7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
1/28
MATRIC NUMBER: 0904872B
SUPERVISORS NAME: Gregor Thut
DATE: 13/03/2013
LEVEL 4 PSYCHOLOGY
The contribution of individual differences, time-on-task and object versusspace-based representations to production of systematic bias in spatial
attention.
Declaration: I have read and understand the School of Psychologys guidelines onplagiarism (found in the class handbook) and declare that this essay (report) is entirely myown work. All sources have been acknowledged in the text and included in the referencesection. All quotations from other authors are marked as such in the text
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
2/28
1
Maxi project
0904872b
Abstract
Processing of visuo-spatial information is strongly modulated by hemispheric specialisation of
associated function. An example is a systematic leftward bias in spatial attention (pseudoneglect)
observed in general population which is believed to be the result of right hemisphere dominance for
visuo-spatial attention. However individual differences have been found both in direction and
magnitude of the bias. The leftward bias has been shown to shift to the left and even reverse in the
opposite direction as a result of increased fatigue. The role of both object-specific and space-specific
representations has been proposed to mediate spatial bias. Current study investigated the effects of
these factors on biases in line bisection and discusses the findings in relation to models of spatial
attention. Participants performed a computerised landmark task for 1 hour to induce a time-on-task
effect. Individual differences were accounted for by dividing participants into groups based on the
direction of their initial bias. The role of space-specific representations was investigated by
measuring the visual field advantages characterised by difference in reaction times to stimuli
presented to the left and right visual field. The results highlight the need for accounting for
individual stimuli in studies investigating biases in line bisection explaining the inability of some
studies to replicate leftward bias in line bisection. More importantly participants differing in initial
bias show different trends for the time-on-task effect in producing shifts in bias on line bisection task
questioning models assuming the involvement of distinct processes. No changes in visual field
advantages were recorded over the course of the experiment suggesting that the time-on-task effect
is better explained by object-specific rather than space-specific attentional processes. The research
invites further exploration into differences in neural connections underlying individual differences in
spatial biases and creation of models which would account for these differences.
1. Introduction
Processing of visuo-spatial information is strongly modulated by hemispheric specialisation of
associated function. As a result the information is not evenly distributed within the brain leading to
biases in spatial attention. A vivid example is unilateral, hemispatial neglect often manifesting after
right hemispheric brain damage characterised by difficulties reacting to and orienting attention
towards stimuli presented on the contralesional side of the visual field (Heilman, Watson, &
Valenstein, 2002). When asked to estimate a midpoint of a line, the majority of neglect patients
exhibit a strong rightward bias (Schenkenberg et al., 1980; Vallar, 1998; Fischer, 2001). In healthy
subjects there is a similar tendency to exhibit a spatial attention bias during a line bisection task
referred to as pseudoneglect(Bowers and Heilman, 1980). The direction of this bias is usually found
to be in the opposite direction (leftward bias) and of smaller magnitude than that of neglect patients
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
3/28
2
Maxi project
0904872b
(Jewell and McCourt, 2000). Pseudoneglect is a robust phenomenon being replicated in studies
differing in stimulus type and context factors. It was found for stimuli differing in length (McCourt
and Olafson, 1997), brightness, numerosity and size (Nicholls et al., 1999). However the magnitude
and direction of bias depend on a number of factors including selected context (Jewell and McCourt,
2000; Heber et al., 2010; Schmitz et al. 2011; Benwell CSY et al., 2012).
While being a relatively reliable phenomenon some studies were unable to find any
significant indication of bias during line bisection suggesting possible influence of individual
differences associated with visuo-spatial bias (Halligan et al., 1990a; Halligan, 1990b, Nichelli, 1989).
Halligan et al. (1990a) demonstrate a high inter-individual variation of subjects performance on a
line bisection task. Apart from a high inter-individual variation only about half of the subjects
demonstrated a typical leftward bias while the other half show bias in the opposite direction. As a
result no statistically significant effect of a bias was found by Halligan et al. (1990a-b) when all the
subjects were considered together, which might account for some of the difficulties replicating
leftward bias in pseudoneglect in some studies. The reported measure of bias will therefore often
depend on the proportion of right shifters and left shifters within the selected sample (Halligan et
al., 1990a). As such these differences pose constraints upon measuring the spatial attention bias in
healthy subjects as well as question some basic assumptions about the magnitude and direction of
such bias.
It is generally believed that pseudoneglect is the result of a leftward bias resulting fromhemispheric differences in attention. The models of spatial attention propose involvement of both
hemispheres but to a different degree (Heilman and Van Den Abell, 1980; Mesulam, 1983, 1990).
Heilman and Valenstein (1979)propose the right visual field to be represented in both hemisphereswhereas the left visual field to be represented in the right hemisphere only. According to this model
the spatial attention biases are a result of the differential activation of the two hemispheres
resulting in an uneven distribution of attention , i.e. biased towards the hemispace contralateral to
the dominant hemisphere (Kinsbourne, 1970). The predominant right hemisphere activation during
a line bisection task (Foxe et al., 2003; Waberski et al., 2008) can therefore account for a leftward
shift of attention causing an increase in the perceived relative length of the associated side of line
thus shifting the mid-point towards the left from the veridical centre. However it is questionable
how this model would account for the variations observed with different context factors and for
individual differences in spatial attention bias demonstrated by Halligan et al. (1990a, 1990b).
An alternative explanation posits that the general (egocentric) location within the visual field
(left or right) and associated hemispheric activation is irrelevant for determining bias in line bisection
and the leftward bias is object-centred, i.e occurs because the left side of objects is in general
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
4/28
3
Maxi project
0904872b
perceived to be longer than the right side. Indeed, spatial attention is not governed solely by general
spatial (egocentric) information regardless of any objects in the scene, as would be suggested by the
widely accepted theory that attention behaves as a spotlight moving through space selecting
specific regions and objects within these regions as they come into focus (Posner, 1980). Research
with both healthy (Duncan, 1984; Kanwisher & Driver, 1992; Gibson & Egeth, 1994) and brain
damaged participants (Driver & Halligan, 1991; Behrmann & Moscovitch, 1994) has shown that
attention might be guided by object-based as well as location-based representations. In terms of
spatial neglect Tipper and Behrman (1996) and Egly et al. (1994) have demonstrated that biases in
attention are mediated based on the availability of object related information and are not limited to
spatial information.
Nicholls and Orr (2005) investigated the effect of object-based and general space-based
reference frames on pseudoneglect. Participants were asked to make luminance judgements
between two mirror reversed greyscales . The location of the greyscales differed throughout the
experiment being placed across various locations between the very-right and very-left side of visual
space. The greyscales were arranged so that when displaced the object and space- specific
information was congruent or incongruent. In the congruent condition the direction of expected bias
based on the luminosity judgement was aligned with that based on object location in space. In the
incongruent condition the bias associated with luminosity of the object was reversed to the bias
produced by the spatial information. Pseudoneglect was observed in the baseline condition and,with increased magnitude, in the congruent condition while no bias was recorded in the incongruent
condition. The results show that both object based and space-based attention processes affect
visuo-spatial bias and operate independently, creating additive effects in the congruent condition to
produce larger bias while cancelling each other out in the incongruent condition. Additionally the
object based bias increased with higher object centrality shedding doubt on models based purely on
spatial attention as proposed by Kinsbourne (1987, 1993) and Heilman (1979). The discussion of the
contribution of object specific and space specific attention creates a framework for the exploration
of some of the differences in direction and magnitude of attention biases as well as difficulties
replicating pseudoneglect across some studies.
Besides stimulus factors and individual differences state of arousal and general fatigue have
also been found to have effect on spatial attention bias. Manly et al. (2005) demonstrate a shift of
leftward bias to rightward bias after a sleep deprivation period in healthy subjects. In addition a
dynamic reversion of attention bias has been found when participants spent an hour practicing a
standardised line bisection task (Manly et al., 2005; Dufour et al., 2007). This time-on-task effect
suggests that pseudoneglect could indeed be a fairly fluid phenomenon modulated by an interaction
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
5/28
4
Maxi project
0904872b
between visuo-spatial and attentional processes. Benwell CSY et al. (2012) investigated the time-on-
task effect on attentional bias using a variation of a landmark task. Participants were asked to carry
out midpoint judgements on a series of short and long lines to account for the effect of stimulus
factors. It was shown that there was a significant time-on-task effect over an hour of trials causing a
rightward reversal of bias when the task was performed with long lines. However no time-on-task
effect was found for short lines despite similar recorded drop in overall vigilance. This lead the
authors to propose a resource based model of attention suggesting that the effect on spatial bias
depends on continuous performance but not on general fatigue. The right hemispheric dominance
causing the leftward attentional biases might decrease as a result of neuronal fatigue produced
throughout the task. Similarly it was assumed that processing of long lines to produce midpoint
judgements would require more attentional resources resulting in higher load on the neurons in the
right hemisphere than processing of short lines resulting in a time-on-task effect for long lines but no
or significantly lower time-on-task effect for short lines.
This was taken as support for study of Schmitz et al. (2011) who found conflicting findings
when replicating the study by Manly et al. (2005) demonstrating no significant shift of participants
bias suggesting general fatigue as produced by sleep deprivation does not affect pseudoneglect . An
alternative explanation however might be the role of individual differences in the direction of bias
and their modulation of the time-on-task effect. Taking into account different distribution of right
shifters and left shifters across studies would potentially account for why Manly et al. (2005) wereable to find an effect of general fatigue on spatial bias and Schmitz et al. were not despite following
a highly similar research paradigm.
Further information on models of attention orienting in line bisection has been provided by
neuroimaging. These studies reveal a predominant involvement of the right hemisphere in line
bisection demonstrating a right-lateralized occipito-parieto-frontal network during task processing
(Cicek, Deouell &Knight, 2009; Fink et al., 2000; Foxe, McCourt, & Javitt, 2003). This was further
supported by studies employing non-invasive methods such as transcranial magnetic stimulation
(TMS) to induce a temporary localised lesion to produce hemispatial neglect. Fierro et al. (2006)
demonstrated the role of posterior right parietal cortex in neglect and pseudoneglect using
repetitive transcranial magnetic stimulation (rTMS). Another rTMS study by Kim et al. (2005) showed
that trains of rTMS delivered over the right posterior parietal cortex (PPC) during a line bisection task
enhance the bias whereas rTMS delivered over the left PPC induces a rightward bias thus providing
support for the activation/orientation model proposed by Kinsbourne (1970). Similarly Giglia et al.
(2011) demonstrated a rightward shift in bias for pseudoneglect after application of dual transcranial
direct current stimulation (tDCS), similar in magnitude to that observed by Benwell CSY et al. (2012)
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
6/28
5
Maxi project
0904872b
as a result of time spent on task. Schmitz et al. (2011) however point out the importance of alertness
and associated alerting and attention orienting networks. This is mainly because noradrenergic (NA)
transmission in the locus coeruleus (LC) have been found to form particularly dense formations in
the right frontal and parietal regions, which are essential for orienting attention and its ability to
select relevant information (Posner & Petersen, 1990).
In summary the research on neglect and pseudoneglect has demonstrated a dominance of
the right hemisphere in mediating spatial bias. Both space and object specific information has been
demonstrated to affect spatial attention bias and are believed to be associated with specific parieto-
frontal regions in the right hemisphere. It is questionable, however, to what degree spatial biases in
line bisection are influenced by visual space based processes and to what degree they are
modulated by object specific orientation of attention. Studies have also recorded a time-on-task
effect on spatial bias however with somewhat mixed findings regarding the effect of general
vigilance. Furthermore there seems to be considerable individual differences which put constraints
upon making generalisations about the mechanisms underlying neglect and pseudoneglect. These
individual variations in spatial biases have received very little attention and to this day their
influence on some of the mechanisms of pseudoneglect outlined by previous studies has not been
studied.
The aim of the current study is to investigate the effect of these individual differences both
on the time-on-task effect and on the influence of space and object-based processes in productionof the attentional biases in line bisection. The study utilises a modification of the landmark task used
by Benwell CSY et al. (2012) to investigate shifts in spatial attention bias due to time-on-task during
line bisection while introducing a simple reaction time task using lateralised, unilateral visual stimuli
assess space-based attention bias by measuring the effect of visual field advantages due to
hemispheric differences in spatial attention. The reaction time task was completed before and after
the landmark task to investigate whether the time-on-task effect found to affect spatial bias in line
bisection will transfer to general spatial attention when no object is present in the scene. The
individual differences were accounted for by dividing participants into two separate groups based on
their initial bias identifying right shifters and left shifters as proposed by Halligan et al. (1990a).
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
7/28
6
Maxi project
0904872b
2. Method
2.1. Participants
Thirty participants, mostly undergraduate students, took part in the experiment. Participants were
recruited using the Glasgow University subject pool and were offered 6 pounds or course credits for
participation. Data from one participant could not be used due to a software malfunction. Nine
participants were removed from the analysis due to unsatisfactory level of performance using a
median absolute difference (MAD) method for outlier removal as a removal criterion (a participants
performance was considered to be an outlier if the test statistic z for MAD was higher than 3.5).
The remaining twenty participants (4 male, 16 female, mean age = 23, SD = 4.6) were all right
handed as confirmed by the Edinburgh Handedness Inventory (Oldfield, 1971) (see Appendix 1). All
participants reported no history of neurological disorder or serious head trauma and had normal or
corrected to normal vision.
2.2. Stimuli and Apparatus
The experimental paradigm partially replicates Benwell CSY et al. (2012) alteration of a
computerised landmark task which was performed for approx. 1 hour to induce a time-on-task effect
resulting in a rightward shift of spatial bias in line bisection. This was preceded and followed by a
simple reaction time task, in which participants responded as fast as possible, using their index
finger, to lateralized, unilateral visual stimuli (henceforth referred to as the general spatial attention
task).
The stimuli used for the landmark task were black and white lines of 100% Michelson
contrast presented on a grey background (luminance = 179, hue = 179). Examples of lines used in the
experiment can be found in Figure 1. All the lines measured 24.3cm in length by .5cm in height
which at a viewing distance of 90 cm subtended 17.3 (width) and 0.36 (height) of visual angle.
Each line was transected at 1 of 17 possible points ranging from 4% of absolute line length
to veridical centre at steps of 0.5% of absolute line length. This represents a range of -0.696 to
0.696 of visual angle with a difference of 0.087 of visual angle between transection locations. All
lines were displayed with the transection point horizontally aligned with the fixation cross which was
located at the midline of the screen.
The lateralized visual stimuli used during the general attention task consisted of a small
white square - 1 (height) 1 (width) of visual angle on black background - presented on the left or
right side from the fixation cross at an eccentricity of 7 along the horizontal meridian .
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
8/28
7
Maxi project
0904872b
Stimuli were presented on a CRT monitor with a 1280 1024 pixel resolution and 85 Hz
refresh rate using E-prime software (Schneider et al., 2002).
Figure 1 - Example of transected line stimuli used in the experiment. Lines A and B are transected in the opposite
direction from the veridical centre. Note that the transection points of the lines are aligned at the centre. Line A and
line B differ in contrast polarity. All trials contained a 1:1 ratio of lines with reversed contrast polarity and were
presented at a random order.
2.3. Procedure
At the beginning and the end of the experiment the participants were asked to fill out a 10 point
Likert-style sleepiness scale, providing a subjective estimate of alertness on a scale ranging from 0
(almost asleep) to 100 (fully alert). Participants were then seated in front of a computer screen at a
viewing distance of 90 cm so that their midsagittal plane was aligned with the centre of the monitor.
A chin rest was used to maintain the participants head in position and prevent distortions to viewing
distance and viewing angle.
In the general attention task each trial began with presentation of a fixation cross [0.39
(height) 0.39 (width) of visual angle] for 1 sec followed by a brief presentation of the lateralised
stimulus. The position of the stimulus (left or right) was randomised but was distributed equally
across all trials in each block. The duration of stimulus presentation was 50 msec and the interval
between stimuli was randomised for 1400 400 msec. Subjects were asked to fixate on the cross at
the middle of the screen and press a button on a computer keyboard as quickly as possible using the
index finger on a given hand upon seeing the stimulus. A total of 2x200 trials were sampled in four
blocks of 100 trials each. These blocks differed in terms of the hand participants used to indicate
their responses. In order to prevent learning effects half of the participants was asked to begin the
first block using their right hand and then switch to using their left hand during the second block
while the second half was instructed to begin the first block using their left. The participants
reaction time was measured for each hand and stimulus location.
In the line bi-section task each trial began with presentation of a fixation cross [0.39
(height) 0.39 (width) of visual angle] for 1 sec followed by presentation of the transected line. The
transection mark was always aligned with the fixation cross in order to prevent the relative position
of the bi-section mark to the fixation cross as a visual aid during bi-section judgements. Participants
were asked to fixate on the cross in the middle of the screen throughout the experiment. The
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
9/28
8
Maxi project
0904872b
transected line was presented for 150ms, followed by a blank grey screen and a response period
during which participants indicated which part of the line appeared shorter (whether the perceived
mid-point of the line was located to the left or to the right of the transection point) by pressing one
of two buttons on a computer keyboard. The participants indicated all their responses using the
index and middle finger on their dominant (right) hand. A new trial began as soon as a response was
made. A total of 1360 trials were presented over 10 blocks of 136 trials.
Each participant completed a practice block of 6 trials of the general spatial attention task
and a practice block of 12 trials of the line bi-section task prior to commencing the experimental
trials to familiarise himself/herself with the task.
Each participant completed two blocks of the general spatial attention task (one using each
hand) followed by 10 blocks of line bi-section task followed again by a general spatial attention task.
Each block of the general spatial attention task lasted roughly 5 minutes (cca. 3 sec per trial) and
each block of the line bi-section task lasted roughly 4 minutes (cca. 2 sec per trial). Participants were
allowed to take short self-paced breaks in between each block. The whole experiment lasted 60-70
minutes.
Figure 2 - A schematic representation of the experimental procedure. Half of the participants were asked to complete
first block of the general spatial attention task (GSA) using their right hand and then use their left hand for block two
(top) while the second half was asked to do the opposite. The general spatial attention task was followed by 10 blocks of
line bi-section task followed by another two blocks of the general spatial attention task. The order of hands in the
second general spatial attention task was kept the same as during the first task.
2.4. Design and Analysis
The experiment followed a within subject repeated measures design. For evaluating the object
specific attention the dependent variable used was the individuals spatial attention bias on the line
bi-section task. Various blocks served as the independent variable to assess the time-on-task effect
on the participants biases. Psychometric functions were derived from the data recorded during the
line bi-section task in order to establish an objective measure of perceived line midpoint which was
then used to calculate individual bias. The functional variables used in the model were the
proportion of all trials in which the participants indicated that the perceived transection point was
GSA 1RH
GSA 1LH
10 blocks of LBSGSA 2
RHGSA 2
LH
GSA 1LH
GSA 1RH
10 blocks of LBSGSA 2
LHGSA 2
RH
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
10/28
9
Maxi project
0904872b
located to the left of the perceived midpoint (left side of the line appeared shorter) and the actual
transection point on the line (based on one of 17 possible transection points). Non-linear least-
squares regression was used to fit a cumulative logistic function to the data for each subject in each
block and to the averaged proportion of left responses for each block. The function used can be
described by the following equation:
( ) ( (
)
where x are the tested transector locations, corresponds to the x-axis location with a 50% left
and 50% right response rate and s is the estimated width of the psychometric curve. The value
corresponding to 50% location on the function is the point of subjective equality (PSE). Individual
PSE was used as an objective measure of participants spatial attention bias. The width of the PF
provides a measure of the precision of participants line midpoint judgements per block. The
direction of the initial bias (estimated based on positivity or negativity of participants PSE) was used
as another independent variable (between subjects) to take into consideration individual variation in
bias effects.
The average reaction time on the general spatial attention task was used as a dependent
measure of general spatial attention. The difference between the average reaction times per each
subject was investigated prior and post completing the line-bisection to assess whether time-on-task
effect in landmark task would also affect visual field advantages in terms of average reaction time in
general spatial attention task (stimulus location was used as the independent variable to analyse
differences in reaction times for left and right visual field). Furthermore the effect of initial bias was
used to investigate individual differences in general spatial attention. Inferential statistical analyses
were performed to investigate relationships between the fitted PSE values, time-on-task, average
reaction time on the general spatial attention task and measures of initial bias as well as Stanford
Sleepiness Scale ratings.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
11/28
10
Maxi project
0904872b
3. Ethics
Participants were fully informed about the purpose of the study and were debriefed at the end of
the experiment. It was made clear to them that their participation in the study is voluntary and they
have the right to withdraw from the study at any time and all of their data will be removed from the
analysis, if they choose to do so. All of the subjects data was kept confidential and any files
containing participants data were kept in a locked filing cabinet or on a password protected hard -
drive (in case of digital files). There were no risks associated with the testing and every effort has
been made that the participants are as comfortable as possible throughout the experimental
procedure. Due to relatively long duration of the experiment participants were given opportunities
to take brief rest between experimental blocks. An example of the consent form, information sheet
and debriefing sheet used in the experiment can be found in Appendix 2.
The experiment was carried out within the School of Psychology at the University of Glasgow and
was approved by the local ethics committee.
4. Results
It was hypothesised that there will be significant differences between right shifters and left shifters
in terms of [1.1] magnitude as well as direction of recorded spatial attention bias and [1.2] the time-
on-task effect in modulating the magnitude and direction of the bias in the landmark task, in line
with the role of individual factors in determining spatial bias in line bisection. It was further
hypothesised that *2.1+ there will be significant differences between right shifters and left shifters
in visual field advantages (characterised by reaction time differences in the general spatial attention
task) and [2.2] that there will be a significant effect of time-on-task in modulating this difference, in
line with both object-based and space-based processes being at play during line bisection.
4.1. General fatigue and alertness
The sleepiness scale ratings confirmed an overall drop in subjects alertness over the course of the
experimental procedure with the mean alertness score dropping from 71.25 (SD=17.76) at the
beginning of the experimental procedure to 53.75 (SD=17.69) at the end. A one way analysis of
variance (ANOVA) of sleepiness ratings pre- and post-experiment shows a significant time-on-task
effect on sleepiness rating [F(7,12) = 5.944, p=0.004].
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
12/28
11
Maxi project
0904872b
4.2. Landmark task, time-on-task effect and individual differences
Figure 3 shows the average proportion of times participants have indicated that the left side of the
line is shorter as a function of the location of the transection point. left 24 represents a stimulus
type where the transection point is located most to the left whereas right 24 represents a stimulus
type where the transection point is located most to the right. It is expected that the ratio of left
responses would increase as the veridical leftward offset of the transector location from the centre
increases, as this should make it easier for the participant to detect the difference between the two
parts of the transected line. The transector (x-axis) corresponding to a 50% response (y-axis) in
relation to the veridical centre represents the participants spatial bias (negative values for leftward
bias, positive values for rightward bias). In order to observe a time-on-task effect resulting in a
rightward shift of the bias, similar to that found by previous studies, there should be a shift of thefunction to the right along the horizontal axis over the course of the experimental procedure. When
all participants are taken into consideration together (left and right biased participants collapsed)
there seems to be no shift of the functions over the course of the experiment (Figure 3).
Cumulative logistic functions were fitted to the data using non-linear least squares
regression and points of subjective equality (PSEs) (indicating the 50% value of the fitted function)
were calculated to provide a quantitative measure of participants bias. Table 1 shows individual PSE
values averaged across all participants for each block. All the average PSEs across all blocks have
negative values indicating leftward bias and there seems to be no shift in the direction of the bias
over time. It is also important to note relatively high values of standard deviation which suggests
considerable inter-individual differences between the participants in terms of the direction and
magnitude of bias.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
13/28
12
Maxi project
0904872b
A repeated measures analysis of variance (ANOVA) for time-on-task (within subjects) on
individual PSEs for each subject shows no significant effect of time-on-task across different blocks
[F(1,19)=0.118, p=0.735]. This confirms the original assumption that in cases of samples with high
inter-subject variability when individual differences between participants are not controlled for
there is no statistically significant shift of participants bias over the course of the experimental
procedure.
Individual differences were then taken into consideration by using individual PSE values at
the starting block 1 as an indicator of participants initial bias (negative PSE = leftward bias, positive
PSE = rightward bias). The group average PSE value for starting block for participants with initial
block 1 block 2 block 3 block 4 block 5 block 6 block 7 block 8 block 9 block 10
Average
PSE -1.5056 -1.4398 -1.7206 -1.5976 -2.8698 -2.1972 -2.4432 -1.8747 -1.0109 -2.1546
Standard
deviation 4.1367 3.7340 3.5365 4.1179 4.8970 5.3215 4.9432 4.5007 3.9721 4.4818
Table 3 Average PSE values for each block and standard deviation.
0
10
20
30
40
50
60
70
80
90
100
le ft2 4 le ft2 1 le ft1 8 le ft1 5 le ft1 2 le ft9 le ft6 le ft3 v er r igh t3 r igh t6 r igh t9 r igh t1 2 r igh t1 5 r ig ht1 8 righ t2 1 r ig ht2 4
block 1
block 2
block 3
block 4
block 5
block 6
block 7
block 8
block 9
block 10
Figure 3 - Functions showing the average proportion of left responses based on the location of transection
point for each block.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
14/28
13
Maxi project
0904872b
rightward bias was 2.861 (SD=1.428) and -3.377 (SD=3.414) for participants with initial leftward bias.
A Mann-Whitney test on the individual PSE values for starting block reveals a significant difference
between the two groups [U(1)=26.325, p=0.001], as would be expected by design. Note that there is
a relatively high proportion of participants with initial rightward bias in the group (30%, 6 of 20).
Together with the lack of an overall leftward bias (see above), this suggests that there are significant
differences between individuals in both the direction and magnitude of bias [hypothesis 1.1].
When considering the time-on-task effect separately for right biased and left biased
individuals, would these groups show different shift of biases across experimental blocks indicating
different time-on-task effect? Figure 4 illustrates the group averaged PSE values over the course of
the experimental procedure based on the participants initial bias. It is possible to observe th at in
right shifters (positive initial bias) the rightward bias gradually decreases with time spent on task
and eventually reverses to the left. In left shifters (negative initial bias) there is no reversal in the
direction of bias however there is a stable decrease in the magnitude of the bias throughout the
experimental session. That is, while the groups have different initial bias both in direction and
magnitude the differences in direction and magnitude of bias decreases throughout the
experimental procedure. A 2 (initial bias) x 10 (experimental block) factorial ANOVA (mixed design)
on individually fitted PSE values revealed no significant main effect of the initial bias [F(1,1)= 1.740,
p=0.204]; no significant main effect of block [F(9,1)=1.187, p=0.306]; and a marginally significant
initial bias block interaction [F(9,1)=1.982, p=0.044] suggesting different time-on-task effects forright shifters and left shifters *hypothesis 1.2+. A follow up analysis of the initial bias block
interaction using linear regression to look for linear trends across time-on-task per group shows no
significant correlation between initial bias and experimental block for the group of left
shifters(n=14) *Pearsons r=0.476, p=0.164+ but reveals a significant correlation for the group of
right shifters (n=6) *Pearson r=-0.721, p=0.019].
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
15/28
14
Maxi project
0904872b
Figure 4 Mean PSE values for each experimental block based on participant's initial bias. Error bars indicate 95%
confidence intervals.
4.3. General attention task, time-on-task effect and individual differences
Figure 5 shows group average times for right shifters and left shifters to stimuli presented to the
left and to the right visual field before and after completing the landmark task. The average reaction
time (msec) of left shifters to stimuli presented to the left versus right visual field prior to
completing the landmark task is 223.554 (SD=10.234) versus 228.705 (SD=12.287). In comparison
the average reaction time (msec) of right shifters to stimuli presented to the left versus right visual
field prior to completing the landmark task is 213.776 (SD=7.904) versus 213.781 (SD=9.069). The
data indicate that prior to completing the landmark task the left shifters may exhibit a slight
hemispheric advantage (equal to 5 msec difference in average reaction time) in favour of the right
hemisphere as indicated by lower reaction times to stimuli presented to the left visual field. The
right shifters demonstrate roughly equal reaction times to stimuli presented to the right and to the
left visual field. After completing the landmark task the reaction time (msec) of left shifters for
stimuli presented to the left versus right visual field increased to 227.859 (SD=9.529) versus 233.036
(SD=10.035). In comparison the reaction times of right shifters for stimuli presented to the leftversus right visual field increased to 218.178 (SD=8.506) versus 218.810 (SD=8.810). While there is a
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
16/28
15
Maxi project
0904872b
general increase in reaction times for both groups to stimuli presented to both right and left visual
field (possibly due to general fatigue) it seems to increase at approximately the same rate per visual
field indicating no change in hemifield advantages over the course of the experimental procedure.
A 2 (initial bias) x 2 (visual field) x 2 (pre vs. post landmark task) factorial ANOVA (mixed design) on
participants reaction times revealed no significant main effect for initial bias *F(1,1)=0.567, p=0.461+,
no significant main effect of visual field [F(1,1)=1.634, p=0.217] and no significant visual field x initial
bias interaction [F(1,1)=1.277, p=0.273] suggesting there are no significant differences between left
shifters and right shifters in terms of visual field advantages (as characterised by reaction time
differences) [hypothesis 2.1]. There was no significant main effect pre vs. post landmark task
[F(1,1)=1.731, p=0.205], no significant pre vs. post landmark task initial bias interaction
[F(1,1)=0.003, p=0.954] and no significant effect for pre vs. post landmark task initial bias
interaction x visual field interaction [F(1,1)=0.011, p=0.920] indicating that there is no significant
time-on-task effect in modulating visual field advantages for either group [hypothesis 2.2].
Left shifters Right shifters
200
205
210
215
220
225
230
235
240
245
250
pre post
LVF
RVF
200
205
210
215
220
225
230
235
240
245
250
pre post
LVF
RVF
Figure 5 -Group average reaction times (msec) for 'right shifters' and 'left shifters' to stimuli presented in the left and
right visual field recorded pre and post completing the landmark task.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
17/28
16
Maxi project
0904872b
5. Discussion
The results of the current study highlight the importance of inter-individual differences in spatial bias
in line bisection. There are considerable differences between participants in both the direction and
magnitude of bias on a landmark task. As a result statistical methods show no shifts in bias displayed
on a landmark task when individual differences are not controlled for. When dividing participants
based on their initial bias it is possible to observe two distinctively different trends for the shifts in
spatial bias over time. Participants with an initial rightward bias show a gradual shift of the bias to
the left as a result of time-on-task. Participants with initial leftward bias maintain the direction of
bias however its magnitude increases as a result of time-on-task. This suggests that the leftward bias
in line bisection, as reported by previous studies is not systematically present in the entire
population but is modulated by inter-individual differences. In addition the shifts of spatial bias inline bisection resulting from time spent on task differ based on the participants initial bias
suggesting inter-individual variability for the time-on-task effect. The results are in line with those
found by Halligan et al. (1990a-b) clearly identifying right shifters and left shifters based on
differences in subjective line midpoint estimation in a line bisection task. In addition it is shown that
the effect of individual differences translates to the time-on-task shifts in spatial bias demonstrated
by previous research (Manly et al., 2005; Dufour et al., 2007; Benwell CSY et al., 2012). In addition
the results demonstrate lack of influence of space-based attention processes in production of
attentional biases in line bisection by showing no difference in reaction times to stimuli presented to
the left and right visual field, even when accounting for individual differences. These findings
contradict models assuming distinct involvement of space-based processes such as those proposed
by Kinsbourne (1987, 1993) and Heilman (1979). Furthermore there were no distinct changes in the
difference between stimuli presented to the right versus left visual field over time questioning the
involvement of hemispheric differences in generating the time-on-task effect in line bisection when
no object is present in the scene.
5.1. Individual differences in bias on landmark task and variations in time-on-task effect
Most models of pseudoneglect emphasise a leftward bias resulting from right hemispheric
dominance in spatial attention and associated neural network connectivity (Jewell and McCourt,
2000; Foxe et al., 2003; Siman-Tov et al., 2007). The current findings show that 30% of participants in
the sample exhibit a reversed rightward bias that is significantly different both in magnitude and
direction from participants exhibiting a leftward bias. It is questionable whether these participants
possess different connectivity patterns of underlying neurological mechanisms than participants
exhibiting an initial leftward bias causing a left hemispheric advantage rather than the right
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
18/28
17
Maxi project
0904872b
hemispheric advantage proposed by most studies. An alternative explanation would be that the
spatial bias in line bisection is indeed modulated by both space-based and object-based
representations (Tipper and Behrman, 1996; Egly et al., 1994; Post, 2001; Nicholls and Orr, 2005)
and there are individual differences in how the attention orienting processes within the visual
system assign weight to space-based and object-based information to produce midpoint line
judgements on line bisection task. In relation to the time-on-task effect majority of studies propose
an initial leftward advantage facilitated by activity in alerting and orienting networks in right
hemisphere (Corbetta and Shulman, 2002; Sturm et al., 2004) which is reduced or even reversed due
to increased fatigue producing a rightward shift in bias on line bisection task (Manly et al., 2005;
Dufour et al., 2007). Benwell CSY et al. (2012) distinguish between the effect of general fatigue and
time-on-task effect by proposing that shifts in line bisection over time are better explained by
uneven neuronal fatigue in right and left hemisphere (depleting neuronal resources in right
hemisphere faster) than general fatigue. The current study has demonstrated an overall increase in
general fatigue across all participants but different time-on-task effects in terms of shifts in
magnitude and direction of bias based on the direction of participants initial bias. The observed
trend for participants with initial leftward bias is in line with previous research showing a steady
decrease in leftward bias however no rightward reversal of bias was found. For participants with
initial rightward bias the trend is reversed showing a steady shift of the bias to the left. This speaks
against theories proposing a rightward shift as a result of decreased activity of alerting and orientingnetworks in right hemisphere due to increased fatigue. An altered version of the neuronal fatigue
explanation proposed by Benwell CSY et al. (2012) could be used to account for these findings
assuming that participants showing an initial rightward bias have different underlying neuronal
connections than participants showing an initial leftward bias and that these connections are
engaging the left hemisphere more than the right hemisphere thus depleting the neuronal resources
in the left hemisphere faster than in the right hemisphere causing a leftward shift in spatial bias on
line bisection task. This assumption, however, remains to be tested.
5.2. Role of object-specific versus spatial (egocentric) information in influencing spatial
biases
Nicholls and Orr (2005) show that both space-based and object-based representations affect visuo-
spatial bias and operate independently on one another. The results from the general spatial
attention task demonstrate that when individual differences are accounted for the general
(egocentric) location of stimulus within the visual field and associated hemispheric activation has no
distinctive effect on producing biases in line bisection. This serves as further evidence against
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
19/28
18
Maxi project
0904872b
models based purely on spatial attention as proposed by Kinsbourne (1987, 1993) and Heilman
(1979). In addition there was no time-on-task effect in modulating visual field advantages as
characterised by reaction times on the general spatial attention task. Considering that there was a
significant time-on-task effect present in the line bisection task when an object (a transected line)
was present in the scene it would seem that location based representations play no role in the time-
on-task effect on line bisection task. Relating this to the neuronal fatigue explanation proposed by
Benwell CSY et al. (2012) it would seem that the differential activation of hemispheres and
associated depletion of neuronal resources occurs only when an object is present. In contrast if an
object is not present in the scene both hemispheres are depleted equally as a result of general
fatigue resulting in no change in hemispheric advantages thus not producing any shift in spatial bias
on line bisection task. This is in line with Benwell CSY et al. (2012) conclusion that the type of
presented stimuli determines the shift of spatial bias over the course of the experimental procedure.
This means that the time-on-task effect is best explained in relation to the object-specific attention
processes.
5.3. Conclusion, Methodological concerns and avenues for further research
Current study emphasises the importance of inter-individual differences for researching the
mechanisms underlying biases in spatial attention in healthy individuals explaining the inability of
some studies to replicate leftward bias in line bisection. In particular it demonstrates the influence of
individual differences on mediating the time-on-task effect in line bisection. The results provide
further support for the role of object-based in addition to space-based representations in producing
spatial biases and suggest that that the time-on-task effect is best explained in relation to the object-
specific attention processes.
The main methodological concern is the uneven distribution of right shifters (n=6) and left
shifters (n=14) within the study with relatively small sample sizes for both groups. It is possible that
the sample might not properly reflect the inter-individual variability in spatial bias in general
population. It is possible to utilise pre-screening methods to identify participants initial bias to
create equally sized groups while also considering larger sample sizes. It is also questionable to what
degree can the differences in reaction times to stimuli presented to the left and right visual field be
taken as indicative of the hemispheric advantages in spatial attention. Neuroimaging methods might
be more suitable for investigating underlying neurological mechanisms than a behavioural paradigm.
Further research can focus on uncovering differences in neurological connectivity underlying
spatial biases between right shifters and left shifters using brain imaging methods. Repeated
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
20/28
19
Maxi project
0904872b
transcranial magnetic stimulation (rTMS) (Kim et al.,2005) and transcranial direct-current stimulation
(tDCS) (Giglia et al., 2011) have been shown to be particularly effective in uncovering neurological
mechanisms associated with biases in line bisection. Testing the applicability of the neuronal fatigue
model proposed by Benwell CSY et al. (2012) for explaining leftward shifts in spatial bias found for
participants showing an initial rightward bias would be particularly useful for providing further
understanding of the role of individual factors in spatial bias in line bisection as well as the
underlying neurological processes. Another avenue for further exploration is the involvement of
object vs. space-based representation in producing special biases particularly the necessity of object
presence in generating the time-on-task effect and the potential role of individual differences in
weighting object and space-based representations in producing subjective line midpoint judgements.
References
Behrmann, M., & Moscovitch, M. (1994). Object-centered neglect in patients with unilateral neglect:Effects of left-right coordinates of objects.Journal of Cognitive Neuroscience, 6, 1-16.
Benwell, C.S.Y., Harvey, M., Gardner, S. & Thut, G. (2012). Stimulus- and state-dependence ofsystematic bias in spatial attention: Additive effects of stimulus-size and time-on-task, Cortex,doi:10.1016/j.cortex.2011.12.007
Bowers, D. & Heilman, K.M. (1980). Pseudoneglect: Effects of hemispace on a tactile line bisectiontask.Neuropsychologia, 18(4-5),491-498.
Cicek, M., Deouell, L. Y., & Knight, R. T. (2009). Brain activity during landmark and line bisectiontasks.Frontiers in Human Neuroscience, 3, 7.
Corbetta, M. & Shulman, G.L. (2002). Control of goal-directed and stimulus-driven attention in thebrain.Nature Reviews Neuroscience, 3(3), 201-215.
Driver, J., & Halligan, P. W. (1991). Can visual neglect operate in object-centered coordinates? Anaffirmative single-case study. Cognitive Neuropsychology, 8, 475-496.
Dufour A, Touzalin P. & Candas V. (2007) Time-on-task effect in pseudoneglect.Experimental BrainResearch, 176(3), 532-537.
Duncan, J. (1984). Selective attention and the organization of visual information.Journal ofExperimental Psychology, 113, 501-517.
Egly, R., Driver, J., & Rafal, R. D. (1994). Shifting visual attention between objects and locations:Evidence from normal and parietal lesion subjects.Journal of Experimental Psychology: General,123, 161-177.
Fierro, B., Brighina, F., Giglia, G., Palermo, A., Francolini, M., & Scalia, S. (2006). Paired pulseTMS over the right posterior parietal cortex modulates visuospatial perception.Journal of theNeurological Sciences, 247(2), 144148.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
21/28
20
Maxi project
0904872b
Fink, G. R., Marshall, J. C., Shah, N. J., Weiss, P. H., Halligan, P. W., Grosse-Ruyken, M., et al.(2000). Line bisection judgments implicate right parietal cortex and cerebellum as assessed by fMRI.
Neurology, 54(6), 13241331.
Fischer, M. H. (2001). Cognition in the bisection task. Trends in Cognitive Sciences, 5(11), 460462.
Foxe, J. J., McCourt, M. E., & Javitt, D. C. (2003). Right hemisphere control of visuospatial attention:Line-bisection judgments evaluated with high-density electrical mapping and source analysis.Neuroimage, 19(3), 710726.
Gibson, B. & Egeth, H. (1994). Inhibition of return to object-based and environment-based locations.Perception & Psychophysics, 55, 323-339.
Giglia, G.,Mattaliano, P., Puma, A., Rizzo, S., Fierro, B. & Brighina, F. (2011). Neglect-like effectsinduced by tDCS modulation of posterior parietal cortices in healthy subjects. Brain Stimulation, 4,294-299.
Halligan, P.W, Manning, L. & Marshall J.C.(1990a). Individual variation in line bisection: a study offour patients with right hemisphere damage and normal controls. Neuropsychologia, 28, 104351.
Halligan, P.W, Manning, L. & Marshall J.C.(1990b). Individual variation in line bisection: a study ofnormal subjects with application to the interpretation of visual neglect.Neuropsychologia, 28, 64755.
Heber I, Siebertz S, Wolter M, Kuhlen T, and Fimm B. (2010). Horizontal and vertical pseudoneglectin peri and extrapersonal space.Brain and Cognition, 73(3), 160-166.
Heilman, K. M. & Valenstein, E. (1979). Mechanisms underlying hemispatial neglect.Annals of
Neurology, 5(2), 166170.
Heilman, K.M. and Van Den Abell, T. (1980) Right-hemisphere dominance for attention: Themechanisms underlying hemispheric asymmetries of inattention (neglect).Neurology, 30(3), 327-330.
Heilman, K. M., Watson, R. T.,& Valenstein, E. (2002). Spatial neglect. In H. O. Karnath, A. D. Milner,
& G. Vallar (Eds.), The cognitive and neural bases of spatial neglect. Oxford, NY: Oxford University
Press, 3-30.
Jewell, G., & McCourt, M. E. (2000). Pseudoneglect: A review and meta-analysis of performance
factors in line bisection tasks. Neuropsychologia, 38(1), 93110.
Kanwisher, N. & Driver, J. (1992). Object, attributes, and visual attention: Which, what, and where.
Current Directions in Psychological Science, 1, 2631.
Kim, Y. H., Min, S. J., Ko, M. H., Park, J. W., Jang, S. H., & Lee, P. K. (2005). Facilitatingvisuospatial attention for the contralateral hemifield by repetitive TMS on the posterior parietal cortex.
Neuroscience Letters, 382(3), 280285.
Kinsbourne, M. (1970). The cerebral basis of lateral asymmetries in attention.Acta Psychologica, 33,
193201.
Kinsbourne, M. (1987). Mechanisms of unilateral neglect. In: Jeannerod M (ed.)Neurophysiologicaland neuropsychological aspects of spatial neglect. Amsterdam: Elsevier (North-Holland), 6986.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
22/28
21
Maxi project
0904872b
Kinsbourne, M. (1993). Orientational bias model of unilateral neglect: evidence from attentional
gradients within hemispace. In: Robertson IH, Marshall JC (eds.) Unilateral neglect: clinical
and experimental studies. Hove UK: Lawrence Erlbaum Associates, 6386.
Manly, T., Dobler, V. B., Dodds, C. M. &George, M. A. (2005). Rightward shift in spatial awareness
with declining alertness.Neuropsychologia, 43(12), 1721
1728.
McCourt, M.E, Olafson, C. (1997). Cognitive and perceptual influences in line bisection: Stimulus
modulation of pseudoneglect.Neuropsychologia, 35, 369380.
Mesulam, M.M. (1983). The functional anatomy and hemispheric specialization for directed attention:
the role of the parietal lobe and its connectivity. Trends in Neurosciences, 6, 384387.
Mesulam, M.M. (1990). Large-scale neurocognitive networks and distributed processing for attention,language and memory.Annals of Neurology, 28, 597613.
Nichelli, P., Rinaldi, M., Cubelli, R. (1989). Selective spatial attention and length representation in
normal patients and in patients with unilateral spatial neglect.Brain and Cognition, 9, 5770.
Nicholls, M.E.R., Bradshaw, J.A., Mattingley, J.B. (1999). Free-viewing perceptual asymmetries forthe judgement of brightness, numerosity and size.Neuropsychologia, 37, 307314.
Nicholls, M.E.R., Orr, C.A. (2005). The nature and contribution of space- and object-basedattentional biases to free-viewing perceptual asymmetries.Experimental Brain Research, 162, 384-393.
Oldfield, R. C. (1971). The assessment and analysis of handedness: The Edinburgh inventory.Neuropsychololgia, 9, 97-113.
Posner, M.I. (1980). Orienting of attention. Quarterly Journal of Experimental Psychology, 32, 325
Posner, M. I., & Petersen, S. E. (1990). The attention system of the human brain.Annual Review ofNeuroscience, 13, 2542.
Post, R.B., Caulfield, K.J., Welsh, R.B. (2001). Contributions of object and space-based mechanismsto line bisection errors.Neuropsychologia, 39, 856864.
Schenkenberg, T., Bradford, D.C.,, Ajax, E.T. (1980). Line bisection and unilateral visual neglect inpatients with neuroleptic impairment.Neurology, 30, 509517.
Schmitz, R., Deliens, G., Mary, A., Urbain, C., and Peigneux, P. (2011). Selective modulations ofattentional asymmetries after sleep deprivation. Neuropsychologia, 49(12), 3351-3360.
Schneider, W., Eschman, A. & Zuccolotto, A. (2002). E-Prime reference guide. Psychology SoftwareTools.
Siman-Tov, T., Mendelsohn, A., Schonberg, T., Avidan, G., Podlipsky, I., Pessoa, L., et al. (2007).Bihemispheric leftward bias in a visuospatial attention-related network.Journal of Neuroscience,
27(42), 11271-11278.
Sturm, W., Longoni, F., Weis, S., Specht, K., Herzog, H., Vohn, R., et al. (2004). Functional
reorganisation in patients with right hemisphere stroke after training of alertness: A longitudinal PET
and fMRI study in eight cases. Neuropsychologia, 42(4), 434-450.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
23/28
22
Maxi project
0904872b
Tipper, S.P, Behrmann, M. (1996). Object-centred not scene-based visual neglect.Journal ofExperimental Psychology: Human., 22(5), 12611278.
Vallar, G. (1998). Spatial hemineglect in humans. Trends in Cognitive Sciences, 2(3), 87-97.
Waberski, T.D., Gobbele, R., Lamberty, K., Buchner, H., Marshall, J.C. & Fink, G.R. (2008). Timing of
visuo-spatial information processing: Electrical source imaging related to line bisection judgements.
Neuropsychologia, 46(5), 1201-1210.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
24/28
23
Maxi project
0904872b
Appendices
Appendix 1 Edinburgh Handedness Inventory
Edinburgh Handedness Inventory1
Your Initials:
Please indicate with a check () your preference in using your left or right hand in the
following tasks.
Where the preference is so strong you would never use the other hand, unless absolutely
forced to, put two checks ().
If you are indifferent, put one check in each column ( | ).
Some of the activities require both hands. In these cases, the part of the task or object forwhich hand preference is wanted is indicated in parentheses.
Task / Object Left Hand Right Hand
1. Writing
2. Drawing
3. Throwing
4. Scissors
5. Toothbrush
6. Knife (without fork)
7. Spoon
8. Broom (upper hand)
9. Striking a Match (match)
10. Opening a Box (lid)
Total checks: LH = RH =
Cumulative Total CT = LH + RH =
Difference D = RHLH =
Result R = (D / CT) 100 =
Interpretation:
(Left Handed: R < -40)
(Ambidextrous: -40 R +40)
(Right Handed: R > +40)
1
Oldfield, R. C. (1971). The assessment and analysis of handedness: The Edinburghinventory.Neuropsychololgia, 9, 97-113.
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
25/28
24
Maxi project
0904872b
Appendix 2.1. Example of consent form
STUDY INFORMED CONSENT(This form must be completed prior to any experiment)
Study title: Testing models of spatial attention through computerized line
bisection and a CUD reaction time task.
I confirm that I have read and understood the Study Information Sheet provided to me for the above studyand have had the opportunity to ask questions.
I understand that my participation is voluntary and that I am free to withdraw at any time, without giving
a reason.
I understand that at all times my personal data will be kept confidential in accordance with data protection
guidelines.
I agree to take part in the study.
I have initialed the above boxes myself and I freely agree to take part in the study.
Signature of Volunteer: ___________________________
Name: ___________________________
Date: ___________________________
Subject ID: ___________________________
Signature of Witness: ___________________________
Name: ___________________________
Date: ___________________________
School of Psychology
University of Glasgow
58 Hillhead Street, Glasgow G12 8QB
Tel: +44 (0)141-330 3395, email: [email protected]
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
26/28
25
Maxi project
0904872b
Appendix 2.2. Information sheet
LETTER OF INFORMATION
Spatial attention and line bisection
Please read this information form carefully. If you have any questions, ask theexperimenter before signing the form.
PURPOSE OF THE STUDY
The purpose of this study is to examine spatial attention and underlying neurologicalmechanisms in healthy subjects.
PROCEDURE
Should you choose to participate, you will be presented with two tasks. During the first taskyou will be briefly presented with a white square on a computer screen. Your task will be toreact as quickly as possible by pressing a button on a keyboard using either your left or righthand. During the second task you will view a series of segmented lines on a computer
screen. Your task will be to indicate which of the two segments presented on the screenappears shorter. After completing the second task you will be asked to repeat the first taskagain using either your left or right hand.
The duration of the experiment is roughly 1 hour, including short self-paced breaks.
The exact predictions being made in this study will be explained to you at the conclusion ofyour participation.
POTENTIAL RISKS AND DISCOMFORTS
The risks associated with testing are no greater than risks you encounter in everyday life.Every effort has been made to ensure that you are as comfortable as possible during testing.You will have opportunities to take brief rests periodically during the experiment.
POTENTIAL BENEFITS
In return for your participation you can choose to be paid 6 or receive 3 e-credits if you area first year Psychology student. You will be debriefed at the end of the experiment, so youmay gain a better understanding of the subject under investigation and enhanced vision offuture scientific avenues in general. In a broader sense, the results from these experimentsmight have implications for the status of scientific knowledge about critical functions of the
visual system and underlying neurocognitive mechanisms in both healthy subjects and,through future extensions of the research into stroke patients, individuals with visual and/or
7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
27/28
26
Maxi project
0904872b
attention deficits. Thus, your involvement in this study might benefit not only the scientificcommunity, but also, in terms of potential applications of this knowledge, the society at large.
PARTICIPATION AND WITHDRAWAL
Your participation in this experiment is voluntary. You are under no pressure to participate inthe experiment, and if you choose to participate you are free to withdraw from theexperiment at any time, with no penalty to yourself. All information about the participants inthe study and collected data will remain fully confidential howeveryou may exercise theoption of removing your data from the study after participation.
If you have any questions or concerns about the research, please contact Martin
Bacik ([email protected])
RIGHTS OF RESEARCH PARTICIPANTS
You may withdraw your consent at any time and discontinue participation without penalty.You are not waiving any legal claims, rights or remedies because of your participation in thisresearch study. This study has been reviewed and received ethics clearance through theFaculty of Information and Mathematical Sciences Ethics Committee. If you have questionsregarding your rights as a research participant, contact:
Klaus Kessler - MREB ChairTel. 0141-330-4774E-mail: [email protected]: 0141-330-4606Room 610, Dept of Psychology, 58 Hillhead Street, Glasgow, G12 8QB
mailto:[email protected]:[email protected]:[email protected]:[email protected]7/28/2019 The effect of individual differences, time-on-task and object versus space-based representations on production of
28/28
27
Appendix 2.3. Debriefing sheet
DEBRIEFING SHEET
Spatial attention and line bisection
Thank you very much for participating in the study!
The aim of the experiment was to investigate a phenomenon called pseudo-neglect and the
underlying neurological mechanisms.
The second task you completed, involving segmented lines is called a line bisection task
and is used to investigate biases in spatial attention. Even when healthy people are asked to
complete the task they tend to exhibit a bias, in other words they tend to see the mid-points
of the lines more to the side than they really are. The interesting thing is that this bias isnt
static but changes depending on how much time we spend on the task and how tired we get
as a result.
It was hypothesised in the current study that these changes in bias might be the result of a
decrease in the ability of the right and left hemisphere to communicate with each other.
The task you have completed at the beginning and end of the experiment was designed to
measure your inter-hemispheric communication.
What was of the interest was the relationship between the changes in inter-hemispheric
communication at the beginning and end of the experiment and the changes in bias
throughout the line-bisection task.
If you have any more questions, please feel free to ask the experimenter.
Top Related