Infant perceptual and conceptual categorization: the roles of static and dynamic stimulus attributes

M.E. Arterberry, M.H. Bornstein / Cognition 86 (2002) 1–24

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COGNITION Cognition 86 (2002) 1–24 www.elsevier.com/locate/cognit

Infant perceptual and conceptual categorization: the roles of static and dynamic stimulus attributes Martha E. Arterberry a,*, Marc H. Bornstein b b

a Department of Psychology, Gettysburg College, Gettysburg, PA 17325, USA Child and Family Research, National Institute of Child Health and Human Development, 6705 Rockledge Drive, Bethesda, MD 20892-7971, USA

Received 3 February 2001; received in revised form 11 April 2002; accepted 23 May 2002

Abstract Infants’ categorization of animals and vehicles based on static vs. dynamic attributes of stimuli was investigated in five experiments (N ¼ 158) using a categorization habituation-of-looking paradigm. In Experiment 1, 6-month-olds categorized static color images of animals and vehicles, and in Experiment 2, 6-month-olds categorized dynamic point-light displays showing only motions of the same animals and vehicles. In Experiments 3, 4, and 5, 6- and 9-month-olds were tested in an habituation-transfer paradigm: half of the infants at each age were habituated to static images and tested with dynamic point-light displays, and the other half were habituated to dynamic point-light displays and tested with static images. Six-month-olds did not transfer. Only 9-month-olds who were habituated to dynamic displays showed evidence of category transfer to static images. Together the findings show that 6-month-olds categorize animals and vehicles based on static and dynamic information, and 9-month-olds can transfer dynamic category information to static images. Transfer, static vs. dynamic information, and age effects in infant categorization are discussed. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Infants; Perceptual and conceptual categorization; Static and dynamic stimulus attributes

1. Introduction The ability to group similar properties, objects, or events into categories is a foundation of cognition, and even young infants have shown facility in forming categories around very basic perceptual dimensions, such as hue, voice-onset-time, form, and spatial relations (e.g. Bomba, 1984; Bornstein, Kessen, & Weiskopf, 1976; Eimas, Siqueland, Jusczyk, & Vigorito, 1971; Mehler, Dupoux, Nazzi, & Dehaene-Lambertz, 1996; Quinn, * Corresponding author. Tel.: 11-717-337-6173; fax: 11-717-337-6172. E-mail addresses: [email protected] (M.E. Arterberry), [email protected] (M.H. Bornstein). 0010-0277/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0010-027 7(02)00108-7

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1994). Soon afterward, infants show increasing sophistication in categorization (see Hayne, 1996; Quinn, 1999, for reviews): between 3 and 6 months, for example, infants categorize simple forms composed of dot patterns (Bomba & Siqueland, 1983; Younger & Gotlieb, 1988), orientations of lines (Bomba, 1984; Quinn, Siqueland, & Bomba, 1985), relations between lines and elements (Cohen & Younger, 1984; Quinn, 1994), different types of animals (Quinn & Eimas, 1996), and animals and other nonliving objects such as vehicles or furniture (Behl-Chadha, 1996). In the second half of the first year, infants categorize the gender of faces (Leinbach & Fagot, 1993) and emotional expressions (e.g. Kestenbaum & Nelson, 1990), and they categorize faces and schematic animals based on correlations among features of objects, such as legs, ears, and tail length in animals and nose size, eye separation, and hairline in faces (Sherman, 1985; Younger, 1985, 1990, 1992; Younger & Cohen, 1983). It has been argued that categorization occurs at multiple levels. Bornstein (1984) suggested four types of categorization: identity categorization, referent equivalence categorization, perceptual equivalence categorization, and conceptual equivalence categorization. Identity categorization is the simplest form and is characterized by perception and recognition of the same object or stimulus across multiple presentations. Referent equivalence categorization is characterized by recognition of a stimulus across variation in appearance such as would be produced by changes in orientation, sensory modality, or dimensionality (i.e. 2D vs. 3D). Perceptual equivalence categorization describes grouping different and discriminable stimuli by qualitative appearance. Finally, in conceptual equivalence categorization different stimuli specified by the same or different dimensions are considered members of the same category. Mandler (1992, 2000) suggested that infants categorize at either a perceptual or conceptual level. Perceptual categorization is based on knowing what objects look like and relies heavily on the physical appearance of objects. Conceptual categorization is based on knowing what objects are and relies on what they do or the roles they play in events. Viewing categorization as a process that can occur at different levels based on the type of information infants use suggests an important role for specific stimulus variables in some types of categorization (e.g. perceptual or referent categorization) but not for others (e.g. conceptual categorization). For example, an adult will recognize a full-color photograph of an animal as representing an object from the same domain as a line drawing of the animal, a three-dimensional replica of the animal, and video footage of the animal. At the surface (proximal) level, these displays look very different; despite their being specified by different stimulus information, however, their resulting representations are common – it is an animal. Infants who categorize at a perceptual level need to match objects based on their physical appearance, and how the object is specified (whether by a line drawing or a full-color moving video) may influence the categorization process. Infants who categorize at a conceptual level need to be able to access object representations to determine what the objects are and what they do. Object meaning and function can be specified in numerous ways, and categorizers (whatever their age) need sometimes to transcend specific stimulus variables when categorizing. Infants’ categorization of animals and vehicles on the basis of static and dynamic information was the specific focus of this study. We were interested in whether certain stimulus variables relative to others facilitate infants’ categorization. To address this

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question, we employed a conservative but robust transfer paradigm (Kaufmann-Hayoz, Kaufmann, & Stucki, 1986; Yonas, Arterberry, & Granrud, 1987). In this design, the information specifying the object is manipulated to test for responding based on higherorder attributes (object category) rather than specific stimulus features (patterns of color or motion). Transfer studies in infant research are typically used to address one of two kinds of questions. The first is whether infants have an understanding of relations between perceptual attributes of objects. For example, do infants know what an approaching object should sound like? In studies addressing these types of questions, researchers investigate the coordination of information from two different modalities, such as vision and audition (e.g. Schiff, Benasich, & Bornstein, 1989). A second set of questions addresses the nature of the representation formed from perceptual information. For example, object shape can be perceived visually and tactually (e.g. Meltzoff & Borton, 1979). Infants’ recognition by sight of an object previously touched provides evidence that infants’ representation of the object is independent of specific stimulus information, such as provided by vision or touch alone. The nature of infants’ representations, and whether they depend on specific stimulus information, also can be addressed in transfer studies within a single modality, such as vision. For example, Kaufmann-Hayoz et al. (1986) showed that infants transferred twodimensional form specified by the accretion and deletion of texture (a dynamic cue) to a static outline of the two-dimensional form, and Yonas et al. (1987) showed that infants transferred three-dimensional shape specified by dynamic information to three-dimensional shape specified by binocular disparity. In both of these studies, the conditions varied dramatically from the habituation to the test phase: a dynamic display became static, subjective contours became explicit, and (presumably) different areas within the visual cortex were activated (e.g. Livingstone & Hubel, 1988). Despite the difference in conditions between habituation and test, infants showed less looking to the object specified during the habituation phase and more looking to an object that presented a change in form or three-dimensional structure in the test phase, leading to the conclusion that infants’ perception of form was independent of the presence of specific information. Using a transfer paradigm, we tested 6- and 9-month-olds’ categorization of animals and vehicles in two conditions. In a Static ! Dynamic condition, infants were habituated to static color images from one category, and following habituation they viewed dynamic point-light displays of novel images from the habituation category (the familiar-category exemplar) and from the category not seen during habituation (the novel-category exemplar). Infants in a complementary Dynamic ! Static condition were habituated to dynamic point-light displays of images from one category, and following habituation they viewed static color images of novel stimuli from the habituation category (the familiar-category exemplar) and from the category not seen during habituation (the novel-category exemplar). This design represents a strong test of infants’ categorization abilities because they cannot merely match perceptual features across exemplars. If infants’ categories are independent of specific stimulus information, then they should show a preference for the novel-category exemplar over the familiar-category exemplar. We studied categorization of exemplars in two common domains, animals and vehicles, because the animal and vehicle domains provide a compelling contrast for addressing

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infants’ categorization based on motion. It has been suggested that older infants categorize animals and vehicles based on an understanding of differences in the types of motion associated with each (Mandler, 1992; Rakison & Poulin-Dubois, 2001). Although the trajectory of the movement of animals and vehicles is horizontal, there are patent differences in the types of motion produced by animals and vehicles. If an animal walks or a vehicle drives “in place”, key parts still specify different kinds of motion. Animals locomote by biological motion, which is characterized by a system of pendular patterns, whereas vehicles move by rotary motion, and we think this key difference in motion pattern may constitute one underlying basis for categorization of animals and vehicles. Before conducting the transfer studies, two control experiments were called for. In Experiment 1, we tested 6-month-olds’ categorization of static color images of animals and of vehicles using a multiple-exemplar habituation-test design. This experiment was conducted to demonstrate that infants categorize the full-color static exemplars created for the transfer design featured in Experiments 3, 4, and 5. Experiment 2 tested 6-month-olds’ categorization of dynamic point-light displays that depicted motion patterns of animals and vehicles created for Experiments 3, 4, and 5. This study was also the first to assess 6month-olds’ ability to categorize animals and vehicles based on motion patterns alone (point-light displays). In Experiments 3, 4, and 5, we then studied 6- and finally 9-montholds’ transfer of object category information across static and dynamic cues using a multiple-exemplar habituation-transfer design. In each experiment, we took as evidence of categorization, first, infants’ habituation to varying stimuli within a category, second, infants’ generalization of habituation to familiar-category novel exemplars and discrimination of novel-category novel exemplars in a subsequent test following habituation, and, third, infants’ preference for a novel-category novel exemplar over a familiar-category novel exemplar in the test. 2. Experiment 1 Experiment 1 tested infants’ categorization of full-color static images from animal and vehicle object domains. Six-month-olds were habituated to different exemplars from the same category (animal or vehicle), and then, on two test trials, infants viewed a familiarcategory exemplar not seen during the habituation phase, and on two other test trials infants viewed a novel-category exemplar. 2.1. Method 2.1.1. Participants Twenty 6-month-olds (M age ¼ 188:2 days, SD ¼ 5:8, eight females) took part in the study. Infants were term and healthy at birth and at the time of testing (M birth weight ¼ 3:7 kg, SD ¼ 0:5, M birth length ¼ 51:8 cm, SD ¼ 1:9). Infants in each experiment were recruited through the use of purchased mailing lists of newborns in a suburban metropolitan area. The majority of infants were of European descent, with 25% of the sample comprising infants of African, Asian, or Latino heritage. An additional eight infants began the procedure, but their data were not included due to fussiness (five), experimenter error (one), or parental interference (two).

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2.1.2. Materials and apparatus The stimuli consisted of 18 full-color images of nine animals and nine vehicles. Fig. 1A shows one example from each category. Each image included the featured object in a naturalistic background. The animals were a bear, bird, cat, cow, deer, dog, horse, rat, and a sheep; the vehicles were a delivery truck, hatchback, mini van, motorcycle, pick up truck, sedan, sports car, tractor, and utility golf cart. The images, figure and background, subtended approximately 20.48 high and 28.18 wide (figures were 17.18 high and 25.98 wide). Each image was presented on a video monitor via videotape (although the images were presented via videotape, they were static). A posttest stimulus of “visual noise” was created by playing the end of a tape without any recording. A test of within-category discrimination was not necessary because the static exemplars and their backgrounds varied substantially and other researchers have demonstrated within-category discrimination of animals and vehicles at 6 months and younger (e.g. Behl-Chadha, 1996; Oakes, Madole, & Cohen, 1991). Infants were tested in a 1.5 by 2.1 m dimly lit room. They sat approximately 50 cm from the video monitor in a high chair, and the parent sat in a nearby chair. The tapes were presented on a 21 by 29 cm video monitor screen which was placed on a table at the infants’ eye level. A video camera (positioned 38 cm behind and 18 cm above the monitor) projected the infants’ face onto another video monitor in an adjacent room. The camera and the rest of the testing room were occluded from the infants’ view by a curtain. A room adjacent to the testing room housed a VCR for presenting the images, a VCR and monitor

Fig. 1. (A) Color static images of exemplars from the animal and vehicle categories and (B) a single frame from the dynamic point-light displays for the dog and sports car.

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for recording the infants’ looking, and a laboratory microcomputer for collecting infants’ fixation duration on each trial. Two experimenters conducted the study. One experimenter recorded the infants’ fixations by depressing a switch attached to the computer. This experimenter did not know which display was presented on each trial, and her data were used in the analyses. The other experimenter operated the VCR to present the stimuli, and she also recorded fixation times to provide a measure of reliability. Infants were judged to look at the stimulus when a corneal reflection of the light from the monitor was in the center of the pupil. The computer calculated the baseline, determined when the infant had met the habituation criterion, and signaled the end of each trial by illuminating an LED. Interjudge agreement (Cohen, 1968, kappa) was obtained for 80% of the sample (k ¼ 0:88); k was based on coder agreement on infant looking at the stimulus for each second.

2.1.3. Procedure Infants were seated in the chair, and parents were asked not to interact with their child during the testing session. Infants were then habituated to stimuli from one domain. Half of the infants were habituated to animals, and half were habituated to vehicles. During the habituation phase, infants had the opportunity to view eight of the nine available images from the habituation category. The presentation order and which eight images were presented were determined randomly. Following an infant-control procedure (Bornstein, 1985; Cohen, 1973), all trials began with a minimum fixation of 0.25 s, and all trials terminated when the infant looked away for 2 continuous s or after 30 s of looking at an image. The next image was presented following a 5 s intertrial interval. Each image was on a separate video tape so rewinding the tapes or searching for the next image during the testing session was unnecessary. Between trials the monitor was dark. The mean of the first two trials determined the habituation baseline. The first trial had to last at least 5 s to be included in the baseline. 1 The habituation phase ended when the mean of two consecutive trials was 50% less than the baseline; these two trials constituted the criterion. Thus, the minimum number of trials in the habituation phase was four. On each habituation trial, infants viewed a different image; if the habituation phase continued past eight trials, the images were re-presented in their original order. All infants were required to reach the habituation criterion before moving on to the test phase. Following habituation, infants were presented four test trials followed by one posttest trial using the same fixation criterion as the habituation trials. On the test trials, infants viewed a familiar-category image (the ninth exemplar from the habituation domain not seen during habituation) and a novel-category image (an exemplar from the domain not seen during habituation). The novel-category image was randomly determined for each infant. The test series employed an ABBA design; half of the infants viewed the familiarcategory stimulus on the first and last test trials, and the other half of the infants viewed the 1

If an initial trial(s) was less than 5 s, the next display was presented. This initial trial(s) was counted in the number of trials to reach habituation, but it was not used to calculate the baseline. Most often initial short fixations were due to the infants’ tendency to reference the parent after the initial appearance of the display on the monitor, and infants’ reached the minimum look time of 5 s on the second habituation trial.

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Table 1 Experiments 1 and 2: mean looking (s) in habituation and test and the proportion of looking to the novel-category exemplar during the test phase by 6-month-olds Experiment

1

Habituation

M SD M SD

2 a

Test

Baseline

Criterion

Familiar-category

Novel-category

Novelty preference

13.43 6.46 19.34 7.77

4.40 2.03 7.38 5.88

4.19 3.53 7.59 5.93

8.02 6.84 12.75 7.51

0.62 a 0.15 0.63 a 0.15

P , 0:01.

novel-category stimulus on the first and last test trials. Following the test phase, infants viewed an unrelated display for one posttest trial. 2.2. Results and discussion Infants’ mean looking during the habituation and test phases and their novelty preference are shown in Table 1, Row 1. Preliminary analyses, here and in subsequent experiments, revealed no effects for sex, and so the data reported were collapsed across girls and boys. 2.2.1. Habituation analyses Infants habituated to varying exemplars in both categories in a mean of 7.50 trials (SD ¼ 3:20). Infants showed a decline in looking to exemplars from the same category across the habituation phase: infants’ looking on criterion habituation trials was significantly less than on baseline trials (tð19Þ ¼ 8:14, P , 0:001). (Because the habituation criterion was 50% of the baseline, achieving criterion perforce means that there is a significant difference in attention to the baseline and criterion trials. By the same token, reaching the criterion means that infants habituated to the multiple exemplar category series.) To rule out factors of fatigue accounting for the decline in attention during the habituation, infants’ looking to the posttest stimulus was compared to the mean of the criterion habituation trials: 6-month-olds showed a significant increase in looking (tð19Þ ¼ 14:33, P , 0:001). 2.2.2. Test trial analyses To assess whether infants generalized to the familiar-category novel exemplar and dishabituated to the novel-category novel exemplar, infants’ looking on the habituation criterion trials was compared to their mean looking to the test exemplars. Six-month-olds generalized habituation to the familiar-category exemplar (tð19Þ ¼ 0:30, n.s.), but they increased looking to the novel-category exemplar (tð19Þ ¼ 2:55, P , 0:05). 2.2.3. Novelty preference analysis To address whether infants showed a preference for the novel-category novel exemplar over the familiar-category novel exemplar, infants’ looking to the familiar-category and

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novel-category stimuli were summed across the two test presentations of each, and a novelty preference score was calculated by dividing the amount of looking to the novel-category stimulus by the infants’ total amount of looking in the test. Fifteen infants showed a novelty preference greater than 0.50, and five showed a novelty preference less than 0.50. The mean novelty preference exceeded chance performance (0.50) (tð19Þ ¼ 3:62, P , 0:01), and it did not vary as a function of habituation category (tð18Þ ¼ 1:34, n.s.). Six-month-old infants categorized primarily four-legged animals and wheeled vehicles when presented with full-color static images of varying exemplars of the category. They habituated to varying stimuli in each category; following habituation, they generalized habituation to a novel exemplar of the familiar-category and dishabituated to a novelcategory exemplar; and they responded with a significant novelty preference for a novelcategory exemplar over a familiar-category exemplar in the test. These findings replicate and extend those of previous work indicating categorization of animals and furniture in 3month-olds and animals and vehicles in 6-month-olds (e.g. Behl-Chadha, 1996; Oakes et al., 1991). The animals and vehicles used in Experiment 1 vary on a number of dimensions, and it is possible that infants responded to differences in one or more of these dimensions. Threemonth-old infants’ categorization of animals at the basic level, such as cat vs. dog, appears to rely on facial configuration (Quinn & Eimas, 1996), and it is possible that the animals and vehicles were differentiated on the basis of having faces or not. Animals and vehicles also differ in texture and contour (Mandler, 2000; Van de Walle, Spelke, & Carey, 1997). Animals covered with fur or feathers appear softer and reflect less light than do the hard metallic surfaces of vehicles. Moreover, the external contour of animals is smoother or less angular than the external contour of vehicles. Older infants’ categorization of animals and vehicles appears to depend on the presence of key parts, legs for animals and wheels for vehicles (Rakison & Butterworth, 1998), and these key parts were available to infants in the static images. Finally, animals and vehicles differ in terms of animacy (e.g. Mandler, 1992, 2000), and it is possible that infants were responding to this difference.

3. Experiment 2 Experiment 2 was designed to test 6-month-old infants’ categorization on the basis of dynamic attributes of animals and vehicles. We used point-light displays to convey motion for two reasons. First, point-light displays isolate motion information from other object features. Adults readily identify dynamic point-light displays of people as humans engaged in various activities, such as walking, dancing, or doing push-ups (Johansson, 1973, 1975, 1977), they recognize themselves and their friends from gait patterns (Cutting & Kozlowski, 1977), they categorize the sex of models (Barclay, Cutting, & Kozlowski, 1978; Kozlowski & Cutting, 1977), and they can estimate the weight of an object a pointlight actor is lifting and even tell if an actor is pretending to lift or really lifting a weighted object (Runeson & Frykholm, 1981, 1983). Also, perception of point-light displays by adults is not specific to human motion; Mather and West (1993) showed that adults identified animal point-light displays as animals when they were in motion but not when static images were presented.

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Second, infants are sensitive to the information contained in dynamic point-light displays. Four-, 5-, and 6-month-olds differentiate upright from inverted dynamic pointlight displays depicting a walking human, and 5-month-olds are sensitive to the phase relations among points (Bertenthal, Proffitt, & Kramer, 1987; Fox & McDaniel, 1982; see Bertenthal, 1993; Bertenthal & Pinto, 1993, for reviews). Moreover, 3-month-olds discriminate dynamic point-light displays created by the facial movements of a person as opposed to deformations of a rubber mask (Stucki, Kaufmann-Hayoz, & Kaufmann, 1987). Together these studies suggest some infant sensitivity to biomechanical motion, at least in displays depicting humans. Experiment 2 tested infants’ categorization of animals and vehicles based on the attribute of motion alone using dynamic point-light displays. Six-month-old infants were habituated to different exemplars from the same category (animal or vehicle) and then on two test trials infants viewed a familiar-category exemplar not seen during the habituation phase and on two other test trials infants viewed a novel-category exemplar. 3.1. Method 3.1.1. Participants Twenty 6-month-olds (M age ¼ 189:2 days, SD ¼ 7:2, ten females, M birth weight ¼ 3:4 kg, SD ¼ 0:4, M birth length ¼ 51:9 cm, SD ¼ 2:3) participated in the study. An additional four infants began the procedure, but their data were not included due to fussiness (three) or experimenter error (one). 3.1.2. Materials and apparatus The stimuli consisted of 18 computer-generated dynamic point-light displays of the same animals and vehicles used in Experiment 1. Fig. 1B displays one frame of the dog and one of the sports car. The stimuli consisted of 0.5 cm black dots (visual angle ¼ 0:68) on a white background which, when in motion, was an animal walking or a vehicle moving in place. All of the displays faced toward the left. The number of dots per display ranged from 11 to 20, and the mean number of dots for the animal and vehicle stimuli was 14.9 (SD ¼ 2:0); there was no difference in the number of dots per category (tð17Þ ¼ 0:62, n.s.). The whole image subtended approximately 8.08 high and 17.18 wide. The time to complete one gait cycle for the animals and one revolution of wheels for the vehicles was approximately 1.5 s. The mean number of frames per second was 19.9 (SD ¼ 11:6), and there was no difference in the number of frames per category (tð17Þ ¼ 0:99, n.s.). The point-light displays were created by videotaping animals and vehicles as they moved perpendicular to the line of sight. The images were digitized, and key joints or intersections were plotted on a frame-by-frame basis through one gait cycle or one wheel revolution. Using a Mac Quadra 840av, the resulting x, y coordinates for each joint or intersection in each frame were loaded into a modified version of an algorithm for generating point-light displays (Cutting, 1978). The algorithm generated a continuous loop display that appeared to be moving in place. The display was output to videotape using a scan converter. To create black dots on a white background, the signal was fed through a video switcher before being recorded on videotape. Prior to presenting the displays to

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infants, six adults were asked to judge whether each stimulus was an animal or a vehicle; they did so with 100% accuracy. 3.1.3. Within-category discrimination Eighteen additional 6-month-olds were tested for within-category discrimination of the animal and vehicle point-light displays. Each infant was habituated to one exemplar following the habituation parameters of Experiment 1. Following habituation, infants viewed the same exemplar and a new exemplar from the same category. Infants showed a significant novelty preference (animal M ¼ 0:69, SD ¼ 0:14, tð8Þ ¼ 4:00, P , 0:01; vehicle M ¼ 0:65, SD ¼ 0:14, tð8Þ ¼ 3:06, P , 0:05; difference between novelty preferences was not significant, tð16Þ ¼ 0:62, n.s.), providing clear evidence that the displays within each category were discriminable from each other. 3.1.4. Procedure The equipment and procedures were the same as in Experiment 1. Interjudge agreement was obtained for 85% of the sample (k ¼ 0:90). 3.2. Results and discussion Infants’ mean looking during the habituation and test phases and novelty preference are shown in Table 1, Row 2. 3.2.1. Habituation analyses Infants habituated to varying exemplars in both categories in a mean of 9.30 (SD ¼ 4:75) trials. Infants’ looking on the criterion habituation trials was significantly less than on the baseline trials (tð19Þ ¼ 6:86, P , 0:001), indicating that they habituated, and they showed a significant increase in looking to the posttest stimulus compared to looking on the habituation criterion trials (tð19Þ ¼ 2:94, P , 0:01). 3.2.2. Test trial analyses Infants’ looking on the habituation criterion trials was compared to their mean looking to the test exemplars. Six-month-olds generalized to the familiar-category exemplar (tð19Þ ¼ 012, n.s.), but increased looking to the novel-category exemplar (tð19Þ ¼ 2:50, P , 0:005). 3.2.3. Novelty preference analysis Fifteen infants showed a novelty preference greater than 0.50, and five showed a novelty preference less than 0.50. The mean novelty preference exceeded chance performance (chance ¼ 0:50) (tð19Þ ¼ 3:94, P , 0:01), and it did not vary as a function of habituation category (tð18Þ ¼ 0:40, n.s.). The findings suggest that 6-month-old infants can categorize animals and vehicles by dynamic information only. Infants showed significant habituation to variation in stimuli within these categories; they generalized habituation to the familiar-category novel exemplar in the test; they dishabituated to the novel-category novel exemplar in the test; and they showed a preference for the novel-category novel exemplar over the familiar-cate-

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gory novel exemplar in the test. Infants’ ability to categorize these motion patterns did not vary as a function of whether they viewed animals or vehicles in motion. 4. Experiment 3 It is possible that in Experiment 1 infants were responding to superficial similarities among the category exemplars depicted in the full-color images, such as shiny surfaces of the vehicles or legs on the animals, and that their understanding of these groups does not go beyond such surface features. Similarly, it is possible that infants in Experiment 2 categorized the dynamic patterns of animals and vehicles without necessarily identifying their respective group membership because objects from a given category (e.g. animals or vehicles) had similar types of motion (e.g. pendular or rotary). In other words, all pendulous displays may have been treated as equivalent and yet different from rotary displays, just as infants discriminate rigid from elastic displays (Gibson & Walker, 1984). Alternatively, it is possible that infants possess deeper conceptual representations of objects from these domains – representations that transcend perceptual variables. More stringent categorization would include a demonstration that infants not only see motion patterns as similar but also recognize that the common motion is characteristic of the domain (animals or vehicles) and recognize stationary examples of the same domain. Conversely, infants would not only see structural similarities among category exemplars but also would recognize how exemplars from the same domains appear in motion. The use of the transfer design in Experiments 3, 4, and 5 permits deeper assessment of the nature of infants’ object categories. Six-month-olds were tested in one of two conditions. In a Static ! Dynamic condition, infants were habituated to static color images from one category, and following habituation they viewed dynamic point-light displays of novel images from the habituation category (the familiar-category exemplar) and from the category not seen during habituation (the novel-category exemplar). Infants in a complementary Dynamic ! Static condition were habituated to dynamic point-light displays of images from one category, and following habituation they viewed static color images of novel stimuli from the habituation category (the familiar-category exemplar) and from the category not seen during habituation (the novel-category exemplar). 4.1. Method 4.1.1. Participants Forty 6-month-olds (M age ¼ 189:5 days, SD ¼ 7:7, 21 females, M birth weight ¼ 3:5 kg, SD ¼ 0:5, M birth length ¼ 51:3 cm, SD ¼ 2:4) participated. An additional three infants began the procedure, but their data were not included due to fussiness (two) or experimenter error (one). 4.1.2. Materials and apparatus Experiment 3 used the same materials and apparatus used in Experiments 1 and 2. 4.1.3. Procedure Infants were randomly assigned to transfer condition. Infants in the Static ! Dynamic

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condition habituated to up to eight color images of either animals or vehicles using the infant-control habituation procedure described in Experiment 1. Following habituation, infants were presented with four test trials of dynamic point-light displays. One of the displays (shown twice) depicted the motion of the ninth exemplar from the habituation category in point-light format rather than as a static image, and the other display (shown twice) was a randomly chosen exemplar from the novel category, also in point-light format. Infants in the Dynamic ! Static condition habituated to up to eight dynamic point-light displays depicting either animal or vehicular motion. Following habituation, infants were presented with four test trials of static color images. One of the displays (shown twice) was the ninth exemplar from the habituation category, and the other display (shown twice) was a randomly chosen exemplar from the novel category. Following the test phase infants viewed one posttest trial of ‘visual noise’ as in Experiment 1. Interjudge agreement was obtained for 68% of the sample (k ¼ 0:92). 4.2. Results and discussion Infants’ mean looking during the habituation and test phases and novelty preference are shown in Table 2. 4.2.1. Habituation analyses Six-month-olds habituated to both static and dynamic variants of the stimuli in equivalent numbers of trials (Static ! Dynamic M ¼ 6:15, SD ¼ 2:56, and Dynamic ! Static M ¼ 7:50, SD ¼ 3:19; tð38Þ ¼ 1:47, n.s.). Infants showed a decline in looking to exemplars from the same category across the habituation phase. A 2 by 2 ANOVA with transfer condition (Static ! Dynamic, Dynamic ! Static) as a between-subjects factor and trial (baseline, criterion) as a within-subjects factor revealed a main effect for trial (Fð1; 38Þ ¼ 141:94, P , 0:001), but no main effect or interaction with transfer condition (all Fð1; 38Þ , 1, n.s.). Infants in both transfer conditions looked significantly more at the posttest display than on the criterion habituation trials (all tð19Þ . 6:37, all P , 0:001). 4.2.2. Test trial analyses In the Static ! Dynamic condition, 6-month-olds showed a significant increase in looking to the familiar-category exemplar (tð19Þ ¼ 5:68, P , 0:001), and to the novel-category exemplar (tð19Þ ¼ 5:37, P , 0:001). In the Dynamic ! Static condition, 6-month-olds did Table 2 Experiment 3: mean looking (s) in habituation and test and the proportion of looking to the novel-category exemplar during the test phase by 6-month-old infants

Transfer Static ! Dynamic Dynamic ! Static

M SD M SD

Habituation

Test

Baseline

Criterion

Familiar-category

Novel-category

Novelty preference

15.76 8.81 17.46 7.40

5.36 2.64 6.73 3.43

14.95 6.67 7.24 4.92

16.29 8.78 8.68 5.56

0.51 0.16 0.54 0.17

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not show an increase in looking to either test display (tð19Þ ¼ 1:22, n.s. for the familiarcategory exemplar and tð19Þ ¼ 0:46, n.s. for the novel-category exemplar). 4.2.3. Novelty preference analysis In the Static ! Dynamic condition, 12 infants showed a novelty preference greater than 0.50 and seven showed a novelty preference less than 0.50 (one infant’s novelty preference was equal to .50). In the Dynamic ! Static condition, 11 infants showed a novelty preference greater than 0.50, and nine showed a novelty preference less than 0.50. In both conditions, infants failed to show a significant preference for the novel-category exemplar (chance ¼ 0:50) (Static ! Dynamic condition tð19Þ ¼ 0:27, n.s. and Dynamic ! Static condition tð19Þ ¼ 1:06, n.s.), and novelty preference did not vary as a function of habituation category (Static ! Dynamic condition tð18Þ ¼ 1:18, n.s. and Dynamic ! Static condition tð18Þ ¼ 1:69, n.s.). It is possible that 6-month-old infants’ novelty preference scores in the Dynamic ! Static transfer condition indicated a response to a change in display direction in the test phase rather than to a change in object category. Recall that the point-light displays all faced to the left and the static images faced either right or left. To address this issue, we reviewed the test displays used for each infant tested in the Dynamic ! Static transfer condition. Twelve infants received a novel-category exemplar that was facing in a different direction from the habituation exemplars, whereas eight infants did not. The novelty preference scores between these two groups of infants did not differ (M ¼ 0:54, SD ¼ 0:16, and M ¼ 0:53, SD ¼ 0:18, respectively, tð18Þ ¼ 0:17, n.s.). The findings provide no evidence that at 6 months infants are able to transfer category information across motion cues. Even though infants of this age have demonstrated categorization of animals and vehicles when presented with static color images or dynamic point-light displays, they appeared to be unable to recognize the familiar-category novel exemplar as a member of the habituation category when it was specified by a different type of information. These results are difficult to interpret. One explanation for the failure to demonstrate transfer of category information may be that infants’ categorization at this age depends on the presence of specific stimulus information and that it is not until infants get older or have more experience that they are able to recognize category members across changes in perceptual information. However, it is also possible that differences in object size and/or background information from the habituation stimuli to the test stimuli may have disrupted transfer. Additionally, in some cross-modal transfer studies, infants show familiarity effects (e.g. Meltzoff & Borton, 1979; Schiff et al., 1989), and it is possible that some infants were responding to category novelty and others were responding to category familiarity. To test the development of this transfer ability, we evaluated 9-month-olds in two subsequent experiments.

5. Experiment 4 Using the same design as Experiment 3, we tested 9-month-olds in one of the two transfer conditions.

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5.1. Method 5.1.1. Participants Forty 9-month-olds (M age ¼ 275:8 days, SD ¼ 4:9, 19 females, M birth weight ¼ 3:5 kg, SD ¼ 0:4, M birth length ¼ 52:3 cm, SD ¼ 2:5) participated. An additional five infants began the procedure, but their data were not included due to fussiness (three), experimenter error (one), or parental interference (one). 5.1.2. Materials and apparatus Experiment 4 used the same materials and apparatus used in Experiment 3. 5.1.3. Procedure The procedure was the same as Experiment 3. Interjudge agreement was obtained for 93% of the sample (k ¼ 0:91). 5.2. Results and discussion Infants’ mean looking during the habituation and test phases and novelty preference are shown in Table 3, Rows 1 and 2. 5.2.1. Habituation analyses Nine-month-olds habituated to both static and dynamic variants of the stimuli in equivalent numbers of trials (Static ! Dynamic M ¼ 6:35, SD ¼ 2:01, and Dynamic ! Static M ¼ 7:40, SD ¼ 3:18; tð38Þ ¼ 1:25, n.s.). Infants showed a decline in looking to exemplars from the same category across the habituation phase. A 2 by 2 ANOVA with transfer condition (Static ! Dynamic, Dynamic ! Static) as a between-subjects factor and trial (baseline, criterion) as a within-subjects factor revealed a main effect for trial (Fð1; 38Þ ¼ 133:28, P , 0:001), a main effect for transfer condition (Fð1; 38Þ ¼ 17:10, P , 0:001), and a Trial by Transfer Condition interaction (Fð1; 38Þ ¼ 11:10, P , 0:01). Infants in the Dynamic ! Static condition showed greater amounts of looking on both the baseline and criterion trials in comparison to infants in the Static ! Dynamic condition. Table 3 Experiments 4 and 5: mean looking (s) in habituation and test and the proportion of looking to the novel-category exemplar during the test phase by 9-month-old infants Habituation Transfer

Static ! Dynamic

Baseline Criterion Familiar-category Novel-category Novelty preference

M 10.10 SD 5.21 Dynamic ! Static M 19.29 SD 8.78 “Moving” Static ! Dynamic M 17.00 SD 5.93 a

P , 0:05.

Test

3.02 1.48 6.47 4.07 6.00 3.23

11.61 7.46 7.36 5.56 16.24 8.06

11.10 8.15 9.24 5.64 14.59 5.89

0.46 0.16 0.59 a 0.14 0.49 0.16

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Moreover, the difference in levels of attention in these two conditions was greatest on the baseline trials. Infants in both transfer conditions looked significantly more at the posttest display than on the criterion habituation trials (all tð19Þ . 5:98, all P , 0:001). 5.2.2. Test trial analyses In the Static ! Dynamic condition, 9-month-olds showed a significant increase in looking to the familiar-category exemplar (tð19Þ ¼ 5:10, P , 0:001), and to the novelcategory exemplar (tð19Þ ¼ 4:37, P , 0:001). In the Dynamic ! Static condition, 9month-olds increased looking to the novel-category exemplar (tð19Þ ¼ 1:87, P , 0:05), but not to the familiar-category exemplar (tð19Þ ¼ 0:62, n.s.). 5.2.3. Novelty preference analysis In the Static ! Dynamic condition, 11 infants showed a novelty preference greater than 0.50, and seven showed a novelty preference less than 0.50 (two infants’ novelty preferences were equal to .50). The mean novelty preference did not exceed chance (chance ¼ 0:50) (tð19Þ ¼ 1:03, n.s.), nor did it vary as a function of habituation category (tð18Þ ¼ 0:72, n.s.). In the Dynamic ! Static condition, 13 infants showed a novelty preference greater than 0.50, and seven showed a novelty preference less than 0.50. The mean novelty preference for 9-month-olds in the Dynamic ! Static condition exceeded chance (tð19Þ ¼ 2:79, P , 0:05), and it did not vary as a function of habituation category (tð18Þ ¼ 0:25, n.s.). In the Dynamic ! Static condition, 13 infants received a novel-category exemplar that was facing in a different direction from the habituation exemplars, whereas seven infants did not. The novelty preference scores between these two groups of infants did not differ (M ¼ 0:55, SD ¼ 0:13, and M ¼ 0:64, SD ¼ 0:14, respectively, tð18Þ ¼ 1:42, n.s.). These findings suggest that 9-month-olds’ categorization abilities go beyond simple stimulus comparisons. In the Dynamic ! Static condition, 9-month-olds generalized habituation to the familiar-category novel exemplar, dishabituated to the novel-category exemplar, and showed a preference for the novel-category exemplar over the familiar-category exemplar. In contrast is infants’ performance in the Static ! Dynamic condition, suggesting that 9-month-olds’ ability with static information is more limited. Habituated to static cues, infants did not show evidence of being able to transfer category information to dynamic displays. Nine-month-olds showed a significant increase in looking to the familiar-category exemplar and to the novel-category exemplar in comparison to their looking on the criterion habitation trials. In addition, they did not show a preference for the novelcategory novel exemplar over the familiar-category novel exemplar. 6. Experiment 5 It is possible that infants’ poor performance in the Static ! Dynamic condition in Experiment 4 was due, not to a failure of transfer of categorization, but to a stimulus confound. Infants often attend more to moving than to static displays (e.g. Kellman, 1984; Slater, Morison, Town, & Rose, 1985), and it is possible that the motion of the test displays in Experiment 4 was in itself more attractive than the static displays. Thus, even if infants

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recognized the new category exemplar, their propensity to look at the new category exemplar may have been overshadowed by the attractiveness of the motion of both the novel-category exemplar and the familiar-category exemplar. We addressed this possibility in Experiment 5. To remove the potential confound of preference for moving displays compared to static displays in the Static ! Dynamic transfer condition, a new set of full-color stimuli was created. The goal was to introduce motion of the full-color images, but not to present motion patterns that infants could use to match with the dynamic point-light displays. In other words, the animals did not “walk” and the vehicles did not “drive” because the pendular limbs or the rotating wheels could be matched to the motion patterns contained in the point-light displays. Instead, we created full-color images and moved them around the video screen to simulate moving a photograph in front of an observer. In other words, the whole image moved rather than key parts, like legs and wheels. With this type of motion, infants still needed to use the static information available in the full-color image to categorize the objects presented in the habituation phase, and they could not match the motion of any one part to the motion patterns available in the point-light displays. Using these “moving” static images, a second group of 9-month-olds was tested in the Static ! Dynamic transfer condition. To exercise caution in selecting the motion pattern of the static images, the animal images moved around the screen in a rotary pattern and the vehicle images moved in a pendular pattern (see Fig. 2A,B). Recall that the animal pointlight displays contained pendular motion patterns and the vehicle point-light displays contained rotary motion patterns. Moving the static images of vehicles in a pendular motion and moving static images of animals in a rotary motion, we were able to assess whether infants would respond to motion patterns in general or to object category. If infants respond to object category, they should habituate to varying stimuli within a category, generalize habituation to a novel image of the familiar category, discriminate a novel image from the novel category, and show a preference for the novel-category novel exemplar over the familiar-category novel exemplar. To show this pattern of results, infants would have to disregard the type of motion shown by the images in the habituation phase and attend to the objects depicted in the photographs. In contrast, if infants respond to motion patterns, they should habituate to varying stimuli within a category, generalize habituation to the novel-category exemplar, show an increase in looking to the familiarcategory exemplar, and show a preference for the familiar-category novel exemplar over the novel-category novel exemplar. To show this pattern of results, infants would need to attend to the motion patterns in both the habituation and test phase and disregard the information specifying object category.

6.1. Method 6.1.1. Participants Twenty 9-month-olds (M age ¼ 280:3 days, SD ¼ 5:6, ten females, M birth weight ¼ 3:64 kg, SD ¼ 0:44, M birth length ¼ 51:73 cm, SD ¼ 2:08) participated. An additional seven infants began the procedure, but their data were not included due to fussiness (four), equipment failure (one), or parental interference (two).

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6.1.2. Materials and apparatus The full-color static images from Experiment 1 were reduced in size and animated using Flash. The images, figure and background, subtended approximately 17.78 high and 24.48

Fig. 2. The motion pattern of the animal (A) and vehicle (B) images used in Experiment 5.

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wide (figures only subtended 14.98 high and 22.58 wide). The animals moved in a counterclockwise circle around the screen, and the vehicles moved in an arc along the bottom half of the screen (see Fig. 2A,B). The speed of the images was 0.33 cycle/s. Each image was output to videotape. In all other respects, the stimuli and apparatus were the same as in Experiment 4. 6.1.3. Procedure The procedure was the same as the Static ! Dynamic condition from Experiment 4. Interjudge agreement was obtained for 80% of the sample (k ¼ 0:90). 6.2. Results and discussion Infants’ mean looking during the habituation and test phases and novelty preference are shown in Table 3, Row 3. 6.2.1. Habituation analyses Nine-month-olds habituated to the full-color images in a mean of 6.3 trials (SD ¼ 3:1). Infants’ looking on the criterion habituation trials was significantly less than on the baseline trials (tð19Þ ¼ 13:61, P , 0:001), and they looked significantly more at the posttest display than on the criterion habituation trials (tð19Þ ¼ 9:96, P , 0:001). 6.2.2. Test trial analyses Nine-month-olds showed a significant increase in looking to both the familiar-category exemplar (tð19Þ ¼ 4:67, P , 0:001) and to the novel-category exemplar (tð19Þ ¼ 6:54, P , 0:001). 6.2.3. Novelty preference analysis Nine infants showed a novelty preference greater than 0.50 and 11 showed a novelty preferences less than 0.50. Infants did not show a significant preference for the novelcategory novel exemplar over the familiar-category novel exemplar (chance ¼ 0:50) (tð19Þ ¼ 0:38, n.s.), and novelty preference did not vary as a function of habituation category (tð18Þ ¼ 1:61, n.s.). In Experiment 5, the static images presented in the habituation phase were moved around the screen in order to introduce infants to motion before the test phase. As a result of this manipulation, infants had the option of responding to object category (by showing a novelty preference) or to motion pattern (by showing a familiarity preference). We found that infants habituated to the moving photographs in the same number of trials as they habituated to the static photographs in Experiment 4 (tð38Þ ¼ 0:06, n.s.), and that their level of attention to the moving photographs in the habituation phase was on par with the level of attention same-aged infants gave when habituated to dynamic point-light displays (Dynamic ! Static condition of Experiment 4). An ANOVA with trial (baseline, criterion) and transfer condition (Dynamic ! Static, “Moving” Static ! Dynamic) revealed no main effect for or interactions with transfer condition (all Fð1; 38Þ , 1:36, n.s.). However, infants showed neither a novelty nor a familiarity preference in the test phase. This finding suggests that, even though motion was present in both the habituation and test phases,

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infants appeared not to transfer category information from static color images of animals and vehicles to dynamic point-light displays. Moreover, infants did not respond based on motion category. Thus, transfer of category information from static to dynamic images appears to be limited at 9 months of age and is not necessarily an artifact of motion per se.

7. General discussion Five experiments were conducted to investigate infants’ use of static and dynamic stimulus attributes in categorizing two domains, animals and vehicles. In Experiment 1, 6-month-olds habituated to static exemplars from the two domains and looked significantly longer at a novel-category novel exemplar than at a familiar-category novel exemplar following habituation. In Experiment 2, 6-month-olds habituated to dynamic exemplars of the two categories and looked significantly longer at a novel-category novel exemplar following habituation. The depth or level of infants’ categorizing (relative to specific information available at acquisition) was tested in Experiments 3, 4, and 5. Sixand 9-month-olds were habituated to static displays and then asked to transfer category information to dynamic displays (Static ! Dynamic condition), or they were habituated to dynamic displays and then asked to transfer category information to static displays (Dynamic ! Static condition). The results of Experiments 3, 4, and 5 indicated that (a) 6-month-olds did not transfer in either the Static ! Dynamic or the Dynamic ! Static conditions, whereas (b) 9-month-olds transferred category information from Dynamic ! Static displays, but not from Static ! Dynamic displays. Together, these findings suggest that 6-month-old infants are able to categorize objects on the basis of static or dynamic information, supporting a view that categorization processes are a fundamental aspect of infant perception and cognition and that, with appropriate exposure, infants group related objects, events, and perceptual qualities (e.g. Bornstein, 1984; Quinn, 1999). Demonstration of infants’ categorization of animals and vehicles using dynamic information is of particular importance given the prominence of this type of information in recent theories of categorization (e.g. Mandler, 2000). The finding that 9-month-olds can transfer category information from dynamic point-light displays to color static images suggests that their categorization abilities are not dependent on the presence of particular stimulus attributes and, hence, may be more conceptual than perceptual in nature. The later emergence of category transfer suggests that transfer is a more difficult task for infants than categorizing stimuli based on a single stimulus attribute (static or dynamic information). Infants must perceive similarity among the varied exemplars presented in habituation, and they need to recognize the similarity of the familiar-category novel exemplar in the test phase even though that exemplar is specified by different attributes. In addition, in the transfer studies, there were changes between the habituation and test phase that were salient but not specific to object category. For example, the static pictures had full-color backgrounds but the point-light displays were presented against a homogeneous white background, and the point-light displays were somewhat smaller overall than the figures in the static images. For successful transfer, infants needed to ignore these

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differences and still recognize the familiar-category exemplar as a member of the habituation category and respond to the novelty of the novel-category exemplar. The failure of the 6-month-olds to transfer category information in Experiment 3 could represent a failure in categorization or a failure in transfer across cues. Although 6-monthold infants habituated to the category exemplars in the same number of trials as the 9month-olds did, 2 it is possible that they did not process the right information and so were not able to recognize the familiar-category exemplar in the test phase. It is unlikely that merely providing 6-month-olds with additional experience with the category (i.e. more habituation trials) would facilitate demonstration of categorization. Six-month-olds met the habituation criterion by showing a decrease in looking well below the required 50% reduction from baseline (34% and 21% for the Static ! Dynamic and Dynamic ! Static conditions, respectively, see Table 2). Alternatively, 6-month-olds may have experienced a failure in the transfer of object information specified by different proximal cues rather than a failure in categorization. To tease apart these possibilities, additional experiments could be conducted in which the categorization component is removed: infants would be habituated to the same stimulus (e.g. a point-light dog) and then tested with a static image of a dog and a static image of a sports car. Recognition of the static dog as the same as the point-light dog would demonstrate that infants at this age can transfer information for a single object across cues. Given the findings of other transfer studies showing that 3- and 4-month-olds transfer object shape (Kaufmann-Hayoz et al., 1986; Yonas et al., 1987), it is likely that 6-month-olds would be able to accomplish this single object transfer task, namely a Dynamic ! Static transfer; however, to our knowledge transfer from Static ! Dynamic information for a single object has not been demonstrated. Transfer of category information as the 9-month-olds did in the Dynamic ! Static condition suggests that older infants’ categories may not depend on surface stimulus information and older infants are able to ignore irrelevant changes between habituation and test stimuli. In other words, the category representation formed during habituation is independent of surface attributes specifying objects in that category, but depends on infants’ abstracting common attributes shared among all habituation stimuli: then, that representation can be accessed via correlated attributes available in the test phase. Evidence of transfer in 9-month-olds, but not 6-month-olds, suggests that with development in the first year infants’ categories begin to reflect amodal representations that are independent of proximal surface features. Successful transfer of category information by 9-month-olds in the Dynamic ! Static but not in the Static ! Dynamic condition is consistent with other research suggesting that dynamic information may have an advantage in perception. Studies of depth and threedimensional object perception have shown that infants are sensitive to dynamic cues earlier than they are sensitive to cues available in static conditions, such as binocular disparity or shading (Arterberry & Yonas, 2000; Gordon & Yonas, 1966; Granrud, 1986; Granrud, Yonas, & Opland, 1985; Nanez, 1988). A similar advantage for dynamic infor2 Mean number of habituation trials for the 6- and 9-month-olds in each transfer condition is reported in the results sections of Experiments 3 and 4 (Sections 4.2.1 and 5.2.1). No age differences were found: Static ! Dynamic condition (tð39Þ ¼ 0:27, n.s.) and Dynamic ! Static condition (tð39Þ ¼ 10, n.s.).

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mation is found in infants’ perception of partly occluded objects. Perception of unity of a partially occluded rod is facilitated by motion (Kellman & Spelke, 1983; Kellman, Spelke, & Short, 1986): when 4-month-olds view the visible ends of the rod translating either laterally or in depth behind a block such that the two ends are moving in the same direction, they perceive the rod as one piece continuing behind the block. This is the case even when static cues, such as shape and texture, conflict. Mandler (1992) and Rakison and Poulin-Dubois (2001) suggested that the trajectory of the path of motion (smooth vs. irregular) and the means for onset of motion (self-initiated vs. other-caused) are two early characteristics of object motion that lead to an understanding of the animacy–inanimacy distinction – a possible underlying basis for categorization of animals and vehicles. The motion used in these experiments differed from those types of motion. The present experiments show that, even when objects do not travel a distance, there are differences in motion patterns that allow infants to discriminate the domains of animals and vehicles. Moreover, young infants are able to categorize these domains based on these differences in motion. How infants transferred category information in the Dynamic ! Static transfer condition may shed some light on the “level” of infant categorization. During the habituation phase, as infants viewed the point-light displays, they may have obtained information from the dynamic displays about what the objects look like, what they do, and/or their meaning. Object motion provides rich information for what an object is and what it can do, what it can be used for, and the like (Gibson, 1982; Gibson, 1979; Ullman, 1979). The fact that infants were able to recognize the familiar-category exemplar and show a novelty effect to the novel-category exemplar despite changes in the type of information available suggests that infants were categorizing at a conceptual level (Bornstein, 1984). One might argue that the transfer task was not a test of conceptual categorization because infants could have perceived the form of the objects specified in the point-light displays using structure from motion processes (e.g. Ullman, 1979). In this case, successful transfer would depend on matching the form from the dynamic point-light displays to the static images (a process more akin to perceptual categorization than conceptual categorization). We do not disagree that structure from motion processes may be at work; however, the resulting representation from these processes is very different from the one specified in the static color images. Thus, successful transfer still must be independent of specific stimulus attributes and be based on some higher-level information. The fact that 6-month-olds did not transfer category information in the Dynamic ! Static condition, but 9-month-olds did, provides additional evidence that the transfer task may have tapped conceptual categorization. Infants younger than 6 months show perceptual categorization, but Mandler and her colleagues (e.g. Mandler, 1992, 2000; Mandler & McDonough, 1993) claim that conceptual categorization emerges only in the second half of the first year of life. Of course, we do not know if all categorization by 9-month-olds is at the conceptual level. This experiment provides one piece of evidence with a limited stimulus set. Moreover, motion is an important cue for some types of object categories but not for others, such as fruit or furniture, and whether conceptual categorization of objects that do not move is possible at 9 months is unknown. Together our findings suggest that categorization processes are relatively flexible as long as the information specifying the objects does not change: 6-month-olds categorized

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animals and vehicles on the basis of static or dynamic information, supporting a view that categorization processes are a fundamental aspect of infant perception and cognition. With age, infants come to be less tied to specific stimulus information and demonstrate the ability to transfer category knowledge across cues, at least in some cases: 9-month-olds showed transfer in the Dynamic ! Static condition but not in the Static ! Dynamic condition. This advance at 9 months may reflect movement from perceptually based categorization to conceptually based categorization. Acknowledgements The authors thank M. Ney, J. Pinto, and W. Wilson for help with stimuli construction, D. Kertes, S. McClure, S. Salkind, and B. Wright for assistance with data collection and manuscript preparation, and an anonymous reviewer for suggesting the single object transfer experiment discussed in Section 7. M.E.A. was a visiting scientist at NICHD during the completion of this work. References Arterberry, M. E., & Yonas, A. (2000). Perception of structure from motion by 8-week-old infants. Perception and Psychophysics, 62, 550–556. Barclay, C. D., Cutting, J. E., & Kozlowski, L. T. (1978). Temporal and spatial factors in gait perception that influence gender recognition. Perception and Psychophysics, 23, 145–152. Behl-Chadha, G. (1996). Basic-level and superordinate-like categorical representations in early infancy. Cognition, 60, 105–141. Bertenthal, B. I. (1993). Infants’ perception of biomechanical motions: intrinsic image and knowledge-based constraints. In C. Granrud (Ed.), Visual perception and cognition in infancy. Carnegie Mellon symposia on cognition (pp. 175–214). Hillsdale, NJ: Erlbaum. Bertenthal, B. I., & Pinto, J. (1993). Complementary processes in the perception and production of human movements. In L. B. Smith & E. Thelen (Eds.), A dynamic systems approach to development: applications (pp. 209–239). Cambridge, MA: MIT Press. Bertenthal, B. I., Proffitt, D. R., & Kramer, S. J. (1987). The perception of biomechanical motions: implementation of various processing constraints. Journal of Experimental Psychology: Human Perception and Performance, 13, 577–585. Bomba, P. C. (1984). The development of orientation categories between 2 and 4 months of age. Journal of Experimental Child Psychology, 37, 609–636. Bomba, P. C., & Siqueland, E. R. (1983). The nature and structure of infant form categories. Journal of Experimental Child Psychology, 35, 294–328. Bornstein, M. H. (1984). A descriptive taxonomy of psychological categories used by infants. In C. Sophian (Ed.), Origins of cognitive skills (pp. 313–338). Hillsdale, NJ: Erlbaum. Bornstein, M. H. (1985). Habituation of attention as a measure of visual information processing in human infants: summary, systematization, and synthesis. In G. Gottlieb & N. A. Krasnegor (Eds.), Measurement of audition and vision in the first year of postnatal life: a methodological overview (pp. 253–300). Norwood, NJ: Ablex. Bornstein, M. H., Kessen, W., & Weiskopf, S. (1976). Color vision and hue categorization in young human infants. Journal of Experimental Psychology: Human Perception and Performance, 2, 115–129. Cohen, J. (1968). Weighted kappa: nominal scale agreement with provision for scaled disagreement or partial credit. Psychological Bulletin, 70, 213–220. Cohen, L. B. (1973). A two-process model of infant visual attention. Merrill-Palmer Quarterly, 19, 157–180. Cohen, L. B., & Younger, B. A. (1984). Infant perception of angular relations. Infant Behavior and Development, 7, 37–47.

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