Categorization studies of 9- to 15-month-old infants: Evidence for superordinate categorization?

NFANT BEHAVIOR AND DEVELOPMENT 12, 265-288 (1989)

Categorization Studies of 9- to 15Month-Old Infants: Evidence

for Superordinate Categorization?

KENNETH ROBERTS Boys Town National Institute for Communication Disorders in Children

MARTIN D. CUFF Kent State University

Four experiments examined the ability of infants 9 to 15 months of age to form a category superordinate to a related basic-level category. Procedures, stimulus conditions, and dependent measures were the same as in previous studies re- porting basic-level categorization (bird) in 9-month-olds. In Experiments 1 through 3, infants were habituated to three different kinds of animals but did not categorize even by 15 months of age. The basic-to-superordinate order of emergence reported for young children and its explanation in terms of category differentiation might be extended into infancy. In Experiment 4, 15month-olds habituated to six different animals provided evidence suggesting successful categorization. Increasing the number of exposure exemplars appears to facilitate categorization. Maternal interviews revealed that 15month-olds appear to comprehend basic-level terms for some of the animal stimuli used. This suggests an assimilation of conceptually independent basic-level categories into a higher order superordinate grouping (Nelson, 1935). Finally, a reliable linear increase in recovery to test stimuli of decreasing prototypicality suggests that graded internal structure may characterize the superordinate category formed.

infants superordinate categorization order of emergence hierarchical relationship

Rosch and her colleagues (Rosch, 1977, 1978; Rosch, Mervis, Gray, Johnson, & Boyes-Braem, 1976) have found that object categories at a basic level of abstraction appear to be more fundamental or useful than categories at other levels of abstraction. For example, bird (basic level) is more fundamental psychologically than robin (subordinate level) or animal (superordinate level). It is

This research was supported by grant HD 20109 from the National Institutes of Health and in part by Biomedical Research Sciences Grant RR07208 from Kent State University. We would like to thank M. Domski and R. Richer of the Akron General Hospital Biomedical Engineering Department for help in configuring some of the instrumentation. We would also like to thank Joseph F. Fagan, Jeanne Montie, and William Merriman for helpful comments on a previous draft.

Correspondence and requests for reprints should be sent to Kenneth Roberts, Boys Town Na- tional Institute, 555 N. 30th Street, Omaha, NE 68131.

265

266 ROBERTS AND CUFF

also well documented that object categories exhibit internal structure. A central theme in categorization research over the last decade has been that internal structure is graded. There are two aspects of the graded structure of object categories. Categories are (a) organized around a prototype(s) with (b) other members forming a gradient that reflects decreasing degrees of “goodness-of- membership” relative to the prototype (Rosch & Mervis, 1975; Rosch et al., 1976).

Data consistent with the developmental implications of these findings have been reported. Children appear to acquire basic-level terms before superordinate terms (Anglin, 1977). Similarly, matching basic-level stimuli has been reported to be easier for young children than matching superordinate or subordinate stimuli (Daehler, Lonardo, & Bukatko, 1979; Mervis & Crisafi, 1982). Ease of matching has been treated as analogous to early acquisition. Moreover, Mervis and Crisafi argued that the basic-to-superordinate order they found in 2%- to 5-year-olds was predicted by the greater differentiation of basic-level categories. That is, members of a basic-level category have low similarity to stimuli outside the category but exhibit substantial similarity to each other. On the other hand, members of a superordinate category, while having a low degree of similarity to stimuli outside the category, also have a lower degree of similarity to each other. In addition, because the use of artificial categories eliminated the influence of linguistic knowledge, Mervis and Crisaf’i suggested that a perceptual-cognitive basis was a sufficient explanation for this basic-to-superordinate order of emergence. Similar parallels have also been reported in studies bearing on the structure of children’s word categories. Like adult categories, children’s early word categories appear to be organized around a prototype(s) (Anglin, 1977; Bower-man, 1978, 1980; Rescorla, 1980).

Recently, there has been considerable interest in the categorization abilities of infants. It appears that infants in the first year of life have the ability to form perceptual categories (e.g., Bomba & Siqueland, 1983; Cohen & Strauss, 1979; Fagan, 1976; Roberts & Horowitz, 1986; Sherman, 1985; Strauss, 1979; Younger, 1985). Moreover, several studies indicate, consistent with studies of adults and young children, that organization of these categories may be around a prototype (Roberts & Horowitz, 1986; Sherman, 1985; Strauss, 1979; Younger, 1985). However, these studies have focused primarily on basic-level categories (e.g., bird) and on categories that appear to be subordinate or even lower level categories (e.g., male face; female face, or simple form categories such as triangle). Only two studies of superordinate categorization in infants have been reported.

Ross (1980) habituated 12-, 18-, and 24-month-old infants to 10 different real object exemplars of superordinate categories. Both visual fixation time and manipulation time combined as dependent measures. In a preference-for- novelty test format, infants showed preferences for an out-of-category exemplar when paired with a novel within-category exemplar for each of the superordi-

SUPERORDINATE CATEGORIZATION IN INFANTS 267

nate categories tested (animal, food, furniture). It was concluded from these preference patterns that a category had been formed. Golinkoff and Halperin (1983), in a case study of a single infant having prior experience with a number of animals, reported formation of the category animal at age 8 months. An affective response served as the dependent measure. These studies suggest that when exposed to a sizable number of real object stimuli, infants may be able to form certain superordinate categories by at least 12 months of age.

However, other issues of interest have not been addressed. One important issue relates to the developmental changes in categorization abilities that may occur as a function of level of abstraction of target categories (e.g., basic vs. superordinate). Specifically, it is not clear whether the basic-to-superordinate order reported for older children, and by implication the explanation of this order in terms of category differentiation, can be extended to infants. In studies of older children, the procedures, the stimulus conditions in exposure and test, and the dependent measures were held constant for both basic and superordinate categories. Thus, direct examination of the developmental changes attributable to level of abstraction, apart from other factors that may affect categorization, was possible in studies of children (Daehler et al., 1979; Mervis & Crisafi, 1982). However, because of substantial differences in procedures, stimulus conditions, and dependent measures in work with infants, comparison of studies of superordinate categorization to existing studies of basic and subordinate categories is difficult. A clearer examination of the order-of-emergence issue in infants would seem to require independent examination of the formation of a basic-level category and a corresponding superordinate category using identical procedures, stimulus conditions, and dependent measures. In addition, data bearing on the conditions affecting the formation of superordinate categories by infants or on the structure of infants’ superordinate categories have not been reported. The experiments reported here provide data bearing on the basic-to-superordinate order of emergence, conditions affecting superordinate categorization, and the structure of a superordinate category in infancy.

The category investigated used line drawings of objects from the adult category of animal. One reason for this choice was consistency with previous studies of infants (Golinkoff & Halperin, 1983; Ross, 1980). In addition, basic- level labels for animals are frequent in the early vocabularies of infants 12 to 24 months of age (e.g., Clark, 1979, 1983; Nelson, 1973; Nice, 1915). This suggests that different kinds of animals are an important focus in the 12- to 24- month-old infant’s expanding physical and linguistic world. Therefore, problems involving these objects would plausibly command processing effort on the part of infants. In the context of a categorization problem, the salience of animal objects may serve to heighten infants’ sensitivity to similarities among different basic-level animal categories (e.g., dog, cat, horse, etc.). Thus, the likelihood of documenting a superordinate category early in development may


be greater with animal objects than with objects of furniture, fruit, vehicle, and SO forth, which at best may have indeterminate importance to infants at this age.

Tversky and Hemenway (1984) have argued that shape information, as influenced by part configuration, is a major factor in distinguishing various levels of abstraction. Shape also appears to represent a primary grouping prin- ciple for young children (Anglin, 1977; Bowerman, 1976, 1978; Clark, 1973, 1983; Rescorla, 1980). Thus, overall shape, particularly early in development, appears to be useful in identifying the level of abstraction of a categorization problem. Because the extension of children’s early words tends to include the more prototypical exemplars from adult categories (Anglin, 1977), the adult extension of the superordinate term animal was used as a guide in choosing familiarization exemplars. Thus, similar to studies of basic-level categories, the familiarization stimuli represented good exemplars of the adult animal category. Furthermore, familiarization exemplars were chosen from different basic-level categories as indicated by the availability of a different basic-level term for each exemplar. It was reasoned that this principled choice of stimuli from different basic-level categories served to preserve the degree of variability in overall shape that characterized a level of abstraction superordinate to the basic level.

However, it should be noted that successful categorization by infants would not necessarily imply full knowledge of the adult category animal nor would it allow a complete specification of infants’ category extension (i.e., the full range of different exemplars included within the category). In addition to key perceptual information, adults appear to have more advanced knowledge of the animal category, such as “has a heart,” “has a nonrigid cell wall structure,” “has a spleen,” which is not readily available through perception. Moreover, practical limitations of design in laboratory experiments with infants limit the number of different exemplars that can be included in a single testing. Nevertheless, successful categorization by infants within the present task would involve formation of a category that embodies important perceptual aspects of adults’ knowledge of a superordinate category involving animal exemplars.

EXPERIMENTS 1 THROUGH 3

Recently, Roberts and Horowitz (1986) and Roberts (1988) reported the formation and memory of a basic-level category (bird) by g-month-old infants. These studies used line drawing stimuli and a multiple habituation-discrimination procedure. In a previous study, using procedures identical to those in the studies of the basic-level category bird, Roberts (1983) had reported successful formation of a related superordinate category involving animal exemplars in g-month-old infants. These findings in infants were inconsistent with several


studies of older children which had reported a basic-to-superordinate order of emergence (Daehler et al., 1979; Mervis & Crisafi, 1982). That is, the findings of Roberts (1983), in conjunction with the studies of the basic-level category bird, suggested that by 9 months of age infants might be capable categorizing nonlinguistically at both the basic and superordinate level of abstraction; the presence of a significant developmental lag between the two levels of abstraction was not indicated. Experiments 1 through 3 extended the studies of a basic-level category to a related superordinate category involving animal exemplars. Experiments 1 through 3 used the same procedures, stimulus conditions, and dependent measures as the studies of Roberts and Horowitz (1986) and Roberts (1988). It is important to note that the present experiments were not conceptualized as studies that would identify the earliest age of onset of superordinate categories. Rather, we were interested in when infants could successfully categorize under certain specified stimulus and procedural conditions (those similar to studies of a basic-level category), even though these conditions may not be maximally facilitative. Experiments 1 through 3 differed from each other only in the age of the infants studied. In Experiment 1, we were interested in replicating Roberts’ (1983) study. The stimuli used in Expe- riment 1 were identical to those used in Roberts’ (1983) study.

Method

Subjects. In each experiment, 36 infants with no known visual problems served as subjects. Infants ages for Experiments 1 through 3 were 9, 12, and 15 months of age (+7 days), respectively. In Experiment 1, an additional 19 infants did not complete the session due to fussing (1 S), equipment failure (3), or failure to habituate (1). Likewise, in Experiment 2, 19 infants did not complete the session due to fussing (13), experimenter error (l), or falling asleep (3). In Experiment 3, an additional 25 infants did not complete the session due to fussing (15), equipment failure (2), failure to habituate (3), or maternal cuing (5). In all instances of maternal cuing, the mother was pointing at the stimuli during the test phase. In three instances, the mothers also named the test stimuli. Infants in all experiments were recruited from the greater Akron, Ohio, area.

Stimuli. Based on Uyeda and Mandler’s (1980) rankings of four-footed animals, three prototypical animals (dog, cat, horse) were chosen for use as habituation stimuli. Test stimuli consisted of a good, but nonprototypical, exemplar of animal (deer), a moderately good exemplar (pig), and a more peripheral two-footed animal (bird; Uyeda & Mandler, 1980). Previous studies of superordinate categorization in infants used test stimuli that were relatively good exemplars of the target category. Consequently, little can be said regarding the breadth of the category formed by infants. Bird was included to obtain a better indication of category breadth and because it represents an exemplar of a basic-level category previously investigated (Roberts, 1988; Roberts &


Horowitz, 1986). It was of interest to determine whether a non-four-legged exemplar which represented a basic-level category successfully formed by young infants (bird) would be included in any category formed. The out-of-category test stimulus used was a car. A 24 x 24 black-and-white checkerboard served as a posttest stimulus. Black-and-white line drawings were made from pictures of these items (see Figure 1). The drawings were equated for size. Slides of these line drawings were presented to the infants.

Setting and Apparatus. Infants were seated on their mother’s lap in a semi- dark testing enclosure consisting of dark curtains on three sides and a large free-standing wall (2.4384 x 1.524 m). A removable board (104.14 x 76.2 cm) containing a rear-projection screen (17.78 x 15.24 cm) was mounted in the free- standing wall. The projection screen was constructed of two single sheets of mat mylar plastic mounted between two squares of brown smoked plexiglass. Mother and infant faced the rear-projection screen from a distance of about 68.58 cm. Infant state and the mother’s behavior were monitored via a TV monitor. Observers wearing headphones through which masking music was played recorded visual fixations using the cornea1 reflection technique. In this technique, points of light appear in the pupil area of an infant’s eye when fixating the screen. Corneal reflections were observed through small holes (1.27 cm diameter) located at the bottom and 11.43 cm to each side of the projection screen. Observers depressed a hand-held button whenever an infant was judged to be fixating the screen. These buttons were hardwired into a computer. The masking music was provided to prevent observers from hearing any noise associated with pressing the hand-held buttons. Stimuli were presented via two Kodak Ectographic III B slide projectors, one for habituation stimuli and the other for test stimuli. Stimulus presentation, timing of fixations and intertrial intervals, and calculation of habituation criterion were computer controlled by an IBM PC equipped with a LABMASTER laboratory control board (Scien- tific Solutions, Inc.).

Procedure. The habituation-discrimination procedure with trials under infant control was identical to that used in previous studies of a basic-level category (Roberts, 1988; Roberts & Horowitz, 1986). Ah infants were habituated to blocks of three different line-drawing exemplars of prototypical animals (dog, cat, horse). These exemplars were presented randomly within blocks of three trials. Mean looking times were computed for each block as the session pro- gressed. A trial consisted of one unlimited look beginning with an initial fixation of at least 1 s and terminating with a look away of 1 s. The intertrial interval was 2.4 s. Habituation criterion was defined as mean looking within a block that represented a 50% decrement from the largest of the first two block means in the habituation phase.

Each infant was randomly assigned to one of two test groups: experimental (n = 18) or control (n = 18). The test phase for the experimental group consisted

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of counterbalanced, single-trial presentations of the three within-category exemplars (deer, pig, bird) followed always by the out-of-category stimulus (car) and then by the 24 x 24 checkerboard. The test phase for the control group consisted of continued counterbalanced presentation of the habituation stimuli (dog, cat, horse) for five additional trials. Categorization was defined as the treatment of discriminably different yet similar stimuli as essentially equivalent while treating other less similar stimuli differently (Bower-man, 1988; Cohen & Younger, 1983; Rosch, 1977). Thus, categorization in these experiments would be indicated by a pattern of responding in which no significant difference existed between the experimental and control group on at least one of the novel within-category test trials (generalized habituation) but with a significant increase in looking time by the experimental group (discrimination) to the out-of-category (car) stimulus. Because generalized habituation is more likely as an infant sees more test stimuli, the fiied ordering of the out-of-category stimulus is considered a conservative test for categorization (Caron, Caron, & Meyers, 1982). The checkerboard was included at the end of testing to control for fatigue or disinterest in looking.

For approximately one fourth of the infants, two independent observers recorded fixations. Observer agreement was sampled every 20 ms. An agreement was defined as both observers recording a fixation. A percentage agreement was calculated for each habituation and test trial. The mean percentage agreement in each experiment was as follows: Experiment l-95070 in habituation and 97% in the test phase; Experiment 2-93070 in habituation and 90% in test; Experiment 3-96070 in habituation and 91% in test.

Analyses of experimental versus control group performance on measures of total fixation time accumulated during habituation, trials-to-criterion during habituation, and mean criterion at habituation revealed no differences between groups within each experiment. This suggests that, within each experiment, experimental and control groups had similar amounts of exposure and had habituated to a comparable level as the test phase began. These analyses are summarized in Table 1.

Graphical inspection of the test trial data in each experiment revealed pro- nounced right skew. A log transformation was applied to the looking times in the test phases of Experiments 1 through 3. This served to equalize the variances and normalize the distributions (Rummel, 1970). All analyses of test data were performed on these transformed scores.

To examine order effects within each experiment, separate one-way repeated- measures analyses of variance (ANOVAs) were performed on the experimental group and control group. No significant order effects were found, p> .32 for all ANovAs.

Categorization in each experiment was examined by way of a Groups (experimental vs. control) x Test Trials ANOVA, with Test Trials as a repeated factor. In Experiment 1, analyses revealed a significant Groups x Test Trials interac-


TABLE 1

Meon Total Fixation Times, Trials to Criterion, and Fixation Times at Habituation by Group in Experiments 1 Through 3

Experiment TFT TC FTH

1 Experimental

Control

2 Experimental

Control

3 Experimental

Control

116.98 (57.20)

105.26 (57.84)

107.68 (49.83) 97.62

(47.41)

112.68

(81 .Bl) 72.87

(39.45)

14.67 (5.72) 15.16

(5.86)

16.50

(7.99) 17.80

(7.31)

16.00

(7.35) 13.67

(6.69)

3.95 (1.59)

3.40

(1 .w

3.14 (1 SO)

(ii)

3.05

(1 .w 2.65

(1.03)

Note. TFT=total fixation time in habituation; TC=trials to criterion in habituation: FTH= fixation time at habituation. Standard deviations are given in parentheses. All contrasts were nonsignificant (p > .05).

tion, F(4,136) = 5.47, p< .OOl, as well as significant main effects for Groups, F(1,34)=6.46, p=.O16, and Trials, F(4,136)=4.11,p=.OO4. Analyses of the Groups x Test Trials interaction using one-way ANOVAs (Bruning & Kintz, 1977) revealed a significant difference between Groups only on Trial 2, F(1,34) = 9.29, p= .004, and Trial 5, the posttest trial, F(1,34) =30.12, p< JO1 (see Figure 2). Because the experimental group did not discriminate the out-of- category stimulus (car), it appears that the infants in this experiment did not reliably distinguish instances from noninstances for the category of interest. However, the experimental group’s discrimination of the posttest checkerboard suggests that the infants were not fatigued or disinterested.

In Experiment 2, analyses revealed significant main effects for Groups, F(1,34)=20.68, p< .OOl, and Test Trials, F(4,136)=2.72,~= .032. The Groups x Test Trials interaction was not significant. The significant Groups main effect indicates that, relative to the control group, infants in the experimental group discriminated both the within-category and out-of-category stimuli (see Figure 3). This failure to generalize habituation to any of the within-category stimuli.indicates that the 12-month-old infants did not form a category under the conditions of this experiment.

Finally, in Experiment 3, analysis revealed significant main effects for Groups, F(1,34)=11.5O,p=.OO2, and Trials, F(4,136)=4.72,~=.001, and a significant Groups x Test Trials interaction, F(4,136) = 3.86, p = .005. Post-hoc analyses of the interaction using one-way ANOVAs (Bruning & K&x, 1977)

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TEST TRIALS

Figure 2. Mean logs of looking times during the test phase of Experiment 1 (9-month-olds). The stimulus designations by trial for the experimental group were: Trial l-deer; Trial 2-pig; Trial 3-bird: Trial 4-tar; Trial 5-checkerboard. The stimulus designations for the control group over Trials 1 through 5 reflect the continued counterbalanced presentations of the habitu-

ation stimuli (dog, cat, horse): thus, there was no change from the habituation phase.

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TEST TRALS

Figure 3. Mean logs of looking times during the test phase of Experiment 2 (12-month-olds). The stimulus designations by trial for the experimental group were: Trial l-deer: Trial 2-pig; Trial 3-bird; Trial 4-tar; Trial g-checkerboard. The stimulus designations for the control

group over Trials 1 through 5 reflect the continued counterbalanced presentations of the habituation stimuli (dog, cat, horse): thus, there was no change from the habituation phase.

274


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0 Experimental

A Control

TEST TFUALS

Figure 4. Mean logs of looking times during the test phase of Experiment 3 (15-month-olds).

The stimulus designations by trial for the experimental group were: Trial l-deer; Trial 2-pig; Trial 3-bird; Trial d-car; Trial 5-checkerboard. The stimulus designations for the control group over Trials 1 through 5 reflect the continued counterbalanced presentations of the habitu-

ation stimuli (dog, cat, horse); thus, there was no change from the habituation phase.

revealed significant differences between groups on test trials 1, 3, and 5 (see Figure 4). There was no significant group difference on test Trial 4 on which the experimental group was shown the out-of-category stimulus (car). How- ever, because there was clear discrimination by the experimental group on test Trial 5 (the posttest checkerboard), it does not appear that infants were fatigued or disinterested. Although response patterns of the experimental and control groups on Trial 2 are consistent with generalized habituation, the failure of the experimental group to discriminate the car on Trial 4 is not consistent with categorization. That is, under the conditions of this experiment, l%month- olds did not appear to reliably discriminate between instances and noninstances of a target category consisting of animal exemplars.

Discussion Infants in Experiments 1 through 3 did not appear to respond in a manner consistent with category formation. Experiment 1 failed to replicate the successful categorization in 9-month-olds reported by Roberts (1983). The reasons for this are not completely clear. One explanation might be the smaller sample (n = 12 in each group) in Roberts (1983). However, the results of Experiment 1 are substantiated by the outcome of Experiment 2 in which 12-month-olds also did not appear to form the category of interest. On the other hand, the clis- crimination of all test stimuli by the 12-month-olds in Experiment 2 does suggest that each test stimulus was discriminable from the habituation stimuli.


Discrimination between habituation and test stimuli is necessary in order to in- terpret generalized habituation as reflecting categorization rather than a simple lack of discrimination (e.g., Cohen & Strauss, 1979; Roberts & Horowitz, 1986). Moreover, this discriminability was demonstrated within the context of a categorization problem. Olson and Strauss (1984) have argued that discrimination under familiarization conditions similar to those used in a categorization problem would provide the clearest demonstration of discriminability.

Studies examining young children’s learning of categories at different levels of abstraction reveal a clear basic-to-superordinate order of emergence (Daehler et al., 1979; Mervis & Crisafi, 1982). By holding other factors constant, these studies directly implicate factors associated with level of abstraction as the explanation for this order of emergence. However, previous studies of infants’ formation of a superordinate category (animal) have differed from studies of corresponding basic-level categories not only in level of abstraction but also in other ways that could affect categorization (e.g., the use of objects, the number of different habituation exemplars). Thus, developmental changes in infant categorization attributable to level of abstraction, apart from other factors, have been difficult to assess.

Recent work using habituation-discrimination of visual fixation to line-drawing stimuli suggests that g-month-old infants can form a basic-level category of bird (Roberts, 1988; Roberts & Horowitz, 1986). However, using identical procedures, dependent measures, and stimulus types, the findings of Experi- ments 1 through 3 indicate that the ability to form a corresponding superordinate category involving animal exemplars may not appear until a number of months later. Because methodological factors were held constant in the present experiments, the developmental difference which is indicated can be reason- ably attributed to the influence of level of abstraction. Thus, in infancy, consistent with studies of older children, there appears to be a consequential age gap between the onset of basic-level categorization and a corresponding superordinate category. Infants appear to be sensitive to the perceptual correlates of level of abstraction. In particular, the earlier emergence of a basic-level category suggests that the psychologically fundamental role of basic-level categorization may be in place early in development.

EXPERIMENT 4

Although not successful under conditions similar to those in studies of basic- level categories, it is possible that under certain other conditions 15month-olds could successfully form a superordinate category involving animal exemplars. One condition reported to affect categorization in adults is the number of exposure exemplars used to introduce the category. Homa and Vosburgh (1976) found that when exposure exemplars are perceptually dissimilar, an increase in number enhances generalization to novel, within-category stimuli. However,


this effect was not observed when the exposure set consisted of highly similar exemplars. Thus, adults’ formation of a category involving perceptually dissimilar exemplars was successful when the exposure set reached sufficiently large size.

Previous studies of superordinate categories in infants have used a large number of different exemplars during familiarization. Ross (1980) used 10 different animals during familiarization. Golinkoff and Halperin’s (1983) description of the single infant they studied indicated that a considerable, although indeterminate, number of different animal exemplars were experienced prior to testing. Thus, the facilitative effect of increasing the number of different familiarization exemplars on infants’ formation of superordinate object categories cannot be evaluated. In addition, previous studies of a category consisting of animal stimuli have used rather prototypical exemplars in both familiarization and testing (Golinkoff & Halperin, 1983; Ross, 1980). Thus, the extension of this category in infants (i.e., the range, broad or narrow, of exemplars that are treated as members) has been difficult to assess. In a category in which perceptual similarity can vary widely among members, it is possible that the category extension in infants will include only exemplars high in similarity to the exposure set (i.e., extreme underextension).

The purpose of Experiment 4 was to examine the effect of increasing the number of habituation exemplars on 15month-olds’ ability to form a category involving animal exemplars. In the event of successful categorization, it was also of interest to determine if generalized habituation would extend to a somewhat more peripheral exemplar of an animal (bird).

Method

Su&cis. Thirty-six full-term 15-month-old infants (+7 days) with no known visual problems served as subjects. An additional 20 infants did not complete the session due to fussing (15), failure to habituate (4), or equipment failure (1). All infants were recruited from the greater Akron, Ohio, area.

Stimuli. The habituation stimuli consisted of the three stimuli used in Expe- riments 1 through 3 (dog, cat, horse) plus three additional stimuli (lion, tiger, cow) (see Figure 1). The test stimuli were identical to those used in Experiments 1 through 3.

Setting and Apparatus. Setting and apparatus were the same as in the previous three experiments.

Procedure. Procedures remained unchanged from Experiments 1 through 3 with two exceptions. First, to reflect the increased number of habituation stimuli, a habituation block was defined in terms of the six habituation stimuli. Pilot testing indicated that the habituation criterion as defined in Experi-


ments 1 through 3 resulted in substantial subject loss due to the minimum 18 trials required and the associated increase in session time. Thus, in contrast to Experiments 1 through 3, the habituation criterion in Experiment 4 was defined as mean looking within a block that represented a 50% or greater decrement from the mean looking during the first habituation block. This reduced the number of required trials to 12 and considerably increased completion rate.

Second, to accommodate to the five-trial test phase, five of the six habituation stimuli were randomly selected for presentation to the control group during test. These five stimuli were dog, cat, horse, lion, and cow. Thus, each infant in the control group received continued presentation of the habituation stimuli (no change) during test with a different habituation stimulus being presented on each of the five test trials. Presentation order was randomly determined for each infant in the control group. Presentation order in the experimental group was determined in the same manner as Experiments 1 through 3.

ReSUltS Mean total fixation times in seconds and trials-to-criterion during habituation for each group were as follows: experimental group, 125.80 (SD = 52.35) and 18.33 (SD=8.10); control group, 142.76 (SD=83.59) and 20.00 (SD=8.97), respectively. There were no significant group differences on either total fixation time, t(34) = .73, p = .47, or trials-to-criterion, t(34) = .59, p= .56.

The mean fixation time in seconds for each group at habituation criterion was as follows: experimental group, 3.58 (SD= 1.26); control group 3.80 (SD = 1.35). A t test for independent groups revealed no significant difference between groups, t(34) = .508, p = .62. This indicates that both groups had habituated to a comparable level as the test phase began.

As in the previous experiments, graphic inspection of the raw test trial data revealed right-skewed distributions. A log-transformation was applied to the test-trial data to normalize the distributions and equalize the variances (Rum- mel, 1970). All analyses were done on the log transformed data. A one-way repeated-measures ANOVA on the experimental group revealed no significant order effects associated with the within-category test stimuli, F(2,34) = 0.09, p = .91. Analysis of the control group also revealed no significant order effects, F(4,68) =0.50, p = .74.

A Groups (experimental vs. control) x Test Trials repeated-measures ANOVA revealed significant main effects for Groups, F(1,34)=9.24, p= .005, and Test Trials, F(4,136) =2.52, p= .044, and a significant Groups xTest Trials interaction, F(4,136) =4.58, p = .002. Post-hoc analyses of the interaction using one-way ANOVAs (Bruning & Kintz, 1977) revealed significant differences (PC .05) between Groups only on Trials 4 (out-of-category car) and 5 (posttest checkerboard; see Figure 5).

In addition, an inspection of Figure 5 suggested the possibility of linearly increasing recovery in the experimental group. Therefore, an orthogonal test for trend was performed on the within-category and out-of-category test trials


t

0 Experimental

3 * Control

TEST TRIALS

Figure 5. Mean logs of looking times during the test phase of Experiment 4 (ISmonth-olds

habituated to six animal exemplars). The stimulus designations by trial for the experimental group were: Trial t-deer; Trial 2-pig; Trial g-bird; Trial 4-car: Trial 5-checkerboard. The stimulus designations for the control group over Trials 1 through 5 reflect the randomized presentation of five randomly chosen habituation stimuli (dog, cat, horse, lion, cow); thus,

there was no change from the habituation phase.

(Trials l-4). This analysis revealed a significant linear trend, F(1,34)=8.68, p = .006. Tests for cubic and quadratic trend were nonsignificant, F(1,34) = 0.38 and F(1,34) = 0.00, respectively.

The pattern of generalized habituation to within-category test stimuli and discrimination of the out-of-category and posttest stimuli is consistent with categorization. However, in discussing linguistic categories, Nelson (1985) has noted that the lower level categories (e.g., basic-level categories) included in the corresponding superordinate category retain their identities. That is, a superordinate category is one which is not just overly broad relative to related basic-level categories, but rather one in which the basic-level subcomponents are independently conceptualized. In other words, superordinate categories are inclusive of a number of lower level and conceptually independent categories (Nelson, 1985). It would be of interest to determine whether this characteristic of superordinates is indicated in the nonlinguistic category evidenced by infants in Experiment 4.

Nelson (1985) regarded children’s comprehension of corresponding basic- level labels as a clear indication of the independent identity of basic-level subcomponents. Consequently, supplemental data bearing on the comprehension of these basic-level labels by 15month-olds were collected. Gleason and Wein- traub (1978) reported that in comparing parental reports on a variety of linguistic abilities to children’s actual test behavior, parents are good predictors


TABLE 2 Number of Infants Reported to Comprehend the Basic-Level Labels

of Stimuli Used in Experiment 4

Stimuli NO

Habituation

Dog Kitty Horse

Lion Tiger

cow

Test Deer

Pig Bird

27** 24*

12 6 4

14

3 10 25’

a Sample size=30. ** p<.ool. * p<.Ol.

of their children’s language comprehension and production. Similarly, Thomas, Campos, Shucard, Ramsay, and Shucard (1981) and Behrend (1988) found that 13-month-olds’ word comprehension as measured by a signal detection procedure confirmed parental reports of words comprehended and not comprehended. Therefore, a separate group of 30 mothers was interviewed in detail regarding their 15month-old infant’s comprehension of the superordinate label animal and of the basic-level terms associated with the habituation and test stimuli used in Experiment 4. The interview followed closely the recognition method used by Snyder, Bates, and Bretherton (1981). If a mother indicated that her infant did comprehend a particular label, she was asked what her infant did that indicated comprehension and about the situational contexts involved. The mother was also probed regarding accompanying gestures. In particular, care was taken to determine if a label was associated with only a single, unique object (e.g., a particular dog) or whether a label was associated with several different objects (e.g., several different dogs including, in some cases, perceptually similar nondogs). Comprehension was credited only if infants indicated appropriate responses without gestural cues in more than one situational context and in association with more than one category member.

Analysis of the interview data indicated that a significant number of these infants were reported to comprehend the words, dog, kitty (or cat), and bird (binomial test, p< .05, see Table 2). However, no infant was reported to comprehend the superordinate label animal. Furthermore, mothers reported that they seldom, if ever, used the label animal when talking to their infants. This is consistent with other studies of early lexical comprehension at this age (Bene- dict, 1979; Hoek, Ingram, & Gibson, 1986) and with reports examining the nature of lexical input to children (Lucariello & Nelson, 1986; White, 1982).


Discussion Infants in the experimental group generalized habituation to novel animal stimuli but discriminated the nonanimal and posttest stimuli. This pattern Of responding is consistent with categorization. Thus, when the number of habituation stimuli was increased sufficiently, performance in the test phase provided evidence suggesting categorization by 15-month-old infants.

In addition, data from a separate group of infants suggested that by 15 months of age, certain basic-level labels for animals appear to be comprehended. These data suggest that the nonlinguistic category formed by infants in Experiment 4 was not simply overly broad. That is, by 15 months of age, subcomponents (basic-level categories) of a category superordinate to the basic level appear to retain their independent conceptual identities. Thus, the nonlinguistic category apparently formed by these infants appears to involve an assimilation of certain known categories into a higher order grouping. This would be consistent with the inclusive nature of superordinate categories (Nelson, 1985). This adds an important dimension to the findings of previous studies, which have not addressed the inclusive nature of superordinate categories in infants.

It is also important to note that because only certain basic-level terms (but not all) were comprehended, the superordinate category formed may not be as extensively differentiated into subcomponents as the adult category. Thus, although the description of the category as superordinate to the basic level appears to be justified, the extent of inclusiveness in the infant’s category is considerably restricted relative to the adult’s superordinate category of animal. A similar point applies to the extension (i.e., the range of different member exemplars) of the category formed by these infants. Practical limitations of design in experiments of this sort limit the number of different exemplars that can be included in testing. Consequently, the degree to which infants’ categories were narrower, equal to, or broader than the adult animal category remains an open question. The interpretation here is that a category consisting of certain an&l exemplars appeared to be formed..No claims are made that the category is coextensive with the adult category of animal. However, the interview data suggest that the category formed appeared to include certain lower level categories, a characteristic consistent with a superordinate relationship.

Moreover, infants generalized habituation to the bird in Experiment 4. This suggests that the category formed extended to more peripheral, two-legged animals and was not restricted only to animals of the four-legged variety. This extends the findings of previous studies, suggesting that the infants’ category, while not coextensive with the adult’s, may nevertheless reflect a somewhat wider scope than previous studies have indicated.

The work of Rosch (e.g., Rosch, 1977, 1978; Rosch et al., 1976) and related studies have amply documented that natural categories exhibit internal structure. An important aspect of such structure is the presence of a membership gradient which reflects the increasing difficulty in making a categorization


decision (instance vs. noninstance) as the degree of prototypicality decreases. Decisions become more difficult as exemplars become less prototypical, reflecting decreasing degrees of goodness-of-membership. In adult studies, a variety of measures have been found to be inversely related to the degree of goodness- of-membership, for example, latepcies to make category decisions, speed of learning, order and probability of item output, and the effects of priming (Rosch, 1978). The significant linear trend for recovery in the test phase of Ex- periment 4 is consistent with the membership gradient reported in studies of adults. That is, as the test exemplars decreased in prototypicality, recovery tended to increase reliably.

GENERAL DISCUSSION

Recently, it has been reported that by 9 months of age infants can both form and retrieve the basic-level category bird (Roberts, 1988; Roberts & Horowitz, 1986). Results of Experiments 1 through 3 suggest that when procedures, stimulus conditions, and dependent measures were used which paralleled these basic-level studies of bird, a superordinate category involving animal exemplars did not appear to be formed even by 15 months of age. Findings of Experiment 2, however, suggested that by 12 months of age infants could discriminate between the habituation and test stimuli in the context of a categorization experiment. Under the familiarization conditions of Experiment 4,15-month-old infants generalized habituation to novel animal stimuli in the test phase but discriminated the nonanimal stimulus. This pattern of responding is consistent with categorization. The results of Experiment 2 suggest that this generalization was of a categorical nature. In addition, interviews of 30 mothers of 15month-old infants indicated that infants at this age appear to comprehend basic-level terms for certain objects within the superordinate category formed by infants in Experiment 4.

Generally, these findings suggest that, when conditions potentially affecting categorization are held constant, there appears to be a prominent developmental gap between the successful formation of a nonlinguistic basic-level category (bird) at about 9 months of age and the formation of a related nonlinguistic superordinate category involving animal exemplars. These findings are consistent with those reported for artificial, nonsense categories in 2%year-olds (Mervis & Crisafi, 1982). Thus, the data from Experiments 1 through 3 extend the basic-to-superordinate order of emergence to a much younger age in the context of two-dimensionaIly represented real-world object categories.

Mervis and Crisafi (1982) argued that the differential ease in forming categories at various levels of abstraction can be explained in terms of the degree of category differentiation. That is, the higher the differentiation, the easier (and earlier) the category should be acquired. While acknowledging the hypo- thetical possibility that subordinate categories could be more differentiated


than superordinate categories, they noted that for the vast majority of natural category hierarchies, differentiation is greatest at the basic level, followed by the superordinate level, and finally by the subordinate level. Thus, degree of differentiation generally predicts a basic-to-superordinate order of emergence. The results of Experiments 1 through 3 taken together with methodologically similar studies of a related basic-level category (Roberts, 1988; Roberts & Horowitz, 1986) are consistent with this prediction. This raises the possibility that the capacities involved in processing information related to category differentiation in the nonlinguistic domain can be extended to infants.

Mervis and Crisafi (1982) also concluded that because the stimuli used with their subjects were from artificial nonsense categories, perceptual-cognitive factors were a sufficient explanation of the basic-to-superordinate order of emergence. However, in a replication of Mervis and Crisafi’s study, using natural categories, Blewitt (1983b; cited in Lucariello & Nelson, 1986) found no difference in the onset of superordinate and subordinate categories even though computed differentiation scores were as predicted by Mervis and Crisafi. Blewitt’s findings raise the possibility that for natural categories other factors may influence order of emergence. As Lucariello and Nelson (1986) noted, Blewitt suggested

. . . that adult speech could raise the salience of levels of categorization for children, and, more specifically, having found that adults use more subordinate than superordinate level words (Blewitt, 1983a), she proposed that a pre- dominance of subordinate terms in adult speech could counteract the effect on child categorization ability produced by the greater differentiation of superordinate level categories. (p. 514)

Similarly, Lucariello and Nelson reported that mothers of 2- to 2’/2-year-olds tended to use more subordinate than superordinate terms when speaking to their children. Although not examining categorization directly, Lucariello and Nelson reported that the children in the study also used more subordinate than superordinate names. They suggested that, indirectly as least, these findings would lend support to the possibility that adult speech may influence young children’s categorizations.

In the present set of experiments, mothers of a group of Is-month-olds reported no comprehension of and little or no exposure to the corresponding superordinate label (animal). Similarly, infants at 9 months of age appear to receive little or no exposure to the basic-level label bird (Roberts, 1988; Roberts & Horowitz, 1986). This suggests a major role for perceptual-cognitive factors as opposed to linguistic factors in explaining the basic-to-superordinate order Of emergence of natural categories in infancy. This is consistent with similar claims made by Mervis and Crisafi (1982) in their study involving toddlers.

Taken together, available data suggest that in the absence of adult labeling, PerCeptUd-COgnitiVe factors are an important influence on the order of the emergence of even natural categories at different levels of abstraction (Mervis


& Crisafi, 1982). Moreover, the present experiments suggest that this influence appears to be functional in infancy. However, as children becor,:,> more linguistic and are thus in a position to better use language to learn language, adult labeling practices may overshadow nonlinguistic factors to change the order in which the words (linguistic categories) for different levels are acquired (Blewitt, 1983b; Lucariello & Nelson, 1986). What is not yet clear is the effect that adult labeling would have on the order of nonlinguistic categories at different levels of abstraction.

The results of Experiment 4 suggest that it is not that 15-month-olds cannot form a superordinate category. Increasing the number of familiarization exemplars to six in Experiment 4 appeared to result in successful categorization by 15-month-olds. Thus, for a superordinate-level category, a level characterized by lower perceptual similarity among members, increasing the number of habituation exemplars appears to facilitate categorization in infants. This is consistent with data reported in studies of adults. Homa and Vosburgh (1976) reported increased classification accuracy with an increase in the number of exposure exemplars when these exemplars exhibited low perceptual similarity. Bomba and Siqueland (1983) reported similar effects for the number of exposure exemplars in 3- to 4-month-old infants in the context of simple-form categories. The results of Experiment 4 extend the facilitative role of increasing the number of exposure exemplars to older infants faced with categorizing rather complex stimuli. Taken together, available data implicate the number of exposure stimuli as an important design consideration, at least when investigating very young infants’ abilities or when examining less perceptually based categories in older infants. In particular, the number of exposure exemplars would be an important consideration in determining the earliest age at which various types of categories could be formed. It seems likely that increasing the number of exposure exemplars alone or in combination with other facilitative factors will move the onset of categorization to earlier points in the age range. This likelihood highlights the importance of identifying other facilitative factors and their cumulative effects on categorization.

The results of Experiment 4 generally support previous findings (Golinkoff & Halperin, 1983; Ross, 1980) which suggest that infants can form a category involving animal exemplars. However, the present study significantly extends previous findings. Comprehension by 15-month-olds of certain basic-level words for the experimental stimuli suggests that the category formed was not simply overly broad. Rather, in the category formed, certain subcomponents (basic-level categories) may have retained their independent conceptual identities, as indicated by infants’ knowledge of a basic-level label (Nelson, 1985). That is, the nonlinguistic category apparently formed may reflect an assimilation of certain known basic-level categories into a higher order grouping in which the conceptual integrity of these basic-level categories is preserved. This is consistent with the inclusive nature of hierarchically related categories and


suggests that the category formed was in a superordinate relationship to related basic-level categories (Nelson, 1985). Thus, it appears that an important characteristic of superordinate categories may have been present in the category formed by infants in Experiment 4. This considerably strengthens the interpretation that infants can apparently form a category involving animal exemplars that is in a nascent superordinate relationship to lower level categories.

Previous studies have used rather prototypical exemplars of animals in testing for the superordinate category of animal (Golinkoff & Halperin, 1983; Ross, 1980). The generalized habituation to the bird in Experiment 4 suggests that the category formed by these infants extended to more peripheral, two- legged animals. This extends the findings of previous reports, suggesting that the extension of infants’ superordinate categories may be broader than previous data have indicated. However, this is not to say that the category formed by infants in Experiment 4 was coextensive with the adult category of animal. Further experiments examining generalization to a wider range of animal stimuli will be necessary to determine more precisely the scope of superordinate categories at this early point in development.

A central theme in recent categorization research has been that categories possess graded internal structure (Rosch, 1978; Smith & Medin, 1981). Infants’ sensitivity to prototypical or best examples has been reported in several studies of basic and subordinate categories (Roberts & Horowitz, 1986; Sherman, 1985; Strauss, 1979; Younger, 1985). However, little is currently known about the structure of infants’ superordinate categories. In Experiment 4, infants evidenced a reliable, linear increase in recovery to test stimuli of decreasing prototypicality. These data appear to suggest the presence of internal structure and thus suggest that infants, like adults, may exhibit a sensitivity to degrees of category membership even in a nascent superordinate-level category. It is important to note, however, that the representational basis for categorization in terms of a summary representation of properties (e.g., an abstracted prototype) versus specific exemplar representation (e.g., best examples) is indeterminate from these data.’ Smith and Medin (1981) have noted that summary

and exemplar accounts will often make identical predictions regarding performance on a dependent measure. Specification of the nature of representation

L One reviewer raised the possibility of an alternative, exemplar-based interpretation in which categorical grouping (e.g., animal vs. nonanimal) may not be present. In this alternative, infants may store memories of individual exemplars grouped by the experimental context and generalize from the total set of memories without regard for animal versus nonanimal (categorical) grouping. It was suggested that performance on the dependent measure (visual futation time) in the test phase of Experiment 4 might be more parsimoniously explained by such an account. Smith and Medin (1981) have noted the difficulty of differentiating categorical accounts involving summary versus exemplar representation, because often the performance predictions are the same for both accounts. An account invoking noncategorical storage of exemplars may represent a third, but no less difficult to isolate, possibility in attempts to characterize the growth of categorical knowledge.


is further complicated by the possibility of mixed (i.e., both summary and exemplar) representations and various learner- and category-related factors that may influence the nature of representation (Smith & Medin, 1981). To clarify these issues, further research will be needed which addresses questions specific to the representation issue. A promising approach may be to use reactivation or reinstatement paradigms to examine the nature of category memory as a function of brief exposure to novel or familiar category exemplars (e.g., Hayne, Rovee-Collier, & Perris, 1987; Roberts, 1987).

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26 July 1988; Revised 21 December 1988 W

Categorization studies of 9- to 15-month-old infants: Evidence for superordinate categorization?

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