the role of shape in 4-month-old infants’ object … · this research examined four-month-old...

THE ROLE OF SHAPE IN 4-MONTH-OLDINFANTS’ OBJECT SEGREGATION

Amy NeedhamDuke University

This research examined four-month-old infants’ use of featural information (shape or color and pattern)to segregate a display into two adjacent pieces. The infants were shown displays consisting of twoobjects that were the same or different in shape and that were decorated with either similar or differentsurface markings such that featural information could suggest that the two portions were eitherconnected or not. Three displays were created that allowed the comparison of infants’ use of shapeinformation and color and pattern information. The results suggest that, at four months of age, infantsare more likely to use shape differences than color and pattern differences to find object boundaries.The results are discussed in the context of infants’ learning about the utility of different sources ofinformation for predicting object boundary locations.

object perception perceptual development cognitive development object features

When we look around us, we see many objectswith ambiguous boundaries due to other ob-jects that are directly adjacent to them or thatpartly occlude them. Adults’ ability to seg-ment a crowded scene into reasonable andrecognizable parts has been extensively stud-ied and has led to a number of interestinghypotheses for how the adult visual systemaccomplishes this task (Braunstein, Hoffman,& Saidpour, 1989; Burbeck & Pizer, 1995;Hoffman & Richards, 1984; Hoffman &

Singh, 1997; Koenderink & van Doorn, 1982;Vaina & Zlateva, 1990). Most of these ap-proaches have explored the possibility thatpart boundaries (which are sometimes objectboundaries, a term that will be used in thepresent paper to refer to places where twoobjects would separate if one were pulled),can be determined by a mathematical analysesof the shape of the object(s) in question.

Research on how these perceptual abilitiesdevelop has been much less extensive than the

● Amy Needham, Department of Psychology, Duke University, Durham, NC 27708-0086; e-mail: [email protected].

INFANT BEHAVIOR & DEVELOPMENT 22 (2), 1999, 161–178 ISSN 0163-6383Copyright © 1999 Elsevier Science Inc. All rights of reproduction in any form reserved.

aforementioned literature, and has focused oninfants’ ability to segment a crowded array,possibly because many of the important devel-opments in the visual system occur within thefirst year of life (Banks, 1983; Maurer & Mau-rer, 1988). The investigation of this issue hasgenerated considerable research over the past15 years, beginning with Kellman andSpelke’s (1983) classic study of 4-month-oldinfants’ perception of a partly occluded object(for reviews, see Needham, Baillargeon, &Kaufman, 1997; Spelke, 1990). It was in Kell-man and Spelke’s paper that a classification ofthe kinds of knowledge that may be useful aswe segregate objects was sketched out. Oneassumption underlying this classification isthat infants must possess knowledge of thefollowing kinds to make use of the relevantinformation that is present in the display. Thekinds of knowledge that have been investi-gated can be organized into three categories:(1) physical knowledge, or knowledge aboutthe physical laws governing the motions ofand interactions between objects; (2) specific-object knowledge, or knowledge based onprior exposures to specific objects or classes ofobjects; and (3) featural knowledge, or knowl-edge about how the shapes, colors and patternsof surfaces are typically organized so thatthese features can be used to determine objectboundary locations. Overall, the literature inthis area has supported the conclusion that,during the first year of life, infants can use allthree sources of knowledge to find objectboundaries.

Many studies have shown that infants canuse a number of different kinds ofphysicalknowledge to determine whether surfacesshould be grouped as part of the same unit. Forexample, infants 2 to 4 months of age perceivesurfaces that undergo common motion as partof the same unit and those that undergo rela-tive motion as belonging to separate units(Johnson, 1997; Johnson & Aslin, 1995; Kell-man & Spelke, 1983; Kellman, Spelke, &Short, 1986; Kellman, Gleitman, & Spelke,1987; Slater et al., 1990; Spelke, von Hofsten,& Kestenbaum, 1989).1 Around 3 months of

age (and possibly younger), infants interpretspatial separations between objects as indica-tors of object boundaries (Kestenbaum, Ter-mine, & Spelke, 1987; Spelke et al., 1989),and by 8 months of age, (and possiblyyounger) infants make use of the apparentsupport or solidity relations between the sur-faces to determine whether two surfaces arelikely to be connected (Craton, 1994; Need-ham & Baillargeon, 1997).

Infants’ use ofspecific-object knowledgetofind object boundaries has been reported byresearchers such as Needham and Baillargeon(1998) and Schwartz (1982). Infants’ use of asingle experience with a particular object wasstudied by Needham and Baillargeon (1998),who found that 4.5-month-old infants coulduse a single brief exposure to a novel object tosee that object as separate from another novelobject, both when the prior exposure cameimmediately prior to test and when it came 24hours prior to test. Use of knowledge of a classof objects, human faces, was studied bySchwartz (1982), who found that 5-month-oldinfants were more likely to see a partly oc-cluded face as a single unit than a partly oc-cluded checkerboard.

Evidence for infants’ use offeatural knowl-edgecomes from studies investigating infants’use of featural information (such as the shape,color and pattern of surfaces) as a signal forobject boundaries. In the first systematic in-vestigation of infants’ object perception, Kell-man and Spelke (1983) presented 4-month-oldinfants with a center-occluded rod or triangle,and assessed whether the infants perceived theobject as connected behind the occluder. Theirfindings suggested that the infants did notmake use of the featural information present inthe display to unify the visible portions of therod or triangle, but instead seemed to regardthese displays as ambiguous. The authors con-cluded that 4-month-old infants do not makeuse of the featural information present in dis-plays to determine object boundary locations.Further research supported the conclusion thatinfants did not use object features to determineobject boundary locations in displays consist-

162 INFANT BEHAVIOR & DEVELOPMENT Vol. 22, No. 2, 1999

ing of partly occluded and adjacent objects(e.g., Kestenbaum et al., 1987; Spelke, Brein-linger, Jacobson, & Phillips, 1993).

However, more recent research, usingslightly different methods and displays, hasproduced evidence that even young infants canmake use of featural information to find objectboundaries (Johnson, 1997; Johnson & Aslin,1996; Needham, 1998; Needham & Baillar-geon, 1997; see Needham, 1998 and Needhamet al., 1997 for possible explanations for thesedifferences). In one set of studies, Needham(1998) presented 4.5-month-old infants withtwo spatially contiguous objects whose fea-tures were quite different: a yellow cylinder onits side and a tall blue box. Infants were firstgiven a 10- to 30-second view of the stationarydisplay and were then shown one of two testevents in which a gloved hand took hold of thecylinder and moved it a short distance to theside. Half of the infants saw the cylinder andbox move together when the cylinder waspulled (move-together event), and half saw thecylinder move away from the box, which re-mained stationary throughout the event(move-apart event).

The rationale behind this design was that ifthe infants used the featural differences be-tween the cylinder and box to perceive thedisplay as consisting of two separate objects,the infants should expect the box to remainstationary when the cylinder was pulled. Be-cause infants tend to look longer at events thatviolate their expectations than those that donot (Bornstein, 1985; Needham et al., 1997;Spelke, 1985), it was expected that infantswho perceived the display as composed of twoseparate units would look reliably longer at themove-together than at the move-apart event.In contrast, if the infants were unable to usethe featural information to find a boundarybetween the objects, the infants should lookreliably longer at the move-apart than at themove-together test event.

The results showed that the infants lookedreliably longer at the move-together than at themove-apart event, indicating that the infantsperceived the cylinder and box to be separate

objects and expected that when one was pulledthe other would not move.

This interpretation was supported by theresults of a control experiment in which in-fants did not receive the trial at the beginningof the session featuring the stationary display.Because the logic of the experiment held thatinfants would look longer at one of the testevents if it depicted an outcome that wascounter to their interpretation of the display, itwas hypothesized that severely reducing thetime available to form an interpretation of thedisplay could prevent infants from forming aninterpretation of the display, and render thetwo test events equally interesting.2 The re-sults of this experiment showed that the in-fants looked about equally at the test eventswithout the prior familiarization trial, indicat-ing that the infants (a) needed some time toview the stationary display in order to form aninterpretation of it and (b) needed to have aninterpretation of the display to evaluate theoutcome shown in their test event. Further,these results support the interpretation offeredfor the results of the initial experiment: thatthe infants looked longer at the move-togetherthan at the move-apart event because they hadsegregated the cylinder and box into separateunits and expected the box to remain station-ary when the cylinder was pulled. This samepattern of withdrawal of the familiarizationtrial leading to a dramatic change in infants’responses to the test events was found for fullyvisible and partly occluded versions of thesame display in this research, lending furthersupport to the explanation offered for the con-sistent pattern of findings (Needham, 1998).

Because the display in the study just de-scribed contained many redundant cues allindicating that the display was composed oftwo separate pieces (including shape, color,pattern, and height), it remains unclear whichspecific cue or cues the infants used to deter-mine the composition of the display. It is pos-sible that at this young age, infants need acombination of cues from a variety of sourcesto determine the composition of a display, butit is also possible that they are using primarily

163Shape in Object Segregation

one of the many available sources of informa-tion to interpret the display. The present re-search was designed to address the question ofwhether 4-month-old infants could make useof a difference in shape alone or a differencein color and pattern alone to segregate a dis-play composed of two adjacent (i.e., spatiallycontiguous) objects. Four-month-old infantswere selected for use in this research becausethis is the youngest age at which infants’ useof features to find object boundaries has beendemonstrated, and the current work sought todetermine what cues are useful for infants asthey are just beginning to make use of objectfeatures for this purpose.

The Use of Individual Sources ofFeatural Information

Why is it important to know which sourcesof information young infants use to segregateobjects? First, if we were to find that infantsfocused on shape when perceiving objectboundaries, this would suggest an importantcontinuity in the way infants and adults ana-lyze the objects they see, as object shape hasbeen identified as a critically important featurefor adults’ perception of object boundaries(Hoffman & Richards, 1984). Secondly,knowing what kinds of information infants useto segregate displays of objects (and whetherthere are some kinds of information that areused developmentally prior to others) couldhelp reveal the ways infants learn about thedifferent facets of object features (and theirrelation to object boundary locations). For ex-ample, if we found evidence that infants useshape to segregate displays prior to the timethat they show evidence of using color andpattern, it could suggest infants begin to learnabout object shape through their oral interac-tions with objects very early in life; whileshape is available through visual or oral ex-ploration, color and (non-raised) pattern isavailable solely through visual exploration.

Alternately, if color and pattern were usedprior to shape, we would more likely evoke

explanations based on visual learning and fo-cus our efforts on trying to determine infants’impressions of the salience of various featuresof objects, and perhaps how infants analyzeand notice the differences between two differ-ent color/pattern combinations. This issue willbe taken up again in the General Discussion.

There are many reasons to suspect thatshape would be more likely to be used by4-month-old infants as information for objectboundaries than color and pattern. Simplybased on what we know about the 4-month-oldinfant’s visual acuity and color vision (Banks,1983), one would expect that (a) large shapescreated by objects’ external contours would bebetter perceived than smaller shapes specifiedby internal pattern elements, (b) some colorsmay still be perceived in a less-than-com-pletely accurate way, and therefore (c) infantswould have had more of a chance to learnmore about the co-occurrences of shape dif-ferences and object boundaries than the co-occurrences of color and pattern differencesand object boundaries. Furthermore, becauseshape is available visually, manually, andorally, while color and pattern are availablejust visually, it seems reasonable to expect thatinfants have more of a chance to notice andlearn about the connections betweenshapeand object boundaries than the connectionsbetweencolor and patternand object bound-aries.

The Present Research

There were three displays involved in thepresent research, all of which were composedof two segments of roughly the same size (seeFigures 1 and 2). The Similar display wascomposed of two segments that were the sameshape, the same color, and decorated with apattern that was symmetric about the boundarybetween them. The Dissimilar-Shape displaywas identical to the Similar display except thatthe right object was a rectangle rather than arounded rectangle. The Dissimilar-Color-and-Pattern display was identical to the Similar


display except that the right object was of adifferent color and pattern than the left object.

Similarities and differences in these twofeatural cues were chosen that would suggestone interpretation over another in a ratherglobal way. We expected that if the infantswere forming interpretations of the displaysprimarily on the basis of shape information,they would perceive both the Similar displayand the Dissimilar-Color-and-Pattern displayas consisting of single units, but would per-ceive the Dissimilar Shape display as com-posed of two separate units. If infants were notusing shape in a rather global way, it is notclear why the Similar display should seem anymore like a single unit than the Dissimilar-Shape display (because both displays wereobviously composed of two potentially sepa-rate parts). In contrast, if the infants wereforming interpretations of the displays primar-ily on the basis of color and pattern informa-

tion, they would perceive both the Similardisplay and the Dissimilar Shape display asconsisting of single units, but would perceivethe Dissimilar-Color-and-Pattern display ascomposed of two separate units. However itwas certainly true that other interpretations ofthese displays were possible. For example, theSimilar display could conceivably be per-ceived as composed of two separate piecesthat would move separately, as there was avisible boundary and an obvious potentialpoint of separation (e.g., minima rule; Hoff-man & Richards, 1984) between the two por-tions that made up the display. Also, if infantsthis age were totally incapable of using fea-tural information to determine object bound-ary locations, they should perceive each dis-play as composed of a single unit, becauseeach display is made up of two parts that arespatially contiguous.

Each infant was shown a test event in

FIGURE 1Schematic representation of the move-apart and the move-together test events featuring the Similardisplay.


which a gloved hand took hold of one segmentof the display and moved it a short distance tothe side. Half of the infants seeing each dis-play saw both segments move together whenone was pulled (move-together event), andhalf saw the pulled segment move away fromthe other segment, which remained stationarythroughout the event (move-apart event). Asin prior experiments using this methodology,the rationale behind this design rested on twoassumptions. First, it was assumed when theinfants saw the stationary display during thefamiliarization trial, they were able to form aninterpretation of the display as composed of asingle unit or of two separate units (given the

appearance of the displays, these were the twomost likely interpretations). Further, it wasassumed the infants would evaluate the likeli-hood of the test event relative to this interpre-tation, and look longer at the test event counterto their interpretation than at one in agreementwith their interpretation (Bornstein, 1985;Needham, 1997, 1998; Needham & Baillar-geon, 1997, 1998; Needham & Kaufman,1997; Needham et al., 1997; Spelke, 1985).Thus, by comparing infants’ looking times atthe move-apart and move-together events foreach display (each infant saw only one event,in a between-subjects design), we may be ableto determine whether the infants tended toperceive each display as consisting of a singleunit or of two separate units.

EXPERIMENT 1

Method

Participants

Participants were 48 healthy, full-term in-fants ranging in age from 3 months, 22 days to4 months, 13 days (M 5 4 months 2 days;SD5 6.1 days). Half of the infants were male,half were female. Each infant was assigned toone of the six experimental conditions formedby crossing the 3 display conditions (Similar,Dissimilar-Shape, and Dissimilar-Color-and-Pattern displays) with the two movement con-ditions (move-apart and move-together testevents) in a pseudo-random fashion, keepingage and sex balanced. Of the infants who sawthe Similar display, half saw the move-aparttest event (M 5 4 months 3 days;SD 5 6.1days) and half saw the move-together testevent (M 5 4 months 0 days;SD5 6.6 days).Of the infants who saw the Dissimilar-Shapedisplay, half saw the move-apart test event(M 5 4 months 1 day;SD5 8.4 days) and halfsaw the move-together test event (M 5 4months 2 days;SD5 7.3 days). Of the infantswho saw the Dissimilar-Color-and-Pattern dis-play, half saw the move-apart test event (M 5

FIGURE 2(a) The Dissimilar-Shape display, (b) The Dissim-ilar-Color-and-Pattern display.


4 months 2 days;SD5 3.5 days) and half sawthe move-together test event (M 5 4 months 2days;SD5 5.4 days). Seven additional infantswere tested and eliminated from the final sam-ple, 3 due to their failure to attend to theexperimental events, 2 due to procedural error,1 due to fussiness, and 1 due to parental in-terference during the testing session.

The infants’ names for this and the nextexperiment were obtained from the DurhamCounty vital records office. Parents were con-tacted via letter and follow-up phone calls.They were offered reimbursement for theirtravel expenses, but were not compensated fortheir participation.

Apparatus

The apparatus consisted of a wooden cubi-cle 201 cm tall, 107 cm wide, and 49.5 cmdeep. The infants faced an opening 56 cm talland 94 cm wide in the front wall of the appa-ratus. On the floor of the apparatus was a pieceof dark blue cardboard covered with a thinpiece of transparent Plexiglas, and the backwall was covered with green poster board.

Each display consisted of two boxes, eachof which was 22 cm tall, 29 cm wide, and 9.5cm deep. The boxes were made of Plexiglasand decorated with construction paper, contactpaper and plastic tape. The boxes in the Sim-ilar display were two curved-top boxes cov-ered with red construction paper, clear contactpaper and black plastic tape arranged in acurved pattern symmetric about the centerboundary. The boxes in the Dissimilar-Shapedisplay were identical to those in the Similardisplay except that the box on the right wasnot a curved box, it was rectangular. Theboxes in the Dissimilar-Color-and-Pattern dis-play were identical to those in the Similardisplay except that the right (curved-top) boxwas covered with light blue contact paper andsmall (about 1 cm) squares of white plastictape.

The bottom of each box was covered withfelt, so that when it was pulled, it slid

smoothly and silently across the Plexiglas onthe apparatus floor. In the move-apart testevent, a heavy weight was placed inside theleft object; in the move-together test event, alarge binder clip (not visible to the infants)was used to clip the objects together frombehind. In both test events, a thin piece ofPlexiglas lay behind the objects and served asa guide to ensure the straight movement of thebox(es).

At the start of the test event, the two boxesstood side by side on the floor of the appara-tus. The front right corner of the right box was34 cm from the right wall and 24 cm from thefront edge of the apparatus. Together, the twoboxes subtended about 52 degrees (horizontal)and 20 degrees (vertical) of visual angle fromthe infant’s viewpoint.

In each test event, the right box was pulledto the side by an experimenter’s left handwearing a 59 cm long lavender spandex glove.The hand entered the apparatus through anopening 55.5 cm high and 37.5 cm wide in theleft wall. This opening was partially hidden bya white muslin curtain; the curtain and theexperimenter were positioned in such a waythat the infant could not see the experimenter’sface through this opening.

The infants were tested in a brightly litroom. Four clip-on lights (each with a 40-Wlight bulb) were attached to the back and sidewalls of the apparatus to provide additionallight. Two wooden frames, each 200 cm highand 69 cm wide and covered with blue cloth,stood at an angle on either side of the appara-tus. These frames served to isolate the infantsfrom the experimental room. At the end ofeach trial, a curtain consisting of a white mus-lin-covered frame 57 cm high and 98 cm widewas lowered in front of the opening in thefront wall of the apparatus.

Events

Move-together event.At the start of eachtest trial, the experimenter’s left hand restedon the floor of the apparatus about half-way


between the edge of the box and the openingin the right wall. After a 1-s pause, the handgrasped the box (1 s) and pulled it 14 cm to theleft at the approximate rate of 7 cm/s (2 s). Thetwo boxes moved as a single, rigid unit withno slight movements of one object relative tothe other. The hand paused for 1 s and thenpushed the boxes back to their starting posi-tions (2 s). The hand then resumed its initialposition on the apparatus floor (1 s). Eachevent cycle thus lasted about 8 s. Cycles wererepeated without stop until the computer sig-naled that the trial had ended (see below).When this occurred, a second experimenterlowered the curtain in front of the apparatus.

Move-apart event.The move-apart eventwas identical to that just described except thatonly the right box moved: the left box re-mained stationary throughout the trial (seeFigure 1 for a depiction of these events).

Procedure

During the experiment, each infant sat onhis or her parent’s lap in front of the apparatus.The infant’s head was approximately 60 cmfrom the boxes.

The infant’s looking behavior was moni-tored by two observers who viewed the infantthrough peepholes in the cloth-covered frameson either side of the apparatus. The observerswere not told which display or event the infantsaw and could not determine which display theinfant saw.3 Each observer held a button boxconnected to a Gateway 2000 4DX2-66 com-puter (Gateway 2000 Inc.; North Sioux City,SD) and depressed the button when the infantattended to the events. Each trial was dividedinto 100-ms intervals, and the computer deter-mined in each interval whether the two ob-servers agreed on the direction of the infant’sgaze. Inter-observer agreement was calculatedfor each trial on the basis of the number ofintervals the computer registered agreement,out of the total number of intervals in the trial.Agreement in this experiment and in subse-quent experiments averaged 96% or more per

trial per infant. The input from the primary(more experienced) observer was used to de-termine the end of the trials. Each infant firstreceived a familiarization trial to acquaint himor her with the boxes in their starting positionsand to allow the infant to produce an interpre-tation of the display as composed of one ortwo units. The experimenter’s hand did notenter the apparatus during this trial, so as notto distract the infant. The trial ended when theinfant either (a) looked away from the boxesfor 2 consecutive seconds after having lookedat them for at least 10 cumulative seconds or(b) looked at the boxes for 30 cumulativeseconds without looking away for 2 consecu-tive seconds.

Following the familiarization trial, each in-fant saweither the move-apartor the move-together test event on three successive trials. Abetween subjects design was employed in thisand the next experiment reported in this paper.Each test trial ended when the infant (a)looked away from the event for 2 consecutiveseconds after having looked at it for at least 8cumulative seconds or (b) looked at the eventfor 60 cumulative seconds without lookingaway for 2 consecutive seconds. Each infant inthis experiment contributed a full set of 3 testtrials to the analyses.

Results

Preliminary Analyses

A preliminary analysis revealed no effectof Condition on the infants’ looking timesduring the familiarization trial (allF’s , 2.71,p . .05). A second analysis revealed no effectof Sex on the infants’ looking times at the twotest events (allF’s , 2.88,p . .05). A thirdanalysis revealed no effect of Trial on theinfants’ looking times at the two test events(all F’s , 1.67, p . .05). The data weretherefore collapsed over these variables forsubsequent analyses.


Main Analyses

The results of Experiment 1 are shown inFigure 3. Inspection of this graph reveals thatthe infants who saw the Similar display andthe Dissimilar-Color-and-Pattern displaylooked longer at the move-apart than at themove-together event, whereas the infants whosaw the Dissimilar-Shape display showed thereverse tendency.

The infants’ looking times were enteredinto a 3 x 2Analysis of Variance (ANOVA)with Display (Similar, Dissimilar-Shape, andDissimilar-Color-and-Pattern) and Condition(move-apart or move-together) as betweensubjects variables. This analysis yielded a sig-nificant Display x Condition interaction,F(2,42) 5 10.52,p , .0005. Planned comparisonsrevealed that the infants who saw the Similardisplay looked reliably longer at the move-apart (M 5 56.1,SD5 4.6) than at the move-together test event (M 5 40.0, SD 5 14.4),F(1, 42)5 6.26,p , .005; and the infants who

saw the Dissimilar-Color-and-Pattern displayalso looked reliably longer at the move-apart(M 5 53.2, SD 5 8.2) than at the move-together test event (M 5 41.9,SD5 8.6),F(1,42) 5 4.36, p , .05. In contrast, the infantswho saw the Dissimilar-Shape display lookedreliably longer at the move-together (M 552.8, SD 5 9.9) than at the move-apart testevent (M 5 37.8, SD 5 13.0), F(1, 42) 55.80,p , .01.

These results were confirmed by nonpara-metric statistics performed on the data. Two-tailed Mann-Whitney tests were conducted onthe infants’ mean looking times over the testtrials at each of the three displays (Conover,1999). These tests revealed that infants whosaw the Similar display and the DissimilarColor-and-Pattern display showed reliablylonger looking times at the move-apart than atthe move-together event (Similar displayT 545.5, p , .05; Dissimilar-Color-and-PatterndisplayT 5 46,p , .05), but infants who saw

FIGURE 3Mean looking times of the infants in the six experimental groups in Experiment 1.


the Dissimilar-Shape display showed reliablylonger looking times at the move-together thanat the move-apart event (T 5 46, p , .05).

Discussion

The infants who saw the Similar displayand those who saw the Dissimilar-Color-and-Pattern-Display looked reliably longer at themove-apart than at the move together testevent, suggesting that they perceived each ofthese displays as consisting of a single unitthat they did not expect to break apart. Incontrast, the infants who saw the Dissimilar-Shape display showed the opposite pattern ofresponse, suggesting that they perceived thisdisplay as consisting of two separate units.These results suggest that, for 4-month-oldinfants, a difference in the shapes of two ad-jacent objects is a clear indication of the pres-ence of two separate objects, whereas a differ-ence in the colors and patterns of the objects isnot.

These results suggest that in previous stud-ies that have revealed young infants’ ability tosegregate displays based on their featuralproperties (Needham, 1998; Needham et al.,1997), the differences in shape may have beenthe most useful for the infants. They also sug-gest that infants use shape in a rather globalway to determine likely points of separationbetween objects. Even though both the Similardisplay and the Dissimilar-Shape display wereobviously composed of two potentially sepa-rate units (i.e., based on a mathematical anal-ysis of shape such as the minima rule), andpossessed visible boundary seams, the infantsseemed to expect the Similar display to becomposed of a single unit and the Dissimilar-Shape display to be composed of two separateunits. Perhaps the symmetry of the shape ofthe display led the infants to perceive it as asingle unit. Researchers have shown that4-month-old infants (Bornstein, Ferdinandsen,& Gross, 1981; Bornstein & Krinsky, 1985;Fisher, Ferdinandsen, & Bornstein, 1981) andeven animals such as honey bees (whose pat-

tern perception abilities have been studied be-cause their efficient foraging behavior dependsupon the recognition of particular flowers—anatural stimulus that almost always is symmet-rical; see Gould, 1988) are sensitive to shapesymmetry in visual displays.

It should be noted that these results are alsoconsistent with the hypothesis that infants de-termined the boundaries in these displaysbased on the overall sizes, rather than theshapes, of the two portions. Because the twoportions of the Dissimilar Shape display weredifferent shapes but also different sizes (i.e.,surface area; the rounded and rectangular por-tions were the same in height), it is impossibleto know which of these factors was most use-ful to infants. It seems most likely to us thatshape is the critical information for infants, inpart because there is extensive evidence thatshape is critical for adults’ object segregation,and in part because the shape difference seemsmuch more salient than the size difference inthis display.

CONTROL EXPERIMENT

Another possible interpretation of these re-sults is that the infants have a simple prefer-ence for watching the move-apart event for theSimilar and the Dissimilar Color and Patterndisplays and the move-together event for theDissimilar Shape display. To investigate thispossibility, a group of infants was presentedwith the test events without any prior exposureto the display. As in previous experiments(Needham, 1998), it was expected that remov-ing the familiarization trial from the procedurewould (a) make it less likely that infants wouldbe able to form an interpretation of the displayagainst which they could evaluate the likeli-hood of the test event they were shown, andtherefore, (b) eliminate the preference infantsdisplayed during the main experiment. Theapparatus, events, and procedure used in thiscontrol experiment were the same as that usedin the main experiment, except that the famil-iarization trial was eliminated.


In contrast, if the results reported abovereflect infants’ inherent preferences for watch-ing the three displays move together or apart,they should show this same preference whenwatching the test events without a previousfamiliarization trial.

Method

ParticipantsParticipants were 30 healthy, full-term in-

fants ranging in age from 3 months, 23 days to4 months, 17 days (M 5 4 months, 3 days,SD 5 7.5 days). Sixteen of the infants weremale, 14 were female. One third of the infantssaw each of the three displays used in Exper-iment 1: the Similar display, the Dissimilar-Shape display, and the Dissimilar-Color-and-Pattern display. Of the infants who saw theSimilar display, half saw the move-apart (M 54 months, 1 day,SD5 7.9 days), and half sawthe move-together event (M 5 4 months, 3days,SD5 6.3 days); of the infants who sawthe Dissimilar-Shape display, half saw themove-apart (M 5 4 months, 7 days,SD5 9.4days) and half saw the move-together event(M 5 4 months, 2 days,SD 5 5.7 days); andof the infants who saw the Dissimilar-Color-and-Pattern display, half saw the move-apart(M 5 4 months, 7 days,SD 5 9.4 days), andhalf saw the move-together event (M 5 4months, 7 days,SD 5 9.4 days). Four addi-tional infants were tested and eliminated fromthe final sample, 2 due to procedural error, 1due to drowsiness, and 1 due to inattentivenessto the experimental event.

Apparatus, Events, Procedure

The apparatus, events, and procedure usedin the Control experiment were the same asthat used in Experiment 1, except that theinfants were not given a familiarization trial(during which the stationary display was vis-ible) prior to seeing the test events.

Each infant in this experiment contributed afull set of 3 test trials to the analyses.

Results

Preliminary Analysis

An analysis revealed no effect of Sex (allF’s , 2.28,p . .05) or Trial (allF’s , 1.87,p . .05) on the infants’ looking times at thetwo test events. The data were therefore col-lapsed over these variables for subsequentanalyses.

Main Analyses

The mean looking times of the infants whosaw the three displays in the Control experi-ment are shown in Table 1. The data wereanalyzed as in Experiment 1, and this analysisyielded a significant Display x Condition in-teraction,F(2, 24) 5 5.09, p , .05. Plannedcomparisons revealed that the infants who sawthe Similar display looked reliably longer atthe move-together (M 5 57.5,SD5 3.4) thanat the move-apart test event (M 5 34.4,SD511.5),F(1, 24) 5 6.26,p , .005; the infantswho saw the Dissimilar-Shape display lookedabout equally at the move-together (M 5 39.3,SD5 16.1) and move-apart events (M 5 39.7,SD 5 12.5),F(1, 24)5 0.12; and the infantswho saw the Dissimilar-Color-and-Pattern dis-play looked about equally at the two events(move-togetherM 5 35.1,SD5 13.2; move-apartM 5 44.2,SD 5 9.2; F(1, 24) 5 2.46,p . .05).

TABLE 1Infants’ Mean Looking Times (in Seconds)

At the Test Events in the Control Experiment

Display

Test Event

Move-together Move-apartSimilar 57.5 (3.4)* 34.4 (11.5)Dissimilar shape 39.3 (16.1) 39.7 (12.5)Dissimilar Colorand Pattern

35.1 (13.2) 44.2 (9.2)

Standard deviations are shown in parentheses.*Mean is reliably different (p , .05) from the other mean in that row.


Discussion

The infants in each of the three displayconditions showed different patterns of re-sponse to the test events in the control exper-iment (after receiving no familiarization) thanthey did in the main experiment (after a famil-iarization trial in which the display was sta-tionary). (1) The infants who saw the Similardisplay looked reliably longer at the move-apart than at the move-together event in Ex-periment 1, but showed the opposite pattern ofresponse in the control experiment. (2) Theinfants who saw the Dissimilar-Shape displaylooked reliably longer at the move-togetherevent in Experiment 1, but looked aboutequally at the events in the control experiment.(3) The infants who saw the Dissimilar-Color-and-Pattern display looked reliably longer atthe move-apart event in Experiment 1, butlooked about equally at the events in the con-trol experiment.

This different pattern of results found withand without familiarization with the stationarydisplay has been found in previous research(Needham, 1998) and suggests the followingconclusions. First, the infants’ responses inExperiment 1 did not reflect a baseline pref-erence for watching the Similar display andthe Dissimilar-Color-and-Pattern displaymove apart and the Dissimilar-Shape displaymove together. If that were the case, the samepattern of results should have been obtained inthe control experiment. Secondly, the patternof results seen in Experiment 1 (consistentwith the explanation that infants used objectshape but not color and pattern to determinethe object boundaries in the display) was ob-tained only after the infants had time to studythe stationary display. This finding supportsthe interpretation that the infants in Experi-ment 1 formed an interpretation of the station-ary display that they then used to evaluate thelikelihood of the test event.

It remains unclear why the infants who sawthe Similar display looked reliably longer atthe move-together than at the move-apartevent in the control experiment, although one

possible explanation will be offered here.Without the opportunity to interpret the sta-tionary display, the infants may have pro-cessed the moving displays in these test eventsat a lower level than they did in Experiment 1.Because the Similar display was the only oneof the three displays that contained real sym-metry in shape and in color and pattern, thesymmetry of the unbroken Similar displaymay have engaged the infants’ attention andled to lengthened looking times (Bornstein etal., 1981; Bornstein & Krinsky, 1985; Fisheret al., 1981). The infants who saw the Similardisplay move apart would not have had asmuch time to attend to the symmetry as theinfants who saw it move together.

Overall, the results of the control experi-ment support the interpretation offered for theresults of the main experiment; that the in-fants’ responses to the test events were basedon their segregation of the display, and that theinfants used a difference in shape, but not adifference in color and pattern, to segregate adisplay into two adjacent objects.

GENERAL DISCUSSION

The main results of this research are thatinfants seemed to interpret the Similar displayand the Dissimilar-Color-and-Pattern displayas composed of a single unit, but the Dissim-ilar-Shape display as composed of two sepa-rate units. These findings are consistent withthe claim that at 4 months of age, infants aremore likely to use differences in the shapes ofobjects than differences in their colors andpatterns to form an interpretation of the com-position of a display. Existing research con-sistent with this interpretation as well as otherpossible interpretations for the results will bereviewed in the following sections.

The Use of Shape in Early Infancy

The results of existing studies indicate that4-month-old infants may use shape but notcolor and pattern in a number of tasks requir-


ing analysis of featural information. For ex-ample, Hespos and Rochat (1997) found that4-month-old infants could use an object’s dis-tinctive shape to keep track of the object’sorientation during a hidden transformation,but they could not use a distinctive color/pattern element on the object for this purpose.However, by 6 months of age, infants coulduse either shape or color/pattern to determinethe object’s orientation.

Similarly, research by Wilcox (in press)indicates that at 4.5 months of age, infants usea change in object shape or size to mark achange in the identity of an object over occlu-sions and reappearances. However, it is notuntil 7.5 months of age that infants use achange in pattern alone and not until 11.5months of age that infants use a change incolor alone as an indication of a change inobject identity. Together with the findings ofthe current experiment, the results of Hesposand Rochat (1997) and Wilcox (in press) sup-port the hypothesis that around 4 months ofage, infants more readily use shape than colorand pattern to accomplish a variety of tasks;keeping track of object identity and orienta-tion, and determining locations of objectboundaries.

Learning about Shape

What would account for this facilitation ininfants’ use of or knowledge about shape rel-ative to color and pattern information? Thebehaviors leading to this facilitation couldstart very early in life. According to Haith(1980), observations of newborn visual behav-ior indicate that very young infants’ eyes aredrawn to areas of high contrast. Because theouter edges of objects typically create contrastrelative to the space around them, this lookingbias would probably produce a great deal ofattention to the external contours of objects,which could in turn lead infants to extractinformation about object shape.

At least two additional aspects of infants’immature visual system could lead to en-

hanced learning about the relation betweenshape and object boundaries compared toother featural cues. Research on the develop-ment of infants’ color vision indicates that thecolor vision system is not fully developed untilsometime around 4 months of age (Teller &Bornstein, 1987). Before this time, infantsmay have difficulty detecting subtle color dif-ferences and would therefore have fewer ob-servations to learn from regarding the relationbetween abrupt changes in color and objectboundary locations. Research also indicatesthat visual acuity does not approach adult lev-els until 6 months of age (Banks, 1983). Notbeing able to clearly resolve the visual imageof an object would likely render its internalpattern elements less clear than its major ex-ternal contours, something that may preventinfants from learning about the relation be-tween similarities and differences in patternelements and how these are related to objectboundary locations. For both of these reasons,the shape of an object may be more accessibleto a young infant than the object’s color orpattern.

Another possibility is that object explora-tion occurring in non-visual perceptual modal-ities contributes to early learning about objectshape. Research conducted by Rochat (1983)indicates that even very early in life, infantsproduce oral activity (sucking and a more var-ied kind of oral activity Rochat refers to asexploration) that allows them to detectchanges in the shape of an object placed in themouth. Rochat’s work also shows that multi-modal exploration of objects (in which oraland visual exploration are well-coordinated)increases significantly between 2 and 5months of age, with a notable increase in theuse of visual exploration strategies around 4months of age (Rochat, 1989). Thus, infantsmay learn first about oral information and onlyonce they are consistently producing more co-ordinated kinds of object exploration do theyuse visual information to learn rules for pre-dicting object boundary locations. In sum, itcould be that the typical development of in-fants’ object exploration strategies encourages


infants to focus on shape information. Therelation between infants’ object explorationstrategies and their ability to segregate dis-plays based on their features is being investi-gated in ongoing research (Needham, 1999).

Although infants must learn about objectsthrough their observations of others’ actionson objects, it also seems likely that an infant’sown actions could be an especially helpfulsource of information (Baillargeon, Needham,& DeVos, 1992). This could be true for anumber of reasons. First, as Baillargeon et al.argue, there are some kinds of observationsthat may be critically important for learning aparticular physical principle but are not typi-cally produced by adults or even children. Forexample, to learn the distinction between sit-uations in which an object is adequately sup-ported and those in which an object is inade-quately supported, infants may need to see theoutcome of events when an object is placed ona surface in such a way that it is inadequatelysupported (i.e., they may need to see the objectfall to learn that this is inadequate support).Because infants may need contrastive evi-dence to learn a physical principle, infantsmay not be able to learn about the adequate/inadequate support distinction just by seeingexamples of an object that does not fall whenit is adequately supported (Baillargeon, 1999).

When learning about object boundaries,contrastive evidence may be collected by in-fants as they explore objects orally and man-ually. An infant may notice that a blanketdraped across his bouncy seat falls from theseat as he thrashes his arms and legs. He mayalso notice that a red rubber teether that isuniform in shape and color does not comeapart into separate pieces even if he bitesdown on one end and pulls on the other. Thislatter kind of information in particular may notbe apparent from others’ actions.

It may also be the case that learning abouthow the features of objects can be used topredict object boundaries is optimal when youhave had a chance to study a display and format least a tentative interpretation of its compo-sition. When observing the actions of caretak-

ers and other people around them, infants’attention may not typically be directed to ob-jects prior to the time that their boundaries arerevealed (much like the control study in thepresent research). If infants do not readilyengage in a post-hoc analysis of the features ofthe objects and the boundaries that were re-vealed (as the infants in the control study inthe present research apparently did not), learn-ing may be hindered when sufficient time isnot available prior to the movement of thedisplay. Thus, being in control of thetimingofthe exploratory actions you observe may alsobe important to effective learning.

Salience Issues

Another interpretation of these findings thatshould be considered is that the salience of theparticular shape difference and color and pat-tern difference chosen for these displays led tothe findings reported here. Specifically, theshape difference chosen for the Dissimilar-Shape display may have been more noticeableto the infants than the color and pattern dif-ference chosen for the Dissimilar-Color-and-Pattern display. Because it is not the goal ofthis research to undertake systematic psycho-physical experiments to determine the range ofshapes that are and are not used and the rangeof colors and patterns that may or may not beused, salience arguments cannot be directlycountered by these data. However, the relatedresearch previously described (Hespos &Rochat, 1997; Wilcox, in press) and additionalresearch from my lab (described below) makesalience arguments seem somewhat unlikely.

First, global judgments of the similaritybetween the two portions of each of the threedisplays were collected from 10 naive adults.The adults were shown each of the three dis-plays (order was randomized) and asked tojudge how similar or different the two portionsof the display were for each display, with 1being very different and 7 being very similar.The average ratings for each of the displayswere as follows: Similar display: 5.9; Dissim-


ilar-Shape display: 3.6; Dissimilar Color-and-Pattern display: 2.3. Thus, according to adults’judgments, the difference in shape present inthe Dissimilar-Shape display seemed less sa-lient or less extreme than the difference incolor and pattern present in the Dissimilar-Color-and-Pattern display.

Of course it is true that infants may not seethese displays in the same way as adults (dif-ferences between the infant and adult visualsystem have been discussed at some length inthis paper), but these data do offer a somewhatobjective indication of the relative salience ofthe shape difference present in the Different-Shape display and the color and pattern dif-ference present in the Different-Color-and-Pattern display. It may be true that infants thisage would consider any shape difference moresalient than any color and pattern difference,but this situation would not be empiricallydistinguishable from the interpretation offeredfor the present results—that at this age infantsare more likely to interpret the compositionsof displays based on their shapes rather thantheir colors and patterns.

Secondly, data collected in my lab in arelated study show that 4-month-old infantsseem to ignore a salient color and patternsimilarity when the shapes of the two portionsof the display are dissimilar (Kaufman &Needham, 1999). Specifically, we created adisplay consisting of two differently shapedportions that were the same in color and pat-tern. The pattern that covered the objects wasvery salient, and consisted of yellow and greenvertical stripes and small white squares; theshapes of the objects were the same rectangleand rounded rectangle used in the Dissimilar-Shape display in the present research. Afterseeing the stationary display, the infantslooked reliably longer at the move-togetherthan at the move-apart event, as did the infantswho saw the Dissimilar-Shape display in thepresent research. These data provide moresupport for the interpretation offered here, be-cause even a very salient similarity in colorand pattern was not used when a conflictinginterpretation based on shape (the same shape

difference used in the present research) wasalso available.

So, although it is true that salience issuescannot be ruled out, it seems unlikely that thepresent results were driven by salience differ-ences between the particular shape differencethat was chosen and the particular color andpattern difference that was chosen.

Integration Strategies

Another way to conceptualize these resultsis in terms of infants’ integration of shape andcolor and pattern information to form an in-terpretation of a display. In prior research, mycolleagues and I have investigated infants’integration of information from differentsources to form an interpretation of a display(Needham & Baillargeon, 1997; Needham &Kaufman, 1997). In these studies, we showedthat, even when conflicting featural informa-tion was available, infants interpreted displaysusing information about (a) the support rela-tions between the objects, (b) the solid con-nection between the objects, or (c) the spatiallayout of the objects. However, infants did usethe featural information to interpret the each ofthese displays when the physical informationwas not available or the spatial informationwas ambiguous. This tendency was explainedby suggesting that infants are sensitive to therelative accuracies (or ecological validities) ofdifferent kinds of information and that infantsmay represent these relative accuracies in ahierarchy that allows them to select the mostaccurate (or ecologically valid) source of in-formation to use to interpret a display.

The present results may bear upon the de-velopment of the hierarchy that was proposedby Needham and Baillargeon (1997) to allowinfants to resolve conflicts between featuraland physical information. These results maysuggest that within one category of informa-tion, such as featural information, infants maylearn how to use some kinds of informationwithin the category prior to others. Alter-nately, it may be the case that even within


featural information (and as soon as they beginto learn about the various featural cues), in-fants have formed a hierarchy that allows themto resolve conflicts between different kinds offeatural information.

Thus, it may not be the case that infants areunable to use color and pattern information,but rather that they do not use it when itconflicts with shape. If this hypothesis weretrue, one would predict that if shape providedonly an ambiguous interpretation of a display,infants may use color and pattern to help themresolve the ambiguity (much like infants usedcolor and pattern differences or similarities tohelp resolve an ambiguity in the spatial layoutof a display in Needham & Kaufman, 1997).As was shown in prior work (Needham &Baillargeon, 1997; Needham & Kaufman,1997), infants will use information fartherdown a hierarchy when their “top choice”information either is not available or providesonly an ambiguous interpretation of the dis-play (a finding that conflicts with Kellman’s(1993) perspective on infants’ use of differentsources of information in object perception).

The results of this research provide furtherevidence that, contrary to the conclusions ofKellman and Spelke (1983) and others,4-month-old infants can use object features todetermine the locations of object boundaries instationary displays. Further, these findingssuggest that, rather than using a combinationof a number of different kinds of featuralindicators for object boundaries, object shapemay provide sufficient information for objectboundaries even in the face of conflicting in-formation from other sources. In sum, theremay be important parallels in the ways inwhich infants and adults perceive the world, assuggested by both age groups’ reliance onobject shape, as information for the locationsof object boundaries.

NOTES

1. Although the present research examines in-fants’ perception of stationary displays, the

method capitalizes on the well-demonstratedfinding that infants use common motion andrelative motion of surfaces as clear indicatorsof object unity and object separation, respec-tively.

2. Without the familiarization trial during whichthe test display was visible stationary for up to30 seconds, there were approximately 2 sec-onds that the display was visible before themotion of the display began and the composi-tion of the display was revealed.

3. Many of the conditions in the experiments re-ported in this paper were run simultaneously(there were always at least 2 simultaneous con-ditions involving different displays, as well astwo different events being run), making it dif-ficult for observers to determine which display(Similar, Dissimilar-Shape, Dissimilar-Color-and-Pattern) or event (move-apart or move-together) a given infant was seeing.

Because the observers would have to knowboth the display and the event to produce thesystematic differences between groups re-ported in this paper, we collected data from ourobservers regarding their ability to determineeach of these aspects of the experimental ses-sion. For 46 of the 78 infants run through theseexperiments, we asked the observers to guesswhich display the infant had been shown. Only14 of the 46 pairs agreed on the correct display,a level not different from chance (15 agree-ments would be expected simply by chancebecause there were two observers guessing and3 possible displays to pick from). These resultsindicate that the observers were not able todetermine which display the infants were pre-sented with, and this information would havebeen critical for the observers to systematicallybias the results.

We also asked each observer to guesswhether each infant had seen the move-to-gether or the move-apart test event. After eachof the sessions of the infants included in thispaper, the two observers were asked to guesswhether the infant was watching the move-apart or the move-together test event. For 26 ofthe 78 sessions, the two observers agreed onthe correct event. Although this number isgreater than one would expect by chance (20agreements would be expected by chance, be-cause each of the two observers had twochoices to pick from), it does not suggest that


observer bias was responsible for these results.Even if every observer knew for sure whetheran infant was seeing the move-apart or themove-together event, they would also have toknow which display the infant was watching toproduce the pattern of results reported in thispaper.

Acknowledgments: This research was sup-ported by a FIRST Award (HD-32129) fromthe National Institute of Child Health and Hu-man Development to the author. I would liketo thank Erika Holz, Scott Huettel, JordyKaufman, Avani Modi, Cynthia Ramirez, andthe undergraduate students working in the In-fant Perception Lab at Duke University fortheir help with the data collection; Jordy Kauf-man and Jim Needham for their statisticalassistance; Avani Modi for her helpful com-ments on the manuscript; and the parents andinfants who gave their time and effort to makethis research possible.

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14 October 1998; Accepted 27 July 1999n


the role of shape in 4-month-old infants’ object … · this research examined four-month-old...

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