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Three-Month-Old Infants' Categorization of Animals and Vehicles Based on Static and Dynamic Attributes
Journal of Experimental Child Psychology 80, 333–346 (2001) doi:10.1006/jecp.2001.2637, available online at http://www.idealibrary.com on
Three-Month-Old Infants’ Categorization of Animals and Vehicles Based on Static and Dynamic Attributes Martha E. Arterberry Gettysburg College and
Marc H. Bornstein National Institute of Child Health and Human Development, Bethesda, Maryland Three-month-old infants’ categorization of animals and vehicles based on static and dynamic attributes was investigated using a multiple-exemplar habituation–test paradigm. Half of the infants viewed static color images of animals and vehicles, and the other half viewed dynamic point-light displays of the same animals and vehicles. Following habituation, infants viewed a novel exemplar from the habituation category and an exemplar from a novel category. Regardless of whether they viewed static or dynamic displays, infants showed habituation to varying exemplars from the same category, generalized habituation to a novel exemplar from the habituation category, dishabituated to an exemplar from a novel category, and showed a significant novelty preference for a novel-category exemplar. These findings suggest that infants categorize animals and vehicles using either static or dynamic information. © 2001 Academic Press Key Words: categorization; infants; static information; dymanic or motion-carried information.
Infants possess remarkable categorization skills. In the first few months of life, infants are able to form categories around very basic perceptual dimensions such as hue and voice onset time (Bornstein, Kessen, & Weiskopf, 1976; Eimas, Siqueland, Jusczyk, & Vigorito, 1971; for a review, see Werker, 1995). In addition, infants 3 to 4 months of age categorize simple forms composed of dot patterns (Bomba & Siqueland, 1983; Younger & Gotlieb, 1988), orientation of lines (Bomba, 1984; Quinn, Siqueland, & Bomba, 1985), spatial relations between
We thank M. Ney, J. Pinto, and W. Wilson for help with stimuli construction and D. Kertes, S. Salkind, and B. Wright for assistance with data collection and manuscript preparation. The first author was a visiting scientist at the National Institute of Child Health and Human Development during the completion of this work. Address correspondence and reprint requests to either author at Child and Family Research, National Institute of Child Health and Human Development, Suite 8030, 6705 Rockledge Drive, Bethesda, MD 20892-7971. E-mail: [email protected] or [email protected]. 333 0022-0965/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.
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lines and elements (Cohen & Younger, 1984; Quinn, 1994), and different types of animals (Quinn & Eimas, 1996; Quinn, Eimas, & Rosenkrantz, 1993; for a review, see Quinn, 1999), and furniture (Behl-Chadha, 1996). However, the type of information infants are able to use in categorization is not well understood. In this study, we take a first step and report an experiment on young infants’ use of static and dynamic attributes of objects in categorization. When the observer and object are stationary, the form of an object can be perceived using static information. The edges and internal contours of objects, and the intersection of these edges and contours, provide information about the threedimensional structure of the objects (e.g., Kavsek, 1999; Waltz, 1975; Yonas & Arterberry, 1994), as do patterns of light and shade (e.g., Granrud, Yonas, & Opland, 1985; Todd & Mingolla, 1983). When an object moves or an observer moves around an object, dynamic or motion-carried information is available. The resulting retinal transformations from observer and object motion provide information about an object’s three-dimensional form (e.g., Braunstein, 1976; Kellman, 1984; Wallach & O’Connell, 1953) and other characteristics such as rigidity and elastisticity (Gibson & Walker, 1984; Todd, 1984). When categorizing, infants (and adults) may use some or all of available object attributes, but it is possible that some attributes may be more useful than others in categorization. The current experiment was designed to investigate and compare the role of static and dynamic attributes in infants’ object categorization. We studied categorization of exemplars in two common object domains: animals and vehicles. These domains were chosen for two reasons. First, whereas much of the work with 2- to 4-month-old infants has addressed categorization of animals such as cats versus dogs or animals versus furniture (e.g., Behl-Chadha, 1996; Quinn & Eimas, 1996; Quinn & Johnson, 2000), no studies have investigated young infants’ categorization of animals and vehicles. However, most of the work on older children’s categorization has specifically contrasted the animal and vehicle domains (e.g., Mandler & McDonough, 1993; Oakes, Madole, & Cohen, 1991; Rakison & Butterworth, 1998); thus, this study provides a downward extension to tell us how early this categorical distinction is made. Second, the animal and vehicle domains provide a compelling contrast for addressing infants’ categorization based on motion. Objects in both of these domains move, but in different ways; animals exhibit biological motion, which is characterized by a system of pendular patterns, whereas vehicles use rotary motion, and this difference in motion pattern may constitute one underlying basis for categorization of animals and vehicles. It has been suggested that older infants’ categorization of animals and vehicles is based on differences in the types of motion associated with each domain (Mandler, 1992, 2000). To assess infants’ categorization of animals and vehicles using information contained in static images, color photographs of the nine animals and nine vehicles were created (Fig. 1A). To assess infants’ categorization of the motion patterns of animals and vehicles, dynamic point-light displays were created. These displays depicted the motions of the nine animals and nine vehicles depicted in
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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 of the dog and the sports car.
the photographs (see Fig. 1B). We used point-light displays to convey motion for several reasons. First, they isolate motion information from other object features and provide rich information on their own. Objects have a number of attributes that provide information as to object identity, and in natural viewing contexts these attributes are available simultaneously. Dynamic point-light displays remove all attributes except motion, thus allowing a direct assessment of the role of dynamic attributes in object identification and categorization. Research with adults, beginning with Johansson (1973, 1975, 1977), has suggested that patterns of motion contained in dynamic point-light displays provide rich information for both object identity (e.g., a person) and action (e.g., walking). For example, responding to only 10 to 12 lights placed on the joints of a human model, people readily identify the moving displays as humans engaged in various activities, such as walking, dancing, and doing push-ups. Further work has shown that adults recognize themselves and their friends from gait patterns (Cutting & Kozlowski, 1977), categorize the sex of models presented in dynamic point-light displays (Barclay, Cutting, & Kozlowski, 1978; Kozlowski & Cutting, 1977), and are able to estimate the weight of an object a point-light actor is lifting and even tell whether an actor is pretending to lift or really lifting a weighted object (Runeson & Frykholm, 1981, 1983). Also, perception of biomechanical motion by adults is not specific to human motion; Mather and West
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(1993) showed that adults readily identify animal point-light displays as animals when they are in motion but not when static images are presented. A second advantage of point-light displays is that infants have been shown to be sensitive to the information contained in dynamic point-light displays. That is, 4-, 5-, and 6-month-olds differentiate upright from inverted dynamic point-light displays depicting a walking human, and 5-month-olds are sensitive to the phase relations among points (Bertenthal, Proffitt, & Kramer, 1987; Fox & McDaniel, 1982; for reviews, see Bertenthal, 1993; Bertenthal & Pinto, 1993). Moreover, 3month-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 infant sensitivity to biomechanical motion, at least in displays depicting humans. Three-month-old infants were tested for categorization of animals and vehicles using a multiple-exemplar habituation-test design. Half of the infants viewed static color images of animals and vehicles, and the other half viewed dynamic point-light displays of the same animals and vehicles. 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 viewed a novel-category exemplar. If infants categorize images of animals and vehicles based on static or dynamic cues, then they should show habituation to the multiple exemplars, generalize habituation to a novel exemplar of the familiar category and discriminate a novel exemplar from the novel category, and show a preference for the novel novel-category exemplar over the novel familiar-category exemplar. METHOD Participants A total of 40 3-month-olds (age M 96.7 days, SD 8.4, 18 females) took part in the study. Infants were term and healthy at birth and at the time of testing (birth weight M 3.6 kg, SD 0.5; birth length M 52.0 cm, SD 6.3). Infants 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 10% of the sample comprising infants of African, Asian, or Latino heritage. An additional 16 infants began the procedure, but their data were not included due to fussiness or falling asleep (15) or parental interference (1). Materials and Apparatus The static stimuli consisted of 18 full-color images of 9 animals and 9 vehicles. Figure 1A shows an 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 sheep; the vehicles were a delivery truck, hatchback, minivan, motorcycle, pickup truck, sedan, sports car, tractor, and utility golf cart. The images, figures and backgrounds, subtended approximately 20.4° high and
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28.1° wide (figures were 17.1° high and 25.9° wide). Each image was presented on a video monitor via videotape (although the images were presented via videotape, they were static). An unrelated posttest stimulus was created for the static condition by playing the end of a tape without any recording; this “visual noise” was equivalent to the fuzz seen on a television screen when the signal is lost. The point-light stimuli consisted of 18 computer-generated dynamic point-light displays of the same animals and vehicles used for the static stimuli. Figure 1B displays one frame of the dog and sports car. The stimuli consisted of 0.5-cm black dots (visual angle 0.6°) on a white background that, when in motion, looked like an animal walking in place or a vehicle in motion in a plane perpendicular to the line of sight. All of the displays faced toward the left. This side view afforded viewing four legs for each animal (with the exception of the bird) and two wheels for each vehicle. The images were scaled so that they all were 15 cm wide. The time to complete one gait cycle for the animals and one revolution of wheels for the vehicles was approximately 1.5 s. There were no significant differences between the animal and vehicle pointlight displays in terms of display height, number of points, number of frames, or frames per second, all ts (16) 1.81, ns. 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 and 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 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 infants, 6 adults were asked to judge whether each stimulus was an animal or a vehicle; they did so with 100% accuracy. A second group of 10 adults was shown the displays and asked an open-ended question regarding the identity of the object depicted (“What is this?”). The animals and vehicles were identified accurately as members from their respective categories (percentage correct for animals M 96.7, SD 5.3; percentage correct for vehicles M 92.3, SD 7.4). Together, these results suggest that the displays were perceived by adults as animals and vehicles. Eighteen additional 3-month-olds were tested for within-category discrimination of the animal and vehicle point-light displays. Each infant was habituated to one exemplar. Following habituation, infants viewed the same exemplar and a new exemplar from the same category. Infants showed a significant novelty preference (animal M 0.61, SD 0.12; t(8) 2.69, p .05; vehicle M 0.58, SD 0.10, t(8) 2.57, p .05; difference between novelty preferences not significant, t(16) 0.48, ns), providing evidence that the displays within each category were discriminable from each other by 3-month-old infants. An additional videotape was created to serve as an unrelated posttest stimulus for the dynamic condition. This stimulus was a static full-color image of the smil-
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ing faces of two young girls. Even though this image was presented on videotape, no motion was present. All infants were tested in a 1.5- by 2.1-m dimly lit room. They sat approximately 50 cm from the video monitor in reclining infant seats, and their parents sat in nearby chairs. The tapes were presented on a 21- by 29-cm video monitor screen that 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’ faces 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 videocassette recorder (VCR) for presenting the images, a VCR and monitor for recording the infants’ looking, and a laboratory microcomputer for collecting infants’ fixation duration on each trial. Two experimenters conducted the study. One operated the VCR to present the stimuli, while the other recorded the infants’ fixations by depressing a switch attached to the computer. Infants were judged to look at the stimulus when a corneal reflection of the light from the display was in the center of the pupil. The experimenter making the fixation judgments did not know which stimulus was being presented in each trial. The computer calculated the baseline, determined when the infants had met the habituation criterion, and signaled the end of each trial by illuminating an LED. To obtain a measure of reliability, the second experimenter also scored infants’ fixations, either during the testing session or at a later time from the video record. Interjudge agreement (Cohen, 1968) was obtained for 72% of the sample (k .91); kappa was based on whether the coders agreed on whether the infant was looking or not for each second. Procedure Infants were seated in chairs, and parents were instructed not to interact with their children during the testing session. Infants were randomly assigned to the static or dynamic condition. Half of the infants in each condition 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 presented were determined randomly. Infants were habituated following an infant control procedure (Bornstein, 1985; Cohen, 1973). In the static condition, all trials began with a minimum fixation of 0.25 s, and all trials terminated when the infants 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 videotape, 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. 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
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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 represented in their original order. Following habituation, infants were presented four test trials followed by one posttest trial. On the test trials, infants viewed a novel 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 test stimuli were determined randomly with the constraint that each display serve as a test stimulus at least once. The test series employed an ABBA design, and half of the infants viewed the novel familiar-category stimulus on the first and last test trials, while and the other half of the infants viewed the novelcategory stimulus on the first and last test trials. Following the test phase, infants viewed the “visual noise” display for one posttest trial. This trial began after the test phase, following the 5-s intertrial interval, and ended when the infants looked away for 2 continuous s or had looked a total of 30 s. Infants in the dynamic condition were tested using the same procedure as described for the static condition except that the trial ended when infants looked away for 1 continuous s rather than 2 s. Pilot-testing revealed that 3-month-olds tended to remain engaged with the point-light displays. These infants viewed an average of 17.3 30-s trials before becoming fussy. RESULTS Preliminary analyses revealed no effects for gender, so the data reported are collapsed across girls and boys. All probability levels reflect two-tailed tests. Habituation Analyses Infants’ mean looking on the baseline, criterion habituation, and test trials for each condition is shown in Table 1. Infants habituated to varying exemplars in both categories, with means of 9.20 (SD 6.34) and 9.65 (SD 4.06) trials in the static and dynamic conditions, respectively. The number of trials taken to TABLE 1 Mean Looking (s) in Habituation and Test and the Proportion of Looking to the Novel Stimulus during the Test Phase by 3-Month-Old Infants Habituation
Condition Static Dynamic Combined
M SD M SD M SD
Test
Baseline
Criterion
Familiar category
Novel category
Novelty preference
20.06 7.53 20.85 6.77 20.45 7.08
7.11 3.36 6.57 3.19 6.84 3.25
12.31 7.61 9.24 6.77 10.78 7.28
18.01 7.70 14.34 9.65 16.17 8.15
.61 .18 .60 .17 .61 .17
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reach the habituation criterion did not differ as a function of condition or category, all Fs(1, 36) .54, ns. Infants’ looking during the habituation phase was analyzed in a 2 2 2 analysis of variance (ANOVA) with trial (baseline or criterion) as a within-subjects factor and condition (static or dynamic) and habituation category (animal or vehicle) as between-subject factors. This analysis revealed a main effect for trial, F(1, 36) 236.31, p .001. No other main effects or interactions were found. The main effect for trial was due to significantly more looking on the habituation baseline trials than on the habituation criterion trials. The lack of a main effect or interaction with condition suggests that adjustment of the time for ending a trial from 2 s to 1 s in the dynamic condition equated infants’ processing time of the dynamic displays to that of the static displays. To rule out factors of fatigue accounting for the decline in attention during the habituation phase, infants’ looking to the posttest stimulus was compared to the criterion habituation trials. Three-month-olds showed a significant increase in looking to the posttest displays in both the static and dynamic conditions, ts(19) 3.91, p .001. Together, these findings indicate that infants showed a decline in looking to the multiple exemplars across the habituation phase, and they did so in the same number of trials regardless of whether they viewed animals or vehicles in static images or dynamic point-light displays. Test Trial Analyses To assess whether infants generalized habituation to the novel familiar-category exemplar and dishabituated to the novel-category exemplar, a 3 2 ANOVA was conducted with trial (criterion, familiar, or novel) as a within-subjects factor and condition (static or dynamic) as a between-subjects factor. The analyses revealed a main effect for trial, F(2, 76) 27.29, p .001. No other main effects or interactions were found. Tukey post hoc tests revealed the main effect for trial to be due to a significant increase in looking to the novel novel-category exemplar compared to looking on the criterion trials, t(19) 7.35, p .001, suggesting that infants discriminated the exemplar from the novel category from the exemplars presented during the habituation phase. In contrast, infants did not look significantly longer to the novel familiar-category exemplar in either condition, t(19) 3.10, ns, suggesting that they treated this new exemplar as familiar and as a member of the habituation category. To address whether infants showed a preference for the novel-category exemplar over the novel familiar-category exemplar, infants’ looking at the novel familiar-category and novel-category stimuli was summed across the two test presentations of each in each condition, 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. The proportion of looking to the novel-category stimulus (novelty preference) in each condition is shown in Table 1. The novelty preferences in each condition exceeded chance performance (.50), ts(19) 2.71, p .05. A 2 2 ANOVA with condition (static or dynamic) and habituation category (animal or vehicle) as between-subjects factors revealed no
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main effects or interactions, all Fs(1, 36) 1.65, ns, indicating that novelty preference did not vary as a function of whether infants viewed static or dynamic displays or were habituated to animals or vehicles. DISCUSSION The current experiment investigated 3-month-old infants’ use of static and dynamic attributes in categorizing exemplars in two domains: animals and vehicles. When infants were presented with static color images and dynamic pointlight displays, they habituated to animal and vehicle exemplars, generalized habituation to the novel familiar-category exemplar, dishabituated to the novelcategory exemplar, and showed a significant preference for the novel-category exemplar over the novel familiar-category exemplar. Together, these findings suggest that infants as young as 3 months of age 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. These findings replicate and extend those of previous work indicating categorization in young babies. Bornstein (1979) found that 4-month-olds, habituated to a variety of hues, generalized habituation to a novel hue in the same category but differentiated a hue from a novel category, Oakes et al. (1991) showed that infants as young as 6 months of age categorized animals versus trucks, and Behl-Chadha (1996) found that 3- to 4-month-olds formed representations of the category of mammals that excluded nonmammals (e.g., birds, fish) and furniture. The static stimuli presented infants with animal and vehicle exemplars depicted in a naturalistic background. Typically, infants have been presented with displays of line drawings, color picture cutouts, or three-dimensional replicas of objects (e.g., Behl-Chadha, 1996; Oakes et al., 1991; Quinn, 1994). Evidence of categorization in the static condition suggests that, even when presented with animals and vehicles depicted in naturalistic settings, 3-month-olds recognize similarities among same-category exemplars. In some ways, the displays used in this study present a more difficult task than the line drawings or color picture cutouts used in other studies because infants must separate figure from ground in the current study; nonetheless, infants categorized, suggesting that the presence of a naturalistic background did not interfere with categorization. In fact, it is possible that divorcing objects from their natural contexts (as was common in previous studies) renders the categorization task more difficult; as a result, findings from studies that do so may underestimate categorization abilities in infants (Bronfenbrenner, 1978). We are currently investigating this possibility. The findings of the dynamic condition are the first we know of to document perception and use of inanimate motion specified by dynamic point-light displays in infants. To date, all of the studies on the perception of point-light displays have used biological motion specifically derived from humans (e.g., Bertenthal, 1993; Bertenthal & Pinto, 1993; Fox & McDaniel, 1982). We extended this work by showing that young infants respond to biological motion that is not human (i.e.,
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animals) and that infants also respond to coherent motion patterns of artifacts (i.e., vehicles). It is possible that infants categorized the dynamic patterns of animals and vehicles without necessarily identifying their respective group membership because exemplars from a given category (e.g., animals, vehicles) showed similar types of motion (e.g., pendular, rotary). From the findings of the dynamic condition, we can conservatively conclude that infants treated the pendular motions as similar even though there was variation among the displays of pendular motion; they did likewise with displays of rotary motion. Thus, even though infants may not have recognized the pendular motion displays as “animals” or the rotary motion displays as “vehicles,” they responded to similarities in these respective motion patterns, which in itself led to correct categorization of animals and vehicles. A remaining question concerns on what basis infants categorized the animals and vehicles. Animals and vehicles differ in a variety of ways including substance and surface texture (i.e., animals are often soft and furry, whereas vehicles are hard and shiny) and the fact that animals have faces and vehicles do not. Threemonth-olds may respond to these differences, at least the presence or absence of faces. Quinn and Eimas (1996) found that 3-month-olds categorized different types of animals based on internal facial features and external head contour. However, if infants relied exclusively on these differences for categorization, then they would not have succeeded in the dynamic condition because only motion information was present. One possible basis for categorization is infants’ knowledge of the differences between animal and vehicle motions or essentially their understanding of animacy. We previously noted that infants may have categorized pendular and rotary motions into two different groups without necessarily recognizing the displays qua animals and vehicles. In addition, it is possible that when infants viewed static images of animals and vehicles, they also grouped them based on the type of motion these objects make, a claim akin to Mandler’s (1992, 2000) claim for older children. To categorize objects based on the type of motion they make, infants need to attend to and recognize key parts, namely those parts that display motion when the objects are moving. The categorization of animals and vehicles by 14- and 18month-old infants appears to be based on just these characteristics: legs and wheels (Rakison & Butterworth, 1998). Even when Rakison and Butterworth (1998) modified three-dimensional replicas of animals and vehicles such that animal legs were fitted to vehicle bodies, wheels were fitted to animal bodies, and infants were presented with a mix of modified and unmodified animals and vehicles, 14- and 18-month-olds grouped all things with legs together and all things with wheels together, a finding that underscores the importance of these key moving parts in young children’s categorization. Three-month-olds in the current study also may have relied on these key parts or similar features. In the static displays, all of the animal exemplars (with the exception of the bird) had visible legs, and all of the vehicle exemplars had visible wheels. In the dynamic displays, legs
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and wheels were specified by the motions of the dots (again with the exception of the bird, whose wings were specified). Also, the leg and wheel dots produced the most motion in the displays, most likely drawing the infants’ attention to these key motion information-specifying regions. The similar categorization performances by 3-month-old infants in the static and dynamic conditions is intriguing. In some domains, such as depth and object perception, infants appear to accomplish perceptual tasks more easily or to a more advanced degree on the basis of dynamic, as opposed to static, information (for reviews, see Bornstein & Arterberry, 1999; Kellman & Arterberry, 1998). For example, 4-month-olds perceive a partially occluded rod as complete behind a block when the visible ends of the rod moved (providing the dynamic cue of common fate) but not when the rod was stationary (providing the static cues of good continuation and similarity) (Kellman & Spelke, 1983). Similarly, 5-month-olds perceive depth separations specified by the dynamic cue of accretion and deletion of texture (Granrud et al., 1984) but not when the separation is specified by static texture gradients (Arterberry, Yonas, & Bensen, 1989; Yonas, Granrud, Arterberry, & Hanson, 1986). Gibson (1979) argued that there are fewer ambiguities in perception under conditions of motion, and young infants’ preparedness to use this type of information is now well documented (e.g., Arterberry & Yonas, 2000; Nanez, 1988). The nature of the task—either categorization in general or categorization of living versus nonliving things specifically—may account for why we did not find an advantage for dynamic information over static information at 3 months of age. Categorization processes may depend less on specific stimulus parameters than other processes, such as depth perception. Alternatively, it is possible that categorization of living versus nonliving things is such an important distinction that infants are prepared to make the categorization distinction using any one of a variety of sources of information. It remains to be seen whether categorization processes are more robust earlier in life or dynamic information again assumes a privileged status. REFERENCES Arterberry, M. E., & Yonas, A. (2000). Perception of structure from motion by 8-week-old infants. Perception and Psychophysics, 62, 550–556. Arterberry, M. E., Yonas, A., & Bensen, A. (1989). Self-produced locomotion and the development of responsiveness to linear perspective and texture gradients. Developmental Psychology, 25, 976–982. 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.
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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. (1979). Effects of habituation experience on posthabituation behavior in young infants: Discrimination and generalization among colors. Developmental Psychology, 15, 348–349. 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., & Arterberry, M. E. (1999). Perceptual development. In M. H. Bornstein & M. E. Lamb (Eds.), Developmental psychology: An advanced textbook (4th ed., pp. 231–274). Mahwah, NJ: Erlbaum. Bornstein, M. H., Kessen, W., & Weiskopf, S. (1976). Color vision and hue categorization in young human infants. Journal of Experimental Psychology: Human Perception & Performance, 2(1), 115–129. Braunstein, M. (1976). Depth perception through motion. New York: Academic Press. Bronfenbrenner, U. (1978). The ecology of human development. Cambridge, MA: Harvard Univ. Press. 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. Cutting, J. E. (1978). A program to generate synthetic walkers as dynamic point-light displays. Behavior Research Methods and Instrumentation, 10, 91–94. Cutting, J. E., & Kozlowski, L. T. (1977). Recognizing friends by their walk: Gait perception without familiarity cues. Bulletin of the Psychonomic Society, 9, 353–356. Eimas, P. D., Siqueland, E. R., Jusczyk, P., & Vigorito, J. (1971). Speech perception in infants. Science, 171, 303–306. Fox, R., & McDaniel, C. (1982). The perception of biological motion by human infants. Science, 218, 486–487. Gibson, E. J., & Walker, A. S. (1984). Development of knowledge of visual–tactual affordances of substance. Child Development, 55, 453–460. Gibson, J. J. (1979). The ecological approach to visual perception. Boston: Houghton Mifflin. Granrud, C. E., Yonas, A., & Opland, E. A. (1985). Infants’ sensitivity to the depth cue of shading. Perception and Psychophysics, 37, 415–419. Granrud, C. E., Yonas, A., Smith, I. M., Arterberry, M. E., Glicksman, M. L., & Sorknes, A. C. (1984). Infants’ sensitivity to accretion and deletion of texture as information for depth at an edge. Child Development, 55, 1630–1636. Johansson, G. (1973). Visual perception of biological motion and a model for its analysis. Perception and Psychophysics, 14, 201–211. Johansson, G. (1975). Visual motion perception. Scientific American, 232, 76–88. Johansson, G. (1977). Studies on visual perception of locomotion. Perception, 6, 365–376. Kavsek, M. J. (1999). Infants’ responsiveness to line junctions in curved objects. Journal of Experimental Child Psychology, 72, 177–192. Kellman, P. J. (1984). Perception of three-dimensional form by human infants. Perception & Psychophysics, 36(4), 353–358.
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Kellman, P. J., & Arterberry, M. E. (1998). The cradle of knowledge: The development of perception in infancy. Cambridge, MA: MIT Press. Kellman, P. J., & Spelke, E. S. (1983). Perception of partly occluded objects in infancy. Cognitive Psychology, 15, 483–524. Kozlowski, L. T., & Cutting, J. E. (1977). Recognizing the sex of a walker from a dynamic point-light display. Perception and Psychophysics, 21, 575–580. Mandler, J. M. (1992). How to build a baby: II. Conceptual primitives. Psychological Review, 99(4), 587–604. Mandler, J. M. (2000). Perceptual and conceptual processes in infancy. Journal of Cognition and Development, 1, 3–36. Mandler, J. M., & McDonough, L. (1993). Concept formation in infancy. Cognitive Development, 8, 291–318. Mandler, J. M., & McDonough, L. (1998). On developing a knowledge base in infancy. Developmental Psychology, 34, 1274–1288. Mather, G., & West, S. (1993). Recognition of animal locomotion from dynamic point-light displays. Perception, 22, 759–766. Nanez, J., Sr. (1988). Perception of impending collision in 3- to 6-week-old infants. Infant Behavior and Development, 11, 447–463. Oakes, L. M., Madole, K. L., & Cohen, L. B. (1991). Infants’ object examining: Habituation and categorization. Cognitive Development, 6, 377–392. Quinn, P. C. (1994). The categorization of above and below spatial relations by young infants. Child Development, 65, 58–69. Quinn, P. C. (1999). Development of recognition and categorization of objects and their spatial relations in young infants. In C. Tamis-LaMonda & L. Balter (Eds.), Child psychology: A handbook of contemporary issues (pp. 85–115). Philadephia: Psychology Press (Taylor & Francis). Quinn, P. C., & Eimas, P. D. (1996). Perceptual cues that permit categorical differentiation of animal species by infants. Journal of Experimental Child Psychology, 63, 189–211. Quinn, P. C., Eimas, P. D., & Rosenkrantz, S. L. (1993). Evidence for representations of perceptually similar natural categories by 3- and 4-month-old infants. Perception, 22, 463–475. Quinn, P. C., & Johnson, M. H. (2000). Global-before-basic object categorization in connectionist networks and 2-month-old infants. Infancy, 1, 31–46. Quinn, P. C., Siqueland, E. R., & Bomba, P. C. (1985). Delayed recognition memory for orientation by human infants. Journal of Experimental Child Psychology, 40, 293–303. Rakison, D. H., & Butterworth, G. E. (1998). Infants’ use of object parts in early categorization. Developmental Psychology, 34, 49–62. Runeson, S., & Frykholm, G. (1981). Visual perception of lifted weight. Journal of Experimental Psychology: Human Perception and Performance, 7, 733–740. Runeson, S., & Frykholm, G. (1983). Kinematic specification of dynamics as an informational basis for person-and-action perception: Expectation, gender recognition, and deception intention. Journal of Experimental Psychology: General, 112, 585–615. Stucki, M., Kaufmann-Hayoz, R., & Kaufmann, F. (1987). Infants’ recognition of a face revealed through motion: Contribution of internal facial movement and head movement. Journal of Experimental Child Psychology, 44, 80–91. Todd, J. T. (1984). The perception of three-dimensional structure from rigid and nonrigid motion. Perception and Psychophysics, 36, 97–103. Todd, J. T., & Mingolla, E. (1983). Perception of surface curvature and direction of illumination from patterns of shading. Journal of Experimental Psychology: Human Perception and Performance, 9, 583–595. Wallach, H., & O’Connell, D. N. (1953). The kinetic depth effect. Journal of Experimental Psychology, 45, 205–217. Waltz, D. (1975). Understanding line drawings of scenes with shadows. In P. H. Winston (Ed.), The psychology of computer vision (pp. 19–91). New York: McGraw–Hill.
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Werker, J. F. (1995). Exploring developmental changes in cross-language speech perception. In D. N. Osherson, L. R. Gleitman, & M. Liberman (Eds.), An invitation to cognitive science: Language (2nd ed., Vol. 1, pp. 87–106). Cambridge, MA: MIT Press. Yonas, A., & Arterberry, M. E. (1994). Infants’ perceive spatial structure specified by line junctions. Perception, 23, 1427–1435. Yonas, A., Granrud, C. E., Arterberry, M. E., & Hanson, B. L. (1986). Distance perception from linear perspective and texture gradients. Infant Behavior and Development, 9, 247–256. Younger, B., & Gotlieb, S. (1988). Development of categorization skills: Changes in the nature or structure of infant form categories? Developmental Psychology, 24, 611–619. Received September 9, 2000; revised April 9, 2001; published online July 25, 2001
Three-Month-Old Infants’ Categorization of Animals and Vehicles Based on Static and Dynamic Attributes Martha E. Arterberry Gettysburg College and
Marc H. Bornstein National Institute of Child Health and Human Development, Bethesda, Maryland Three-month-old infants’ categorization of animals and vehicles based on static and dynamic attributes was investigated using a multiple-exemplar habituation–test paradigm. Half of the infants viewed static color images of animals and vehicles, and the other half viewed dynamic point-light displays of the same animals and vehicles. Following habituation, infants viewed a novel exemplar from the habituation category and an exemplar from a novel category. Regardless of whether they viewed static or dynamic displays, infants showed habituation to varying exemplars from the same category, generalized habituation to a novel exemplar from the habituation category, dishabituated to an exemplar from a novel category, and showed a significant novelty preference for a novel-category exemplar. These findings suggest that infants categorize animals and vehicles using either static or dynamic information. © 2001 Academic Press Key Words: categorization; infants; static information; dymanic or motion-carried information.
Infants possess remarkable categorization skills. In the first few months of life, infants are able to form categories around very basic perceptual dimensions such as hue and voice onset time (Bornstein, Kessen, & Weiskopf, 1976; Eimas, Siqueland, Jusczyk, & Vigorito, 1971; for a review, see Werker, 1995). In addition, infants 3 to 4 months of age categorize simple forms composed of dot patterns (Bomba & Siqueland, 1983; Younger & Gotlieb, 1988), orientation of lines (Bomba, 1984; Quinn, Siqueland, & Bomba, 1985), spatial relations between
We thank M. Ney, J. Pinto, and W. Wilson for help with stimuli construction and D. Kertes, S. Salkind, and B. Wright for assistance with data collection and manuscript preparation. The first author was a visiting scientist at the National Institute of Child Health and Human Development during the completion of this work. Address correspondence and reprint requests to either author at Child and Family Research, National Institute of Child Health and Human Development, Suite 8030, 6705 Rockledge Drive, Bethesda, MD 20892-7971. E-mail: [email protected] or [email protected]. 333 0022-0965/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.
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lines and elements (Cohen & Younger, 1984; Quinn, 1994), and different types of animals (Quinn & Eimas, 1996; Quinn, Eimas, & Rosenkrantz, 1993; for a review, see Quinn, 1999), and furniture (Behl-Chadha, 1996). However, the type of information infants are able to use in categorization is not well understood. In this study, we take a first step and report an experiment on young infants’ use of static and dynamic attributes of objects in categorization. When the observer and object are stationary, the form of an object can be perceived using static information. The edges and internal contours of objects, and the intersection of these edges and contours, provide information about the threedimensional structure of the objects (e.g., Kavsek, 1999; Waltz, 1975; Yonas & Arterberry, 1994), as do patterns of light and shade (e.g., Granrud, Yonas, & Opland, 1985; Todd & Mingolla, 1983). When an object moves or an observer moves around an object, dynamic or motion-carried information is available. The resulting retinal transformations from observer and object motion provide information about an object’s three-dimensional form (e.g., Braunstein, 1976; Kellman, 1984; Wallach & O’Connell, 1953) and other characteristics such as rigidity and elastisticity (Gibson & Walker, 1984; Todd, 1984). When categorizing, infants (and adults) may use some or all of available object attributes, but it is possible that some attributes may be more useful than others in categorization. The current experiment was designed to investigate and compare the role of static and dynamic attributes in infants’ object categorization. We studied categorization of exemplars in two common object domains: animals and vehicles. These domains were chosen for two reasons. First, whereas much of the work with 2- to 4-month-old infants has addressed categorization of animals such as cats versus dogs or animals versus furniture (e.g., Behl-Chadha, 1996; Quinn & Eimas, 1996; Quinn & Johnson, 2000), no studies have investigated young infants’ categorization of animals and vehicles. However, most of the work on older children’s categorization has specifically contrasted the animal and vehicle domains (e.g., Mandler & McDonough, 1993; Oakes, Madole, & Cohen, 1991; Rakison & Butterworth, 1998); thus, this study provides a downward extension to tell us how early this categorical distinction is made. Second, the animal and vehicle domains provide a compelling contrast for addressing infants’ categorization based on motion. Objects in both of these domains move, but in different ways; animals exhibit biological motion, which is characterized by a system of pendular patterns, whereas vehicles use rotary motion, and this difference in motion pattern may constitute one underlying basis for categorization of animals and vehicles. It has been suggested that older infants’ categorization of animals and vehicles is based on differences in the types of motion associated with each domain (Mandler, 1992, 2000). To assess infants’ categorization of animals and vehicles using information contained in static images, color photographs of the nine animals and nine vehicles were created (Fig. 1A). To assess infants’ categorization of the motion patterns of animals and vehicles, dynamic point-light displays were created. These displays depicted the motions of the nine animals and nine vehicles depicted in
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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 of the dog and the sports car.
the photographs (see Fig. 1B). We used point-light displays to convey motion for several reasons. First, they isolate motion information from other object features and provide rich information on their own. Objects have a number of attributes that provide information as to object identity, and in natural viewing contexts these attributes are available simultaneously. Dynamic point-light displays remove all attributes except motion, thus allowing a direct assessment of the role of dynamic attributes in object identification and categorization. Research with adults, beginning with Johansson (1973, 1975, 1977), has suggested that patterns of motion contained in dynamic point-light displays provide rich information for both object identity (e.g., a person) and action (e.g., walking). For example, responding to only 10 to 12 lights placed on the joints of a human model, people readily identify the moving displays as humans engaged in various activities, such as walking, dancing, and doing push-ups. Further work has shown that adults recognize themselves and their friends from gait patterns (Cutting & Kozlowski, 1977), categorize the sex of models presented in dynamic point-light displays (Barclay, Cutting, & Kozlowski, 1978; Kozlowski & Cutting, 1977), and are able to estimate the weight of an object a point-light actor is lifting and even tell whether an actor is pretending to lift or really lifting a weighted object (Runeson & Frykholm, 1981, 1983). Also, perception of biomechanical motion by adults is not specific to human motion; Mather and West
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(1993) showed that adults readily identify animal point-light displays as animals when they are in motion but not when static images are presented. A second advantage of point-light displays is that infants have been shown to be sensitive to the information contained in dynamic point-light displays. That is, 4-, 5-, and 6-month-olds differentiate upright from inverted dynamic point-light displays depicting a walking human, and 5-month-olds are sensitive to the phase relations among points (Bertenthal, Proffitt, & Kramer, 1987; Fox & McDaniel, 1982; for reviews, see Bertenthal, 1993; Bertenthal & Pinto, 1993). Moreover, 3month-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 infant sensitivity to biomechanical motion, at least in displays depicting humans. Three-month-old infants were tested for categorization of animals and vehicles using a multiple-exemplar habituation-test design. Half of the infants viewed static color images of animals and vehicles, and the other half viewed dynamic point-light displays of the same animals and vehicles. 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 viewed a novel-category exemplar. If infants categorize images of animals and vehicles based on static or dynamic cues, then they should show habituation to the multiple exemplars, generalize habituation to a novel exemplar of the familiar category and discriminate a novel exemplar from the novel category, and show a preference for the novel novel-category exemplar over the novel familiar-category exemplar. METHOD Participants A total of 40 3-month-olds (age M 96.7 days, SD 8.4, 18 females) took part in the study. Infants were term and healthy at birth and at the time of testing (birth weight M 3.6 kg, SD 0.5; birth length M 52.0 cm, SD 6.3). Infants 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 10% of the sample comprising infants of African, Asian, or Latino heritage. An additional 16 infants began the procedure, but their data were not included due to fussiness or falling asleep (15) or parental interference (1). Materials and Apparatus The static stimuli consisted of 18 full-color images of 9 animals and 9 vehicles. Figure 1A shows an 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 sheep; the vehicles were a delivery truck, hatchback, minivan, motorcycle, pickup truck, sedan, sports car, tractor, and utility golf cart. The images, figures and backgrounds, subtended approximately 20.4° high and
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28.1° wide (figures were 17.1° high and 25.9° wide). Each image was presented on a video monitor via videotape (although the images were presented via videotape, they were static). An unrelated posttest stimulus was created for the static condition by playing the end of a tape without any recording; this “visual noise” was equivalent to the fuzz seen on a television screen when the signal is lost. The point-light stimuli consisted of 18 computer-generated dynamic point-light displays of the same animals and vehicles used for the static stimuli. Figure 1B displays one frame of the dog and sports car. The stimuli consisted of 0.5-cm black dots (visual angle 0.6°) on a white background that, when in motion, looked like an animal walking in place or a vehicle in motion in a plane perpendicular to the line of sight. All of the displays faced toward the left. This side view afforded viewing four legs for each animal (with the exception of the bird) and two wheels for each vehicle. The images were scaled so that they all were 15 cm wide. The time to complete one gait cycle for the animals and one revolution of wheels for the vehicles was approximately 1.5 s. There were no significant differences between the animal and vehicle pointlight displays in terms of display height, number of points, number of frames, or frames per second, all ts (16) 1.81, ns. 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 and 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 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 infants, 6 adults were asked to judge whether each stimulus was an animal or a vehicle; they did so with 100% accuracy. A second group of 10 adults was shown the displays and asked an open-ended question regarding the identity of the object depicted (“What is this?”). The animals and vehicles were identified accurately as members from their respective categories (percentage correct for animals M 96.7, SD 5.3; percentage correct for vehicles M 92.3, SD 7.4). Together, these results suggest that the displays were perceived by adults as animals and vehicles. Eighteen additional 3-month-olds were tested for within-category discrimination of the animal and vehicle point-light displays. Each infant was habituated to one exemplar. Following habituation, infants viewed the same exemplar and a new exemplar from the same category. Infants showed a significant novelty preference (animal M 0.61, SD 0.12; t(8) 2.69, p .05; vehicle M 0.58, SD 0.10, t(8) 2.57, p .05; difference between novelty preferences not significant, t(16) 0.48, ns), providing evidence that the displays within each category were discriminable from each other by 3-month-old infants. An additional videotape was created to serve as an unrelated posttest stimulus for the dynamic condition. This stimulus was a static full-color image of the smil-
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ing faces of two young girls. Even though this image was presented on videotape, no motion was present. All infants were tested in a 1.5- by 2.1-m dimly lit room. They sat approximately 50 cm from the video monitor in reclining infant seats, and their parents sat in nearby chairs. The tapes were presented on a 21- by 29-cm video monitor screen that 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’ faces 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 videocassette recorder (VCR) for presenting the images, a VCR and monitor for recording the infants’ looking, and a laboratory microcomputer for collecting infants’ fixation duration on each trial. Two experimenters conducted the study. One operated the VCR to present the stimuli, while the other recorded the infants’ fixations by depressing a switch attached to the computer. Infants were judged to look at the stimulus when a corneal reflection of the light from the display was in the center of the pupil. The experimenter making the fixation judgments did not know which stimulus was being presented in each trial. The computer calculated the baseline, determined when the infants had met the habituation criterion, and signaled the end of each trial by illuminating an LED. To obtain a measure of reliability, the second experimenter also scored infants’ fixations, either during the testing session or at a later time from the video record. Interjudge agreement (Cohen, 1968) was obtained for 72% of the sample (k .91); kappa was based on whether the coders agreed on whether the infant was looking or not for each second. Procedure Infants were seated in chairs, and parents were instructed not to interact with their children during the testing session. Infants were randomly assigned to the static or dynamic condition. Half of the infants in each condition 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 presented were determined randomly. Infants were habituated following an infant control procedure (Bornstein, 1985; Cohen, 1973). In the static condition, all trials began with a minimum fixation of 0.25 s, and all trials terminated when the infants 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 videotape, 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. 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
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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 represented in their original order. Following habituation, infants were presented four test trials followed by one posttest trial. On the test trials, infants viewed a novel 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 test stimuli were determined randomly with the constraint that each display serve as a test stimulus at least once. The test series employed an ABBA design, and half of the infants viewed the novel familiar-category stimulus on the first and last test trials, while and the other half of the infants viewed the novelcategory stimulus on the first and last test trials. Following the test phase, infants viewed the “visual noise” display for one posttest trial. This trial began after the test phase, following the 5-s intertrial interval, and ended when the infants looked away for 2 continuous s or had looked a total of 30 s. Infants in the dynamic condition were tested using the same procedure as described for the static condition except that the trial ended when infants looked away for 1 continuous s rather than 2 s. Pilot-testing revealed that 3-month-olds tended to remain engaged with the point-light displays. These infants viewed an average of 17.3 30-s trials before becoming fussy. RESULTS Preliminary analyses revealed no effects for gender, so the data reported are collapsed across girls and boys. All probability levels reflect two-tailed tests. Habituation Analyses Infants’ mean looking on the baseline, criterion habituation, and test trials for each condition is shown in Table 1. Infants habituated to varying exemplars in both categories, with means of 9.20 (SD 6.34) and 9.65 (SD 4.06) trials in the static and dynamic conditions, respectively. The number of trials taken to TABLE 1 Mean Looking (s) in Habituation and Test and the Proportion of Looking to the Novel Stimulus during the Test Phase by 3-Month-Old Infants Habituation
Condition Static Dynamic Combined
M SD M SD M SD
Test
Baseline
Criterion
Familiar category
Novel category
Novelty preference
20.06 7.53 20.85 6.77 20.45 7.08
7.11 3.36 6.57 3.19 6.84 3.25
12.31 7.61 9.24 6.77 10.78 7.28
18.01 7.70 14.34 9.65 16.17 8.15
.61 .18 .60 .17 .61 .17
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reach the habituation criterion did not differ as a function of condition or category, all Fs(1, 36) .54, ns. Infants’ looking during the habituation phase was analyzed in a 2 2 2 analysis of variance (ANOVA) with trial (baseline or criterion) as a within-subjects factor and condition (static or dynamic) and habituation category (animal or vehicle) as between-subject factors. This analysis revealed a main effect for trial, F(1, 36) 236.31, p .001. No other main effects or interactions were found. The main effect for trial was due to significantly more looking on the habituation baseline trials than on the habituation criterion trials. The lack of a main effect or interaction with condition suggests that adjustment of the time for ending a trial from 2 s to 1 s in the dynamic condition equated infants’ processing time of the dynamic displays to that of the static displays. To rule out factors of fatigue accounting for the decline in attention during the habituation phase, infants’ looking to the posttest stimulus was compared to the criterion habituation trials. Three-month-olds showed a significant increase in looking to the posttest displays in both the static and dynamic conditions, ts(19) 3.91, p .001. Together, these findings indicate that infants showed a decline in looking to the multiple exemplars across the habituation phase, and they did so in the same number of trials regardless of whether they viewed animals or vehicles in static images or dynamic point-light displays. Test Trial Analyses To assess whether infants generalized habituation to the novel familiar-category exemplar and dishabituated to the novel-category exemplar, a 3 2 ANOVA was conducted with trial (criterion, familiar, or novel) as a within-subjects factor and condition (static or dynamic) as a between-subjects factor. The analyses revealed a main effect for trial, F(2, 76) 27.29, p .001. No other main effects or interactions were found. Tukey post hoc tests revealed the main effect for trial to be due to a significant increase in looking to the novel novel-category exemplar compared to looking on the criterion trials, t(19) 7.35, p .001, suggesting that infants discriminated the exemplar from the novel category from the exemplars presented during the habituation phase. In contrast, infants did not look significantly longer to the novel familiar-category exemplar in either condition, t(19) 3.10, ns, suggesting that they treated this new exemplar as familiar and as a member of the habituation category. To address whether infants showed a preference for the novel-category exemplar over the novel familiar-category exemplar, infants’ looking at the novel familiar-category and novel-category stimuli was summed across the two test presentations of each in each condition, 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. The proportion of looking to the novel-category stimulus (novelty preference) in each condition is shown in Table 1. The novelty preferences in each condition exceeded chance performance (.50), ts(19) 2.71, p .05. A 2 2 ANOVA with condition (static or dynamic) and habituation category (animal or vehicle) as between-subjects factors revealed no
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main effects or interactions, all Fs(1, 36) 1.65, ns, indicating that novelty preference did not vary as a function of whether infants viewed static or dynamic displays or were habituated to animals or vehicles. DISCUSSION The current experiment investigated 3-month-old infants’ use of static and dynamic attributes in categorizing exemplars in two domains: animals and vehicles. When infants were presented with static color images and dynamic pointlight displays, they habituated to animal and vehicle exemplars, generalized habituation to the novel familiar-category exemplar, dishabituated to the novelcategory exemplar, and showed a significant preference for the novel-category exemplar over the novel familiar-category exemplar. Together, these findings suggest that infants as young as 3 months of age 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. These findings replicate and extend those of previous work indicating categorization in young babies. Bornstein (1979) found that 4-month-olds, habituated to a variety of hues, generalized habituation to a novel hue in the same category but differentiated a hue from a novel category, Oakes et al. (1991) showed that infants as young as 6 months of age categorized animals versus trucks, and Behl-Chadha (1996) found that 3- to 4-month-olds formed representations of the category of mammals that excluded nonmammals (e.g., birds, fish) and furniture. The static stimuli presented infants with animal and vehicle exemplars depicted in a naturalistic background. Typically, infants have been presented with displays of line drawings, color picture cutouts, or three-dimensional replicas of objects (e.g., Behl-Chadha, 1996; Oakes et al., 1991; Quinn, 1994). Evidence of categorization in the static condition suggests that, even when presented with animals and vehicles depicted in naturalistic settings, 3-month-olds recognize similarities among same-category exemplars. In some ways, the displays used in this study present a more difficult task than the line drawings or color picture cutouts used in other studies because infants must separate figure from ground in the current study; nonetheless, infants categorized, suggesting that the presence of a naturalistic background did not interfere with categorization. In fact, it is possible that divorcing objects from their natural contexts (as was common in previous studies) renders the categorization task more difficult; as a result, findings from studies that do so may underestimate categorization abilities in infants (Bronfenbrenner, 1978). We are currently investigating this possibility. The findings of the dynamic condition are the first we know of to document perception and use of inanimate motion specified by dynamic point-light displays in infants. To date, all of the studies on the perception of point-light displays have used biological motion specifically derived from humans (e.g., Bertenthal, 1993; Bertenthal & Pinto, 1993; Fox & McDaniel, 1982). We extended this work by showing that young infants respond to biological motion that is not human (i.e.,
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animals) and that infants also respond to coherent motion patterns of artifacts (i.e., vehicles). It is possible that infants categorized the dynamic patterns of animals and vehicles without necessarily identifying their respective group membership because exemplars from a given category (e.g., animals, vehicles) showed similar types of motion (e.g., pendular, rotary). From the findings of the dynamic condition, we can conservatively conclude that infants treated the pendular motions as similar even though there was variation among the displays of pendular motion; they did likewise with displays of rotary motion. Thus, even though infants may not have recognized the pendular motion displays as “animals” or the rotary motion displays as “vehicles,” they responded to similarities in these respective motion patterns, which in itself led to correct categorization of animals and vehicles. A remaining question concerns on what basis infants categorized the animals and vehicles. Animals and vehicles differ in a variety of ways including substance and surface texture (i.e., animals are often soft and furry, whereas vehicles are hard and shiny) and the fact that animals have faces and vehicles do not. Threemonth-olds may respond to these differences, at least the presence or absence of faces. Quinn and Eimas (1996) found that 3-month-olds categorized different types of animals based on internal facial features and external head contour. However, if infants relied exclusively on these differences for categorization, then they would not have succeeded in the dynamic condition because only motion information was present. One possible basis for categorization is infants’ knowledge of the differences between animal and vehicle motions or essentially their understanding of animacy. We previously noted that infants may have categorized pendular and rotary motions into two different groups without necessarily recognizing the displays qua animals and vehicles. In addition, it is possible that when infants viewed static images of animals and vehicles, they also grouped them based on the type of motion these objects make, a claim akin to Mandler’s (1992, 2000) claim for older children. To categorize objects based on the type of motion they make, infants need to attend to and recognize key parts, namely those parts that display motion when the objects are moving. The categorization of animals and vehicles by 14- and 18month-old infants appears to be based on just these characteristics: legs and wheels (Rakison & Butterworth, 1998). Even when Rakison and Butterworth (1998) modified three-dimensional replicas of animals and vehicles such that animal legs were fitted to vehicle bodies, wheels were fitted to animal bodies, and infants were presented with a mix of modified and unmodified animals and vehicles, 14- and 18-month-olds grouped all things with legs together and all things with wheels together, a finding that underscores the importance of these key moving parts in young children’s categorization. Three-month-olds in the current study also may have relied on these key parts or similar features. In the static displays, all of the animal exemplars (with the exception of the bird) had visible legs, and all of the vehicle exemplars had visible wheels. In the dynamic displays, legs
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and wheels were specified by the motions of the dots (again with the exception of the bird, whose wings were specified). Also, the leg and wheel dots produced the most motion in the displays, most likely drawing the infants’ attention to these key motion information-specifying regions. The similar categorization performances by 3-month-old infants in the static and dynamic conditions is intriguing. In some domains, such as depth and object perception, infants appear to accomplish perceptual tasks more easily or to a more advanced degree on the basis of dynamic, as opposed to static, information (for reviews, see Bornstein & Arterberry, 1999; Kellman & Arterberry, 1998). For example, 4-month-olds perceive a partially occluded rod as complete behind a block when the visible ends of the rod moved (providing the dynamic cue of common fate) but not when the rod was stationary (providing the static cues of good continuation and similarity) (Kellman & Spelke, 1983). Similarly, 5-month-olds perceive depth separations specified by the dynamic cue of accretion and deletion of texture (Granrud et al., 1984) but not when the separation is specified by static texture gradients (Arterberry, Yonas, & Bensen, 1989; Yonas, Granrud, Arterberry, & Hanson, 1986). Gibson (1979) argued that there are fewer ambiguities in perception under conditions of motion, and young infants’ preparedness to use this type of information is now well documented (e.g., Arterberry & Yonas, 2000; Nanez, 1988). The nature of the task—either categorization in general or categorization of living versus nonliving things specifically—may account for why we did not find an advantage for dynamic information over static information at 3 months of age. Categorization processes may depend less on specific stimulus parameters than other processes, such as depth perception. Alternatively, it is possible that categorization of living versus nonliving things is such an important distinction that infants are prepared to make the categorization distinction using any one of a variety of sources of information. It remains to be seen whether categorization processes are more robust earlier in life or dynamic information again assumes a privileged status. REFERENCES Arterberry, M. E., & Yonas, A. (2000). Perception of structure from motion by 8-week-old infants. Perception and Psychophysics, 62, 550–556. Arterberry, M. E., Yonas, A., & Bensen, A. (1989). Self-produced locomotion and the development of responsiveness to linear perspective and texture gradients. Developmental Psychology, 25, 976–982. 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.
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