = .26).

Gaze-following task Infants fixated the actor’s face and then made a saccade to either of the two toys in the majority (63%) of trials. There were no learning effects; therefore all data across trials were collapsed. The first gaze shift from the actor’s face to a toy was analyzed, revealing that infants’ first gaze shifts tended to be directed to the attended toy, with a mean difference score of 1.10 (SE = .29), t(53) = 3.7, p < .001, d = 1.02, indicating that infants were reliably following the actor’s gaze (zero indicates random performance).

EEG task As reported in Bakker et al. (2015), there was a main effect of condition on the P400 amplitude, p < .001, with a mean amplitude of 15 μV (SD = 6 μV) in response to the give-me gesture and 9 μV (SD = 7 μV) in response to the control condition.

Behavioral task at 9 months None of the infants responded to the give-me gesture request as determined by the ESCS scale. Four infants responded by moving the hand with the object to the experimenter but did not release it. Two infants moved the hand away from the experimenter when seeing the request. None of the caregivers re-ported that their infant was able to produce or respond to the give-me gesture outside the laboratory. Therefore, no statistical analysis was performed.

Behavioral task at 2 years In the giving behavior task, 37 children (88%) displayed giving or non-giving helping, 24 children displayed non-giving helping (57%), 15 children dis-played giving (36%), and 5 children produced none of these behaviors (12%). The four dependent variables of interest included the proportion of trials with giving (M = .29, SD = .41), proportion of trials with non-giving helping (M = .45, SD = .41), latency in seconds to the first giving (M = 16.22, SD = 14.94), and latency in seconds to the first non-giving helping (M = 19.63, SD = 17.07; excluding children who did not perform the actions).

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Correlations of giving related measures at 9-months and 2-years-old Correlations are presented in Table 1. Infants with a greater P400 amplitude when viewing the give-me gesture compared with a control hand-shape had significantly greater pupil dilation, t(220) = -3.855, p < .001, d = 1.06, when viewing an inappropriate give-me gesture(M = .069, SD = .137) response than an appropriate response (M = -.078, SD = .145). See Figure 8.

Infants showing stronger neural processing of the give-me gesture at 9 months had a greater proportion of trials with giving and decreased latency to giving at 2 years (r = .47). Infants with a greater difference in pupil dilation during observation of an appropriate and inappropriate give-me gesture re-sponse at 9 months also had a greater proportion of trials with giving (r = -.45) and decreased latency (r = .42) to giving at 2 years (statistics here).

None of the giving-related variables correlated with any of the non-giving related variables at any age. Performance on the gaze-following task corre-lated positively with language ability at 9 months (r = .30), indicating that children with greater accuracy in the gaze-following task had greater reported language ability at 9 months. Performance on the gaze-following task was un-related to any of the other variables at 9 months or two years of age. Neither gaze following nor language at 9 months correlated with language at 18 months or two years, likely due to attrition in the later language measures which reduced power. There was continuity in language ability from 18 months to two years, suggesting validity of the measure (r = .57).

To establish that intra-giving-related-variable correlations were stronger than potential undetected correlations between intra-giving-related-variables and non-giving-related variables, we utilized a method suitable for comparing correlations (including non-parametric correlations) by z-transforming r (My-ers & Sirois, 2004). We compared a series of correlations that are comparable, for example, the correlation between pupil dilation and behavioral giving was compared with the correlation between pupil dilation and non-giving helping. Correlations between the three giving-related variables were stronger than comparable potential undetected correlations between these variables and be-tween non-giving related variables (four ps < .1, three ps < .05, one p < 0.01).

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Table 2. Correlations between Non-Giving Helping (placing in the box) and Giving at Two Years, P400 Amplitude, Pupil Difference Scores and Gaze Following at 9-months, and Language Scores at 9 months, 18 months, and Two Years of Age. Age Measure

1 2 3 4 5 6 7 8 9 10

24 1 Non-giv-ing help-ing (pro-portion tri-als)

- -.84** -.47** -0.42** -.19 -.16 .09 .13 .03 -.12

24 2 Non-giving hel-ping (la-tency)

- .45** -.41** .13 .25 -.10 -.07 -.01 .07

24 3 Giving (proport-ion trials)

- -.97** .51* .42* -.10 -.07 -.05 .19

24 4 Giving (latency)

- -.53* -.45* .07 .09 .03 -.26

9 5 P400 - .47* -.02 -.23 -.09 -.21 9 6 Pupil - -.09 .11 -.11 -.19 9 7 Gaze

following - .30* -.04 -.02

9 8 Language - .17 .09 18 9 Language - .57** 24 10 Language -

Note: All coefficients are Pearson’s r, except for proportion and latency correlations, for which Spearman’s rho is used due to heavy skew. *p < .05, **p < .01

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Figure 8. Difference scores in P400 mean amplitude in the neural correlates of give-me gesture perception task is positively correlated with difference scores in pupil size between the appropriate and inappropriate conditions in the evaluation of give-me gesture interactions task.

Conclusions This study demonstrated longitudinal effects of individual differences in early perception. The same infants that showed greater pupil dilation also showed an increased sensitivity to the give-me gesture, and later in development, the same infants were more likely and quicker to comply with a give-me gesture by giving. Importantly, none of the individual differences in giving perception and performance were associated with non-giving helping behavior or other non-giving-related socio-cognitive measures. These findings provide strong evidence for early maturing cognitive mechanisms for processing and produc-ing giving-related behaviors that generalize across different giving-related tasks and show longitudinal continuity through the early years.

Study III Background Allocation of selective attention resources in infants varies as a function of an infants’ emotional contexts, and certain children will also vary in how they process and understand information throughout development depending on

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early experiences (Pollak, Cicchetti, Hornung, & Reed, 2000; Amso, Haas & Markant, 2013). In adults, modulations of selective attention resources have been shown by presenting threat signaling stimuli, such as fearful faces, re-sulting in different patterns of subsequent environmental scanning (Becker, 2009; Haas et al., 2016). There is very limited examining how emotional con-texts affect selective attention during infancy, a period of paramount im-portance for neural development and learning (Markant & Amso, 2016).

Throughout development, these modulations of selective attention affect how infants and children perceive and interpret the world around them. The present study takes an integrative approach to selective attention by consider-ing three emotional contexts that have been found to modulate allocation se-lective attention early in life: maternal negative affect, infant negative affect, and the immediate emotional context. While there is evidence that each of these contexts separately impact attention to social stimuli, their joint impact on selective attention in a larger sense is not known. Thus, it is imperative to understand how infants who develop within an integrated system of caregivers with high levels of negative affect, certain temperamental traits, and exposed to negative emotional expressions, are differentially (and perhaps sub-opti-mally) impacted in their ability to selectively encode and learn from their en-vironment.

Procedure One experimenter obtained informed consent and photographed the mother’s facial expressions, while another experimenter accompanied the child in a sep-arate waiting room. The mother then returned to the waiting room with the infant and completed the affect measures (PANAS and IBQ-R), which took approximately 20 minutes to complete. The infant then participated in the eye-tracking task. This task began with an attention grabber followed by a random emotional face from either the mother or stranger presented for 1000 ms. Face emotion and familiarity were randomized and counterbalanced across trials. Trials advanced either from the infant’s fixation to the target, or automatically without a reward if the infant failed to fixate on the target within 6s. An atten-tion-grabber appeared on the screen between each trial and the next trial only began when the infant’s gaze was fixated on the center of the screen. The in-fants viewed a total of 64 trials (32 per set size, 8 per emotion). The task lasted on average 10 minutes with the total visit lasting approximately 45 minutes.

Data analysis We conducted a LMM with the AR1 covariance structure for the random and repeated fixed-effects. Each model was fitted using the restricted maximum likelihood criterion. There was a correlation between maternal negative affect and infant negative affect, yielding a Pearson r = .21, p < .001. Only maternal self-report on the PANAS was examined in the final LMM model due to the correlation and concerns of reliability of maternal report of infant behavior.

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We then conducted a LMM with participant as a random effect, emotion (an-gry, fearful, happy, blurred) as a categorical fixed effect, maternal negative affect (PANAS) as a continuous fixed effect, as well as coded familiarity (mother = 0, stranger = 1), and set size type (no distractors = 0, 4 distractors = 1) as continuous fixed effects. The dependent measure was the latency in milliseconds to locate the target in the visual search. We examined parameter estimates and follow up multiple comparisons with Bonferonni adjustments to determine significant main effects and interactions.

Results There was an emotion and set size interaction, F(3, 2402.69) = 6.09, p < .001, with faster times to locate the target for Set Size 5 only (see Figure 9a). There was also a significant two-way interaction of emotion and familiarity, F(3, 2395.89) =3.00, p < .05), with fearful and angry primes having opposite ef-fects on attention depending on whether the primes were of the mothers or a strangers face. Post hoc analyses for the angry emotion condition revealed that infants had shorter latencies to find the targets when they were primed with a mothers angry face, compared to a stranger’s, F(1, 2405.33) = 3.801, p = .051 (see Figure 9b). Infants had longer latencies to find the targets when they were primed with a mothers fearful face, compared to a stranger’s, F(1, 2405.87) = 4.058, p < .05. Post hoc analyses for the maternal prime condition, F(3, 2411.59) = 6.54, p < .001, revealed that infants were fastest to locate the target after seeing the mothers angry face, compared to seeing the mothers fearful (p < .05) or happy face (p < .001).

There was a three-way interaction of emotion, maternal negative affect, and set size, F(3, 2400.75) = 4.07, p < .01. For the fearful condition subset of the set size 5 condition, infants with mothers who scored low on negative affect demonstrated the slowest latencies to detect the target, while infants of moth-ers with high negative affect scores had the shortest latencies to detect the targets, t(121.46) = -2.92, p < 0.001 (see Figure 10).

Figure 9. Latency to detect target by (a) emotion for set size 5, and (b) emotion and face familiarity.

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Figure 10. Change in reaction time from set size 1 to set size 5 as a function of emo-tion and maternal negative affect (PANAS).

Conclusions Study III shows that the emotional context can impact infants’ on multiple dimensions, some immediate (the emotion of a face) and other reflecting as-pects of the infant’s environment or relationship to their caregiver. The results demonstrate specifically that maternal negative affect, a measure and used to assess the emotional environment provided by mothers, and the immediate emotional context from an emotional prime, have the capacity to impact real-world perception. If the face prime was the infant’s mother, then an angry emotion prime also resulted in faster search times. Together, the immediate emotional face prime, the familiarity of the face, and maternal negative affect impacted infants’ performance during visual search, demonstrating their ca-pacity to influence what infants attend to in their environment.

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General Discussion

The overarching aim of this thesis was to understand the development of ac-tion perception and how individual differences contribute to the perception and processing of actions. Research has demonstrated that the perception of actions and interactions is a dynamic process linked with perceptual processes, the internal and external states of the individual, prior experiences, and the immediate environment (Jacobs et al., 2004). Therefore, a particular interest for this thesis was also to examine action perception and action understanding in relation to various contexts of the child.

This discussion section will take a look at the main findings from each study and then examine the broader umbrella of context and what it means for action perception. First, the discussion looks in depth at the findings showing that infants, like adults, can differentiate between physically possible and physically impossible apparent motion paths. Additionally, the discussion ex-amines what these results might reveal about perceiving biological and non-biological motion (Study I). Next, the discussion focuses on the processing of actions in the context of a social interaction, and how infants differentiate ap-propriate and inappropriate responses to a giving exchange (Study II). By spe-cifically examining giving-related behavior, this part of the discussion allows predictions related to individual differences and the relation to later behaviors, offering insight into possible specialized mechanisms. The discussion then takes an integrative perspective on the joint impact that internal and external emotional contexts have for a child (Study III). Finally, we move to a broader discussion about the integrative role of context in perception, whether percep-tion can ever be truly decontextualized, and the theoretical and practical im-plications for acknowledging the experiences and environments of a child in understanding perception.

How do infants differentially process simple human versus non-human motion? Study I began at one of the simplest forms of motion perception – so simple, in fact, that no motion actually occurs, but is merely perceived. Even in still images, humans will tend to anticipate motion where there is none, reflecting

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a hard-wired bias to be sensitive to motion in our environment (David & Sen-ior, 2000). But not all motion is equal. Study I examined the how an apparent motion stimulus itself – either biological or non-biological – impacted the per-ceived motion in 12-month-olds and adults. When infants observed a biolog-ical apparent motion stimulus (an apparent motion stimulus with human fig-ures), they were sensitive to impossible motion paths and reacted with sur-prise, as indicated by pupil dilation. The same was not true if the apparent motion stimulus was an object; in this case, they failed to make any distinction between two different motion paths. This finding shows that infants at 12-months take the type of motion - either biological or non-biological - of ap-parent motion into account when constructing the motion paths from two still images. Furthermore, it also suggests that infants, like adults, take into account the constraints of motion paths, such as solidity and continuity, in beta appar-ent motion with a human figure. There are a few ways to interpret these find-ings.

Core knowledge One possibility is that physical and social knowledge (i.e, knowledge about object and human movement) are two separate systems that provide a founda-tion for all new knowledge and skills pertaining to physical or social con-structs. According to Spelke and colleagues’ core system theory, infants are born with a small number of dedicated systems that are at the foundation of human knowledge and perception (Kinzler & Spelke, 2007). The core system of object representation and the core system of agents and goal-directed ac-tions are two such systems. The principles within each specific system, such as continuity and solidity, are therefore more dedicated to interpretation of events within those domains. The different systems may involve separate mechanisms and the flexibility of beliefs in each system may vary. For exam-ple, it is clear that a physical object cannot pass through a physical barrier under any circumstance. However, the perception of a social agent’s action under the core social principles may be more flexible and the rules that govern the social world are far more complex and relative than those for inanimate objects. The social core system allows for beliefs that an agent can change its direction, path, and acceleration at will. In this view, the principles and mech-anisms of each separable core system may lead to very different interpreta-tions.

Top-down modulation of motion perception While it may be possible that such core systems may be present from birth, another possible interpretation of the findings is that this simple form of per-ception is impacted by top-down modulation at 12 months. Infants at this age

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have first-hand knowledge about the constraints of the body based on experi-ences. It has previously been demonstrated that infants are sensitive to viola-tions of constraints, such as solidity, continuity, and timing (Spelke, 1994), and form expectations based on this knowledge (Hespos & vanMarle, 2012). From own-experience interacting with the physical world and becoming fa-miliar with the constraints of human movements, infants may update expecta-tions about events based on the outcome of their own observed and experi-enced events. Indeed, there is evidence for an association between action eval-uation and action prediction, showing that infants become surprised when an action does not go the way they expect. A top-down modulation of biological motion therefore seems plausible.

Why is there no motion path differentiation with an object stimulus? One noteworthy finding in both Stevens et al. (1999), and replicated here in Study I, is that there was no differentiation between possible or impossible motion paths when the apparent motion stimulus was geometrical objects. Ste-vens and colleagues interpret this as adults processing human and object ap-parent motion stimuli differently. In fact, they go on to suggest that the spe-cific brain regions observed in their study are selectively activated to process actions which conform to the capabilities of the observer. We succeeded in the aim of Study I, which was to replicate these findings and extend them to infants. Therefore, it is possible that the differences in pupil dilation in the current study may represent differential processing of apparent motion by a human figure compared to an object. An alternative explanation, however, is that the same object control used in both Stevens et al., (1999) and Study I does not account for an important perceptual cue for motion, resulting in the differences found in both Study I and in Stevens et al. In the human figure condition, the arm begins on the other side of the leg with part of the arm hidden, signaling depth. But in the object condition there is no such cue, there-fore resulting in different basic perceptual information than the human stimuli. Because the object control does not account for the depth cue that is present in the human figure condition, it is possible that a depth cue is important for perceiving solidity and continuity in these stimuli. In order to rule out this explanation, a control would be needed that is perceptually visually similar to the human figure where the objects are placed behind one another and slightly occluded, signaling depth. This limitation in Stevens et al.’s original work (and replicated here) means we cannot rule out that the depth cue is responsi-ble for the observed differences in both studies.

The results of Study I nonetheless highlight that, similar to adults, infants can indeed differentiate between physically possible and physically impossi-

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ble apparent motion paths. But future work addressing the limitations of Ste-ven and colleague’s control is needed to validate their original conclusions regarding whether apparent motion operates differently for objects and human figures.

What the pupil indicates: Potential neural underpinnings Adults in Study I demonstrated similar results for the biological apparent mo-tion as the 12-month-old infants. When the apparent motion path was per-ceived to violate one of the assumptions of human movements, adults in-creased their arousal as indicated by greater pupil dilation. When asked, adults also reported as seeing a motion path that violated physical constraints when it was impossible for the path to be completed, given the amount of time the stimuli were presented for. This did not happen when observing the apparent motion stimuli that were possible given the constraints of the human body. The similar findings in adults replicate and extend the findings from Stevens et al. (1999), suggesting that it may be possible to measure a perceptual shift between possible and impossible human movements via pupil dilations.

The similar findings to Stevens et al., (1999) allow us to speculate as to the potential neural underpinnings of possible and impossible motion. Stevens and colleagues found a shift in activity between the ventral prefrontal cortex (associated with perceptions of violations of normal motor action path in adults; Nobre, Coull, Frith, & Mesulam, 1999) and motor and sensory regions in the brain when adults perceived the possible versus impossible path. In the impossible path, the adults perceived the hand passing through the leg, a movement that violates the constraints of human movement (solidity) and that the perceiver cannot perform themselves. Because there was no activation of the motor and parietal cortex during observations of the impossible path, this could mean that these regions of the brain are only activated when observing actions that do not violate constraints of movement.

Given the larger pupil dilations when perceiving a physically impossible motion path, changes in the pupil size in infants and adults may be indicative of a switch between activation in the ventral prefrontal cortex and the motor and sensory regions of the brain. While this interpretation is highly specula-tive, changes in the pupil may represent this dynamic activation because of the close link between the pupil and LC system in the brain (Laeng et al., 2012). Larger pupil size in the impossible conditions may reflect activation of the ventral prefrontal cortex, providing a marker for differentially processing physically possible and impossible apparent motion when performed by a hu-man figure.

Motion perception: looking aheadThe majority of prior research has demonstrated violations of constraints with stimuli that display object motion, not human motion (Spelke, Phillips, & Woodward, 1995). In the current study,

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we can only speculate that infants’ observation of an apparent biological mo-tion is specifically held to human action constraints, and that these specific constraints shape their actual perception of that action. However, the exact constraints that infants perceived to have been violated cannot be concluded from the current study. Additionally, additional object control conditions are needed to examine if these findings replicate to other stimulus types in both infants and adults. Examining apparent motion of objects provides its own challenges, as seen in Stevens et al., (1999) and in Study I. There is a still-not-entirely-understand line for when an object in motion becomes perceived as an intentional agent and no longer an inanimate object. The more information and expectations that are communicated through an object’s motion, the more that object may become perceived as an intentional agent. The question then becomes to what extent is an apparent motion path affected by the perception of an intentional agent?

Changes in the pupil observed in the current study may be indicative of a dynamic activation of different brain areas, with larger pupil size in the im-possible conditions reflection activation of the ventral prefrontal cortex. This speculation is supported by the close link between the pupil and LC system in the brain (Laeng et al., 2012). It is also supported by previous work showing differential brain activation when processing possible and impossible biolog-ical actions. If this speculation were to hold true, it would also provide support for a top-down modulation of perception that changes depending on the type of stimuli. Clearly, however, more research is needed to test this idea.

The results from Study I show that by 12 months of age, infants differenti-ate between physically possible and physically impossible apparent motion paths. It may also be that this differentiation may be dependent on if it is ap-parent biological motion. It is important for infants to have the ability to detect human biological motion, even with the most limited amount of information (Johansson, 1973). This ability aids not only their survival (Miller & Saygin, 2013), but also forms foundations for later perceptual and cognitive domains. For example, individual differences perceiving biological motion are corre-lated with later attention and executive function (Chandrasekaran, Turner, Bulthoff, & Thornton, 2010). The results from Study I suggest that there are important constraints, such as understanding that an arm cannot pass through a leg, that aid in this processing of motion performed by human figures. This finding could have implications for a wide range of processes related to action understanding. For example, these data suggest early perceptual based mech-anisms for how infants’ view human stimuli, and could have implications for how infants’ make sense of goal-directed actions, identifying agency, and pre-dicting motion paths. If there is indeed differential activation already in in-fancy, it would suggest an interaction between the brain, perception, and own experiences that underlie the foundations of action understanding. The results

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from Study I also pose an important question as to what the long-term conse-quences for individual differences in action perception and processing might be.

What are the long-term consequences of individual differences in early action understanding? In Study II, 9-month-old infants showed greater pupil dilation in response to social actions that violated their understanding of basic giving behavior, and the same infants that showed greater pupil dilation also showed an increased neural sensitivity to the give-me gesture. Finally, the same infants at 9 months who showed increased pupil dilation to an inappropriate giving response, and increased neural processing to the give-me gesture, were more likely and quicker to comply with a give-me gesture by giving a toy at 2 years of age. None of these measures were related to other socio-cognitive measures, in-cluding language ability or gaze-following ability. By focusing on a particular type of social interaction that included giving, it allowed us to make more specific predictions regarding mechanisms that might explain individual dif-ferences in early action understanding. Together, these findings demonstrate for the first time that early pre-verbal individual differences in giving percep-tion are stable over time and linked to actual behavioral differences in child-hood, providing evidence of for a system that has a large impact on how we perceive, interpret, and interact with others. This is very much in line with a neoconstructvist account, which emphasizes that the brain, biology, environ-ment, and learning mechanisms are continuously interacting. Because of these interactions, the growth of the system of the child is inevitably context bound (i.e., the child cannot be decontextualized), with new representations increas-ingly dependent on prior ones (Mareschal, 2011).

There has been a severe lack of research examining what the long-term consequences of differences in early action understanding are, and what vari-ations in development and experiences mean for long-term stability. Study II demonstrates the long-term consequences of early action understanding for later behavior. These findings may not come as a surprise if one considers the large body of work demonstrating longitudinal links in other areas of devel-opment. For instance, infants’ early gaze-following behavior (Brooks & Melt-zoff, 2008), join attention (Charman et al. (2000), and novelty responses (Woodward & Sommerville, 2000). But the longitudinal results shown here further emphasize how early knowledge is actively shaped through experi-ences that guide early reasoning, and that infants’ early reasoning is related to later more mature knowledge concepts in development.

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Does early action-experience drive the understanding of others’ actions? At 9 months of age, none of the infants themselves produced giving in re-sponse to the give-me gesture, or did the parents report behaviors associated with responding to the give-me gesture. Despite this inability to perform or respond to the give-me gesture themselves, infants in Study II were able to demonstrate understanding by evaluating actions that violated the norms of the give-me gesture, as indicated by greater pupil dilation in response to inap-propriate as compared to appropriate actions. Therefore, it appears that the cognitive mechanisms for the perception and understanding of giving-related behavior develop before overt responses to other people’s give-me gestures. This represents an example of early understanding through observation being important for later action ability, rather than early action-experience driving the understanding of others’ actions (Gredebäck & Falck-Ytter, 2015; Gott-wald, Achmermann, Marciszko, Lindskog, & Gredebäck, 2016). This finding supports earlier joint action work, showing that joint action understanding may precede and perhaps even form the basis of own joint action ability (Sebanz et al., 2006).

Most prior “action first” studies focus primarily on basic motor actions. In Study II, infants can physically give things, but the results demonstrate it is about knowing the social circumstances for giving and being motivated to complete the giving action. This would imply more of an importance for top-down knowledge rather than basic motor skills. Actions of this nature, i.e. giving, might be understood in observation before they can be carried out be-haviorally. Similar findings have also been demonstrated in children’s helping and cooperation abilities (for review, see Holvoet, Scola, Arciszewski, & Pi-card, 2016), with early perceptual understanding in infancy but only later be-havior.

The importance of giving-related behavior Previous studies have speculated about the major role that giving and resource transfer play in human evolutionary history, and have even suggested that the assertion that “humans possess an early conceptual knowledge of social goals that should be extended to include basic social interactions based on giving” (Tatone et al., 2015). The findings from Study II may in part be explained by the particular importance of giving-related behavior for humans. One possi-bility is that there is an underlying specialized mechanisms that accounts for giving understanding and behavior that are already developing early in in-fancy, before the emergence of language and other advanced social exchange behaviors. Infants are able to use these specialized mechanisms when observ-ing third-party social relations (Mascaro & Csibra, 2012). Children then use

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these mechanisms later in development to guide their own first-person behav-iors in giving interactions.

The nature of such cognitive mechanisms cannot be established more pre-cisely than that they appear specialized in some way for giving-related behav-ior. Furthermore, it is difficult to separate infants’ understanding of giving behavior from understanding of the give-me gesture, although understanding of the connection between the gesture and giving is clear. However, we spec-ulate that there are at least two possibilities for explaining infants’ variability in understanding and correlation to later behavior. One possibility is that these mechanisms relate specifically to give-me gesture processing – all tasks in-volved understanding of the give-me gesture and this explanation is therefore parsimonious. However, fundamental to performance in both the eye-tracking task and the behavioral task is an appreciation of the connection between this gesture and the appropriate response of giving, so the understanding demon-strated here relates not only to the gesture but also to giving itself, including the tendency to produce it.

A second possibility worth considering is that the variation in understand-ing may reflect more general gesture processing mechanisms not specifically related to the giving-related behavior. This would suggest that the associations between understanding and production of giving-related behavior may be due to individual variation in mechanisms not specific to giving-related behavior, but rather a more general social cognitive ability. This explanation does not seem very likely, however, there were no associations between giving-related measures and any other variables. This includes giving-related measures and measures of language ability, a measure that included gestures. Furthermore, behavior at 2 years was separated into two types: giving (in response to the give-me gesture) and non-giving helping (putting the toy in the box). Both of these behaviors are prosocial in nature. Although individual differences in so-cial cognition could account for the correlation between giving understanding at 9 months and giving at 2 years, it would also predict a similar correlation between giving understanding and non-giving helping, which was not found.

Finally, we cannot fully exclude the possibility that the results demonstrate a broader understanding of ‘action between two people’. One could argue that two of our control measures do not account for a broad category that includes helping, cooperation, and comforting; gaze following is just one person and language is verbal, not active. However, our third control measure at 2 years accounts for general helping behavior that is not specific to giving. This meas-ure was not correlated with any other measures, but was inversely correlated with giving-related helping.

Long-term consequences and looking ahead In summary, the evidence from Study II suggests that giving plays an im-portant role in human development. Individual variations in early perceptions of giving-related actions had long-term consequences for actual giving-related

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behavior. Prior work has demonstrated that giving behavior emerges earlier than other forms of instrumental helping (Warneken & Tomasello, 2007) and other forms of prosocial behavior (Dunfield & Kuhlmeier, 2013). The exist-ence of cognitive mechanisms for giving in infancy that continue through childhood is therefore consistent with the apparent importance of giving for human development (Godbout & Caillé, 1998; Mauss, 1954; Rochat, 2009).

As discussed, additional research should revisit these questions incorporat-ing other domains of action understanding and behaviors, not just giving. Fu-ture research should also further examine early underlying mechanisms for giving related behavior. Studies should include measures that account for a broader category if two people interacting, such as measures of cooperation and comforting. Furthermore, methods that are more naturalistic at 9 months would be valuable for looking at the relationship between early understanding and later behavior. Finally, different contexts of the child may influence both understanding and behavior. For example, although giving appears culturally universal (Rochat, 2009; Tatone et al., 2015), the forms in which infants are exposed to it may differ in ways that could influence the findings.

How do differences in infants’ environmental and internal contexts influence their perception of the world? Study I and Study II both examined specific types of perceptions and their individual relation to infant’s understanding or performance of an action. Study III instead aimed to take an integrative approach to the relationship be-tween perception and context – how do multiple contexts interact and affect infants’ perceptions in the world? In Study III, we therefore examined multi-ple contexts of an infant, both internal (infant temperament) and external (ma-ternal affect and an emotional face prime). The immediate emotional face prime, the familiarity of the face, and maternal negative affect each impacted 9-month-old infants’ performance during visual search, demonstrating their capacity to affect infants’ visual attention. Specifically, infants had faster vis-ual search times on a non-emotionally valenced search task after being primed with a fearful face. This effect was even greater for infants of mothers with higher negative affect scores, resulting in even faster visual search times after being primed with a fearful face. If the face prime was the infant’s mother, then an angry emotion prime also resulted in faster search times.

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The emotional context Immediate emotional context Fear is an emotion with a particularly powerful impactful on infants’ attention in the immediate emotional context. Being primed with a fearful face resulted in more vigilance in the perceptual field, resulting in increased attentional re-sources and faster search times. This makes sense, as social stimuli provide useful cues to identify the immediate emotional context and can modulate in-fants’ attention. For example, when a child observes a parent or other relevant individual displaying a fearful expression, he or she quickly becomes attuned to identifying potential threat in the environment. In such scenarios, it is most adaptive to be vigilant of their surroundings, heed caution, and stay close to their caregiver.

Prior work has demonstrated that adults also show more efficient visual search for nonthreatening objects after being primed with a fearful face (Becker, 2009). Furthermore, an emotional arousal from a fearful face results in attending to fewer environmental cues and attending more to information that is most salient (Cohen, 1978; Baron-Cohen et al., 1994). Together, the results from the immediate emotional context in Study III, along with prior studies, suggest that it is important from an early age to utilize social context cues from caregivers and other individuals in the environment.

Familiarity Prior work has shown that the face of a child’ caregiver is a particularly im-portant source of information for the child (Gredebäck et al., 2011). It is there-fore surprising that there were very few findings in study III based on the fa-miliarity of the presented face prime. Anger was the only emotion for which familiarity played a role. While a fearful prime was important regardless of the familiarity of the face, anger was specific to the mother, suggesting that a mother’s emotionally angry face may be particularly salient for an infant. This could be due to either a particular sensitivity to this emotional expression for the child (an angry mother is triggering for the infant and may signal sever consequences for the child), or a novelty effect (the infant typically does not see their mother angry).

Maternal affect A history of an affective environment experienced by the child or caregiver, such as stress (Pine et al., 2005; Pollak, 2008), has a critical impact on emo-tional development and selective visual attention. Indeed, the results of Study III showed that for infants of mothers with higher negative affect scores, visual search times become even faster subsequent to being primed with a fearful face. Infants learn affective signals in their environment and individual differ-ences in affective environments result in differences in the perception of emo-tional signals. For example, infants of mothers with higher negative affect may

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have increased exposure to negative emotions, such as fear. This interpretation is supported by a large body of work showing that stress in early childhood may result in attention bias to threat cues and changes in attention allocation (Mogg, Bradley, & Hallowell, 1994; Pine et al., 2005; Pollak, Cicchetti, & Klorman, 1998; Romens & Pollak, 2012).

An integrative approach to the impact of emotional contexts Study III provides strong evidence for context-dependent development and the complexity of the system of the child. In Study III, the system of the child includes both the immediate emotional context, in this case the emotional face primes, and the prior emotional context, in this case the mother’s negative affect. Prior work has shown that young children with histories of atypical emotional experiences show differential cognitive processing for facial ex-pressions, such as a heightened ability to identify fearful faces (Masten et al., 2008; Pollak & Sinha, 2002). Another more recent study showed that exposure to mothers’ negative emotions plays a role in the development of infants’ at-tention to facial expressions in typical development (Aktar et al., 2018). One possible interpretation is that this is an example of a child’s adaption to the emotional environment. This would mean that an increase in attention to the emotional face could carry over to attention within the environment beyond face processing, serving as an adaptive mechanism by increasing vigilance in the environment.

The results in Study III that demonstrate an additive effect of maternal neg-ative affect on infants’ visual attention. This means that the system of a child includes multiple contexts. It is clear that the various prior and immediate emotional contexts of a child are not independent from one another, but rather are integrated in a system that has the capacity to influence infants’ selective visual attention. For the big pictures, how a child learns to view and interact with the world is no doubt shaped in part by the influence of these various contexts. This of course is not a new concept and in fact has been a focus in many fields, for example attachment research. Studies show the importance of early interactions for later life. The results from Study III extend this work to perception and attentional processes of young infants.

One could rightfully argue that the dependent measure (time to locate tar-get) was assessed only seconds after the emotional prime, therefore only demonstrating short-term effects on subsequent visual attention. However, maternal negative affect reflects a broader environmental context of the infant – one that is far more constant than an emotional prime. Therefore, even if the effect on visual attention is short-term, the child may be regularly exposed to this environmental context, creating a potential for more long-term conse-quences.

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The importance of context From the findings of each study included in this thesis, we can see that infants’ perceptions are integrated with their various contexts and experiences. In Study I, the findings demonstrate how infants’ perception of human motion is specifically integrated with knowledge and experiences regarding the con-straints of human movements. Study II demonstrates that individual variations in early perceptions of giving-related actions had long-term consequences for actual giving-related behavior. Finally, Study III shows the joint functional role of emotional contexts (such as an emotional prime and maternal affect) on how infants’ view the world.

The context of the child includes the broader sense of the child’s life – prior experiences, interactions with people and objects, and the various environ-ments they experience. Previous work has shown that certain children will vary in how they process and understand information throughout development depending on specific environments and experiences (Pollal et al., 2000), sug-gesting that investigating the role of a child’s broader context is important for understanding the development of action and interaction perception. Yet de-spite the importance, very few studies factor in the context in examining the perception of actions and interactions. Even fewer studies address the need to understand how infants who develop within an integrated system of various contexts are differentially impacted in their ability to selectively encode and learn from their environment.

In the three studies included in this thesis, we have learned that perception cannot be decontextualized; the context is an always present and an integrated part of perception. The context has the capacity to actively influence how the child perceives the world. This is especially clear in Study III, where infants’ visual attention is influenced not only by an immediate affective context, but also from the negative affect of their mother.

Moving forward with context There is a greater need to acknowledge the context of the child in research on the perception of actions and interactions in development. This may include direct measurements of various contexts, like what was done in Study III. It can also include acknowledging the prior experiences of a child, or acknowl-edging that placing an action within a context may alter children’s perception and subsequent processing of that action. There is a need for research that considers the perception of actions not in a vacuum, but in the broader context of children’s life experiences and environments. For example, would children of refugees, where families may regularly experience higher levels of stress, show a similar relationship between visual attention and various emotional contexts?

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There have multiple reports about a replication crisis in psychology (Earp & Trafimow, 2015; Maxwell, Lau, & Howard, 2015). These reports claim that there is a lack of replication among many important psychological findings, bringing these findings into question. This could possibly be due to poorly designed measures and studies, inconsistencies in experimental procedures, and publication bias. While these may certainly be factors, the results from the individual differences and importance of context in the current thesis offer another perspective: perhaps the lack of replication is also due to differences in samples. Samples with variability in context may also have an effect on the findings in psychology, particularly in infant studies, which typically have rel-atively small sample sizes. By failing to account for differences in contexts, studies may be missing important information for replication. The majority of studies in psychology fall into the WEIRD category (White, Educated, Indus-trialized, Rich, and Democratic), with estimates around 99% of all published studies relying samples that fit those criteria (Henrich, Heine, & Norenzayan, 2010). More consideration is needed toward accounting for sample differences and including more diverse samples. This is especially true for studies that report on averages in very small samples. Results from small samples are highly dependent on the characteristics of the sample.

Children’s contexts are interactive and constantly changing. This means that there is also a need for more longitudinal work examining the stability of psychological mechanisms. Longitudinal investigations are important for un-derstanding the mechanisms underlying the relationship between early per-ception and later complex behaviors. Longitudinal work should take into con-sideration both immediate and cumulative effects of various contexts, such as stress and negative affect. It should also look toward relations to other cogni-tive processes, such as attention and executive functions. It is important to examine such long-term consequences as they may have implications for be-havioral and socio-cognitive developmental outcomes. This is particularly im-portant for children that might be in certain impoverished environments or at greater risk for early exposure to increased stress (Pollak, 2008).

Theoretical Implications In the introduction I reviewed several theoretical perspectives that account for the development of action understanding: the teleological stance, statistical learning, the embodied account, and neoconstructivism. The three studies here do not provide unilateral support for one theoretical perspective over another. This is partly due to the fact that these theoretical perspectives are not mutu-ally exclusive, nor do their predictions necessarily compete with one another. Therefore, instead of arguing for one dominant perspective, I will instead highlight how the results from each study argue that each theoretical perspec-tive requires a consideration of context in action perception and processing.

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In Study I under the teleological stance, one may conclude that the percep-tion of differential motion paths was a result of a rational interpretation of the action, goal, and constraints. In the object condition, not enough information was present to make a rational interpretation. Similarly, in Study II, a teleo-logical interpretation would be that the goal, action, and constraints are being interpreted through a rational system, allowing for prediction. The results from Study III might suggest that the experiences of the child, such as the emotional context, may actually affect this rational predictive system in a significant way. Together, each study points to a rational interpretive system that is not only affected by context, but that context may actively shape rational interpre-tations of events.

A statistical learning stance might interpret the results from Study I and Study II as a computation of the motion paths and giving behaviors based on experience with elements in the environmental input, allowing infants to ex-tract the appropriate pattern of human movement and constraints for further processing. The results from Study III clearly support the importance of the environment input for subsequent processing and, ultimately, learning.

From an embodied account, the infant’s own experiences with his/her body and movements inform the infant about what is possible and impossible in Study I. Therefore, perceptions of a human figure’s actions may change de-pending on an individual’s own-experience with that action. The results from Study II, however, suggest that perhaps own-experience is not essential for understanding appropriate giving behavior. In this study, the infants demon-strated an understanding before they were able to perform the action them-selves. In this case, perhaps an infant’s experiences with observations are im-portant. In Study III, the results would be interpreted by an embodied account as demonstrating that the context of the infant, such as the emotional input by the mother, is highly relevant to what the infant perceives and processes in the environment.

Finally, from a neoconstructivism perspective, each one of the studies demonstrates an interaction between neurological mechanisms, such as the differential activation of brain areas, and experiences of the infant, such hu-man movements and constraints. In this sense, a neoconstructivist perspective is the most parsimonious with the findings from each Study. In sum, the results from each Study add varying support to these perspectives. It is clear, how-ever, that whichever perspective one takes, the dynamic role of the child in his/her context is essential when considering action perception and processing and subsequent learning.

Final Thoughts Individual differences are related to differences in the perception and pro-cessing of actions. This is because the perception of an action is structured not

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only by intentions, goals, and extensions of thoughts and feelings, but also by the broader context of the action. The findings from this thesis show that, from an early age, the environment and experiences actively shape infants’ percep-tion of the world, in the both the short- and long-term. Each of the three studies included in this thesis speak toward a context-dependent system for develop-ment and are compatible with several different theoretical perspectives. Though additional research is warranted, the findings also support the idea that infants might be equipped with specific content (such as a bias for human motion) that aids in and forms the basis for later learning (such as the con-straints involved in human motion).

In summary, it is not just the action alone that is important for perception; it is also the context that constrains the action. The results here suggest that given differential environmental contexts and experiences, there will be dif-ferences in how individuals perceive and interpret actions and interactions (e.g., Pine et al., 2005). There is a functional role of context in infants’ learning and interpreting the world around them; acknowledging infants’ broader con-texts will aid researchers in understanding the development of action percep-tion, and how individual differences contribute to the perception and pro-cessing of actions.

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Acknowledgments

The journey that has brought me to the completion of my doctoral thesis has been long and challenging. When I first started this quest for a PhD, I could never have anticipated where it would take me. From the rainy forests of the Pacific Northwest, to the neon deserts of Las Vegas, across the world to the little student town of Uppsala, the medieval island of Gotland, and even to the remote and isolated country of Bhutan. I have always worked toward the end goal of receiving that document granting me PhD status, but I never could have imaged I would also receive another document along the way equally as valuable – a document granting me citizenship in Sweden. Today, I call Swe-den home. And as with any home, I have also been blessed with people here I can truly call my family. So, to my family in Sweden, to my family in the US, my dear friends, colleagues, mentors and supervisors– none of my accom-plishments would ever have been possible without you.

First and foremost, I certainly would never have achieved any of this with-out my supervisor, mentor, and friend, Gustaf Gredebäck. My decision to quit my program in Vegas and instead stay in Sweden was a direct consequence of your incredibly contagious warmth and welcoming spirit from the very mo-ment I arrived. And I am a better person for it. You have been a constant source of positive encouragement and inspired ambitions and passions I never even knew I had in me. You made it impossible to ever leave. And since that day I first arrived, you have continued to support me, encourage me, challenge me, sometimes comfort me, and always have my back, even during the most trying times, both in academia and in life. I am so grateful for the many expe-riences we have had and I think we made a great team, although in many ways we were always perhaps a bit too much alike for our own good – as evidenced by our inability to ever say ‘no’ to new cool research ideas. But because of that, I have grown to respect you beyond words as both a researcher and a friend. I am looking forward to our continued teamwork together and the bril-liant studies that will come out of our collaborations and the gaming lab. Oh, and I still think we can do some kind of eye-tracking study on babies and archery, perhaps related to core knowledge.

I have also had an immense amount of support and encouragement from other supervisors, collaborators, and mentors. Christine Fawcett, thank you for not only your supervision but also for helping me adjust to the Swedish way of things. Agneta Herltiz, you quite literally adopted me and kept me alive when I first arrived in Sweden. Thank you for everything you have done for

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me, you are my family here. To my committee members, Leo Poom and Gunilla Stenberg, thank you for taking the time to carefully and thoughtfully help me improve my work for thesis. And Gunilla, it has truly been a pleasure working with you. Thank you to my collaborators at the department, Ben Ken-ward, Marcus Lindskog, Pär Nyström, Fredrick Åhs, and Jörgen Rosén, you were always more than just co-authors. You each individually helped me grow as a researcher and it was a blast working with each one of you. Also to the brilliant Sara Haas, another victim of Sweden’s allure. To Marta Bakker, you were a true friend and guide for me in the beginning when I was terrified of teaching –you helped me get past that! To Håkan Nilsson and Ingrid Lagerlöf who also helped me discover my love for teaching and always made sure I had plenty of it! A huge appreciation for Cecilia Sundberg who seriously put up with a lot of weird and complicated paperwork over the years (from immigra-tion and visa forms and housing paperwork, to US student loans, US taxes, and strange documents for Bhutan). Thank you for your patience – I surely would have been deported long ago if it weren’t for you! And finally, thank you Magnus Johansson for believing in me, I look forward to what the future brings.

To my other colleagues, it has been a blast to share this road with you. I will miss our office parties, gaming, and AWs. Kim, Kahl, Anton, Philip, and Joakim – it has been an absolute pleasure sharing an office with each of you and I am grateful for those times! Except when Kahl played his music. Ma-tilda, Janna, Viktor, Linn, Elin, Johan, Maleen, Dorota – I am thankful for memories with each one of you at conferences, in classes, or after work. Ben-jamin, you were the best ‘mentor’ any new doktorand could ask for!

To my mom and dad – I know this has been just as trying of a journey for you as it has been for me. Thank you for the unconditional love and support, even across the world. It gave me confidence knowing that no matter what happened, no matter where in the world I was, you were always behind me cheering me on. I would not be who I am today if not for you and the way you raised me to be. Believe it or not, but I am finally –finally– no longer a student! Now that I am finished…I am going to Disneyland!

To Justin, thank you for always making me smile and laugh. Stress always disappears when we were together, there is nobody I can escape with as I can with you, and I go back to the good ol’ days. Thank you for being on this journey with me, all the way. Don’t forget these wise words: One thing you must never lose is courage. If you believe in the goal you are striving for, you will be courageous. There are many difficult times ahead, but you must keep your sense of humor, work through the tough situations and enjoy yourself. When you have finished this cup of coffee, your adventure will begin again…

Tommie, it is simply not possible for me to fully express to you or for you to fully understand how important you have been to me over these years. I have my citizenship, I have my PhD - but I think the most important and treas-ured thing to come out of all this is you. Thank you for all the laughs, the tears,

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and the beers. Well, I hope that someday, buddy, we have peace in our lives. Together or apart, my best unbeaten brother.

To all my friends, I don’t even know where to begin. When I came here I knew no one. Now it feels like I have the biggest family on earth. And without the joy my friends bring me, there is no doubt in my mind I would never sur-vive academia. Julia Brehm, thank you for being my first friend in Uppsala and teaching me the ways of Flogsta and the Nations. My life has never been the same since. Henrik, truly the punniest guy I know. Thank you for making me laugh, for the best Valborgs ever, and for just being a good person in my life. Claire, my best dancing buddy and dear friend, you have always been there for me from the beginning. I will miss our office couch! Isabel and Jakob, the life of my parties (and Mario Party)! Benjamin, our long talks over whiskey and cigars always helped me keep things straight. Isabella, thank you for a special bond that has brought me joy and comfort and amazing good times. Anna, even though you moved away to the black hole of Stockholm, I am so happy whenever we can catch up and it feels like no time has passed. Nils, you are both a friend and an inspiration and I am proud to have collabo-rated with you! Hannes, I have always been able to count on you – thank you for unforgettable memories, far too many to list here. Jon and Andy, even though you are across the sea, you have never felt far from me, thank you for your love and support. Babsi, you have been the best flatmate anyone could ever ask for. I owe you so much just for putting up with me through these final years of my PhD. To all my other friends, there simply just is not enough space to write all the incredible memories we have shared – at Halloween and Amer-ican parties, at the nations, midsummers, and during travels - but I am so thankful for each and every one of you throughout these years.

Last but not least, to Krystal and Laila, who started this long journey with me from the very beginning back in Vegas and let me drag them here with me to Sweden. I would never have been able to do this alone, and your support has been nothing but sacrificial and endless. It is crazy to look back and see how far we have come. It has not always been easy, but your unconditional belief in me got me through the worst of times. I know I am not the easiest person to deal with under stress, and it turns out a PhD is pretty stressful. So thank you, from the bottom of my heart, for always being there no matter what. I would never have been able to do this without you.

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