paper how infants grasp two adjacent objects: effects of ...paper how infants grasp two adjacent...

Developmental Science 2:2 (1999), pp 219±233

PAPER

How infants grasp two adjacent objects: effects of perceiveddisplay composition on infants' actions

Amy Needham

Duke University, USA

Abstract

In this research, 12.5- and 9.5-month-old infants' use of information about the composition of a display to guide theiractions on the display was examined. The display was composed of two pieces that were either glued together (the one-object display) or not (the two-object display). At the beginning of the session, the composition of the display wasrevealed to the infant and then the display was placed on the table within the infant's reach. When resting on the table,these two versions of the display were indistinguishable. The results showed that the 12.5-month-old infants reached withone hand after they had experience with the display as a single object, and with two hands if they had experience with thedisplay as two separate objects. The 12.5-month-old infants also placed their grasps differently depending upon theexperienced composition: infants who experienced a single object tended to distribute their reaches evenly along thedisplay, whereas infants who experienced two separate objects placed their reaches toward the outer ends of the display,avoiding the boundary between the two pieces. In contrast, the 9.5-month-old infants reached with one hand for both theone-object and the two-object displays, and their reaches did not show a difference in the distribution pattern that couldbe traced back to the infants' prior experience with the composition of the display they were grasping.

Once infants begin to reach for objects, how are theiractions tailored to the objects they reach for? Somestudies have shown that just as soon as infants begin toreach, their actions on objects are appropriate in anumber of ways. Successfully reaching for and graspingan object depends upon the infant's accurate estimationof the location of the object relative to their body in allthree dimensions (Lockman, Ashmead & Bushnell,1984; Yonas & Granrud, 1984). As soon as infantsbegin reaching, they tend to produce reaches for objectsthat are accurate in the ways just described whether theyare produced in the light or in the dark (Clifton, Muir,Ashmead & Clarkson, 1993), and whether the object isstationary or moving (Hofsten, 1980). Thus, in a generalway, infants produce actions that are appropriate for theobjects they are acting upon as soon as they can act onthem.However, more detailed adjustments (which also tend

to involve movements in the hand and wrist rather thanthe whole arm or torso ± see Lockman and Ashmead

(1983) for elaboration of this perspective) do not occuruntil later in the first year of life. For example, prior tograsping a rod in a vertical or horizontal orientation, 9-month-old infants tend to align the grasping hand withthe orientation of the rod (Hofsten & Fazel-Zandy,1984; Lockman et al., 1984), and they tend to producean opening between the thumb and fingers that is scaledto the size of the object (Hofsten & Ronnqvist, 1988).Developmental studies have shown that at 5 monthsof age these preparatory behaviors are less common andless sophisticated (Hofsten & Fazel-Zandy, 1984;Lockman et al., 1984; Hofsten & Ronnqvist, 1988;Newell, Scully, McDonald & Baillargeon, 1989).Although no definitive conclusions have been agreed

upon about the source of these developments, Lockmanand Ashmead (1983) suggested that the developmentaltrend toward more extensive manual anticipations couldbe linked to improvements in visuomotor coordination.Thus, perhaps 9-month-old infants are better than 5-month-old infants at determining the motor movements

Address for correspondence: Department of Psychology, Experimental, Duke University, Durham, NC 27708-0086, USA.

# Blackwell Publishers Ltd. 1999, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.

appropriate for acting upon an object with certainvisually specified characteristics (e.g. orientation, size,shape). Although improvements in visuomotor coordi-nation are undoubtedly occurring during this timeperiod, additional factors that could be involved in thedevelopment of these abilities, such as improvements ininfants' representation and planning abilities (DeLisi,1987; Mandler, 1988; Baillargeon, DeVos & Graber,1989; Baillargeon, Graber, DeVos & Black, 1990;McCarty, Clifton & Collard, in press) or developmentaldifferences in the kinds of visual processing required forcognitive tasks versus action tasks (Milner & Goodale,1995), should also be explored.The fact that infants' actions are (or at least can be)

mediated by something other than continuously avail-able visual information about the object was demon-strated in a clever study conducted by Clifton and hercolleagues (Clifton, Rochat, Litovsky & Perris, 1991). Inthis study, the researchers showed that after a verylimited exposure to two sounding objects, 6-month-oldinfants made a connection between the size of eachobject and the sound it created (e.g. the big objectproduced a jingle bells sound and the small objectcreated a rattle sound). This conclusion was supportedby the infants' behavior in the second phase of the studyduring which trials that took place in total darknesswere introduced. When the infants heard one of thesounds (the jingle bells or the rattle), they reached outfor the object in different ways depending upon whetherthey expected to find the large object or the small object.This study provides evidence that, at 6 months of age,infants form multimodal representations of objects (i.e.in this case the infants must have stored informationabout the size of the object with information about thesound produced by that object) which they can use toproduce object-appropriate actions.The present research was designed to explore further

this issue of how infants' stored interpretation of adisplay could impact their actions on the display. In thisresearch, infants were given information about thecomposition of the display prior to their action on thedisplay. At the time that the reach was executed, thedisplay looked exactly the same for the infants in allgroups. Thus, like the Clifton et al. (1991) study,differences between groups in the way infants reachedfor the display would be due to an influence of the priorexperience with the display. However, two aspects of ourevents probably resulted in a more difficult situation forthe infants than in the Clifton et al. study. First, thesituation presented to the infants in our study is likely tobe more difficult for the infants than the Clifton et al.situation for the same reason that reaching for a hiddenobject whose cover is visible is more difficult than

reaching for an object hidden by a completely darkenedroom. Having a visible display to reach for may makekeeping in mind the information presented at thebeginning of the trial (e.g. the composition of thedisplay in our situation) more difficult than being forcedto rely on your visual image of the display. Second, theinfants in the Clifton et al. study did reach in differentways for the large and small displays, but these infantscould have been primarily concerned with getting theirhands to the objects. In contrast, the events included inthe present research involved additional decisionsregarding the most effective way to pick up a singlelong object versus two smaller adjacent objects. Becauseof these differences (and based on our pilot observa-tions), we tested infants older than those in the Cliftonet al. study.

Object segregation in infancy

Before addressing that particular question, findingsfrom studies using visual measures to investigate whatkinds of information infants use to segregate displaysshould be mentioned. In general, the experimentalfindings in this literature suggest that infants withinthe first year of life make use of a number of differentkinds of information when segregating displays, includ-ing spatial separations between surfaces (Kestenbaum,Termine & Spelke, 1987; Spelke, Hofsten & Kesten-baum, 1989), the motions of surfaces (Kellman &Spelke, 1983; Kellman, Spelke & Short, 1986; Kellman,Gleitman & Spelke, 1987; Johnson & Aslin, 1995), otherphysical relations between objects (such as support andsolidity; see Craton, 1994; Needham & Baillargeon,1997), at least some featural properties of objects, suchas object shape (Needham, 1997, 1998; Needham &Baillargeon, 1997), and prior experiences with objects(Schwartz, 1982; Needham & Baillargeon, 1998). Thus,by 12 months of age, infants have presumably learnedquite a bit about the regularities and behavior of objectsand how these characteristics can be used to predictobject boundary locations.

Effects of display composition on infants' actionson objects

The effects of infants' perception of a display ascomposed of one or two objects on their actions onthe display was investigated in a series of studies bySpelke, Hofsten and their colleagues (Hofsten & Spelke,

220 Amy Needham

# Blackwell Publishers Ltd. 1999

1985; Spelke, Hofsten & Kestenbaum, 1989). In thesestudies, 5-month-old infants produced different kinds ofactions on a display depending upon whether theyperceived it as a single unit or as two separate units. Theinfluence of spatial information (adjacency or separate-ness of the surfaces) and motion information (common,relative, or no motion) on infants' actions on thedisplays were examined, leading to the conclusion thatboth kinds of information affect infants' actions on thedisplays (and presumably their interpretations of thedisplays, a conclusion supported by looking time studiesinvolving the same displays). More recently, studies oninfants' actions on various adjacent and partly occludeddisplays (e.g. squares, faces etc.) also suggest that whenthe relevant information is available at the time of thereach, infants as young as 6 months of age adjust theirgrasps to reflect the composition of the display (Vishton,Stulac & Calhoun, 1998).Together, these last two sections suggest that there

may be considerable overlap in the kinds of informationthat are useful in visual and manual tasks, although thishas not been explicitly studied (e.g. by presenting thesame infants with both a visual and a manual task). Theextent to which the interpretations infants form basedon a given source of information are also useful forguiding their actions is as yet unclear, although recentclaims made by Milner and Goodale (1995) suggest thatthere could be dissociations between visual processingfor the purposes of identification or representationversus visual processing for the purposes of action.Developmental research has led to conflicting views onthe extent to which these differences may appear inhumans during the first year of life (Aguiar &Baillargeon, 1997; Bertenthal, 1997).

The present research

The present research sought to explore the question ofhow infants' actions on a display are affected by theirknowledge of the composition of the display ± informa-tion that was available only prior to their grasp of thedisplay. In this experiment, a single display was used thathad two versions which were indistinguishable when inplace on the table prior to the infants' reach. The displaywas a three-dimensional wooden rod made up of tworectangular blocks. The entire display was painted brightblue and decorated with vertical white stripes (seeFigure 1). The two-object display consisted of these twoblocks; the one-object display consisted of two identicallydecorated blocks that were glued together. Thus, either

display could conceivably be seen either as a single objector as two separate objects placed side by side.There were two different events: visible motion and

hidden motion (see Figures 1 and 2). In the visiblemotion event, the experimenter demonstrated thecomposition of the display and then immediately placedthe display on the table within the infant's reach. Thequestion addressed here was whether infants wouldreach differently for a display depending upon whetherthey had just been shown that it consisted of one pieceor two separate pieces. We presented three separategroups of infants with the display using the visiblemotion method: one group saw the experimenterdemonstrate the composition of the two-object displayand present it using two hands, a second saw theexperimenter demonstrate the composition of the one-object display and present it using one hand, and the

Figure 1 Display presentation during the visible motion event.In the left panels, note that the experimenter reveals thecomposition of the display as one or two units and then placesthe display within the infants' grasp. In the right panels, note thatthe appearance of the two displays after the experimenterdeposited them on the table was indistinguishable.

How infants grasp two adjacent objects 221


third saw the experimenter demonstrate the compositionof the one-object display and present it using two hands(the third condition was included to see whether theinfants might simply imitate the experimenter).Next, the infants were presented with the display in

the hidden motion event, during which the compositionof the display was revealed only when the infantproduced his or her own actions on the display. Thedisplay was switched between the two trials so we coulddetermine whether there were visible signs of surprisewhen the composition of the display changed from onetrial to the next. A separate group of infants received thedisplay via hidden motion without receiving any priorexperience with the display. This group was included asa control and to determine whether infants who had notbeen shown the composition of the display neverthelessformed an interpretation based on the features of thedisplay and used this interpretation to guide theiractions on the display.The focus of our observations was on infants'

preparations for their actions on the displays. Becausewe were interested in how infants' knowledge about thedisplay's composition would guide their actions on thedisplay, it was essential to look at how the infants'actions differed prior to their initial contact with thedisplay. Thus, two of the measures we used to assess theinfants' behavior were the number of hands used to reachfor the display and the locations of the infants' initialgrasps of the display. Both of these features of theinfants' actions were presumably affected by the infants'interpretation of the display, but could not be influencedby any feedback the infants received about the displayafter their initial contact with it. However, infants'expectations about the display could also potentially bereflected in their initial reaction to the display eitherduring or immediately following their pick-up of thedisplay. To assess this factor, we made note of acollection of behaviors that seemed likely to be a resultof the infants' reaction of surprise at the composition ofthe display.

Experiment 1

Method

Participants

Participants were 40 healthy, full-term infants (22 female,18 male) ranging in age from 11 months 15 days to 12months 29 days (M� 12 months 8 days; SD� 10 days).Three groups of ten infants first received a visible motiontrial: one saw the two-object display presented with twohands (M� 12 months 6 days; SD� 11 days), one sawthe one-object display presented with two hands (M� 12months 13 days; SD� 12 days), and one saw the one-object display presented with one hand (M� 12 months10 days; SD� 7 days). Two of these groups (the first andthird listed in the prior sentence) next received the hiddenmotion event featuring the display which had surrepti-tiously undergone a change in composition between trials.The fourth group of ten infants received the hiddenmotion event without first receiving any prior experiencewith the display (M� 12 months 2 days; SD� 8 days).Three additional infants were tested but excluded fromthe final sample, two because of fussiness and onebecause of experimenter error.The infants' names for this experiment and the next

experiment were obtained from the Durham Countyvital records office. Parents were contacted via letter andfollow-up phone calls. They were offered reimbursementfor their travel expenses but were not compensated fortheir participation.

Apparatus

The infant was seated in a trayless high chair and placedin front of a table measuring 115 cm� 77 cm. Theinfants' position at the table was high enough that theyhad a good view of the objects on the table and their armswere well above the table's surface. A wooden presenta-tion platform measuring 49 cm� 77 cm� 4 cm lay ontop of the table and extended all the way across the table

Figure 2 Display presentation during the hidden motion event. In this event, the experimenter did not reveal the composition of thedisplay to the infant.

222 Amy Needham


from the infant to the experimenter. The platform wascovered with a brightly lined white contact paper.The experimenter sat across the table from the infant,

and in the hidden motion event she used a screen to hidethe motion of the display during presentation (seeFigure 2). This screen was a rectangular box(17 cm� 39 cm� 12 cm) with an open back and bottomand was covered with light blue contact paper. Thescreen hid the objects from any view the infant mighthave had: front, top or side.There were two possible displays: the one-object or

the two-object display. Each display consisted of two24 cm� 3 cm� 3 cm blocks of wood. The two blockswere glued together for the one-object display and wereleft as separate pieces in the two-object display. Bothdisplays were painted bright blue and decorated withthin (1 cm) vertical strips of white plastic tape (seeFigure 1).A mirror was positioned beside and slightly behind

the infant, to allow the single camera to capture anotherview of the infant's actions on the objects. The mirrorview was used in coding when the front view of theinfant was less than optimal (e.g. when the experimenterinadvertently obstructed the front view of the infant).The sessions were videotaped with a Panasonic AG 195video camera.

Events

Visible motion event In the visible motion event, theexperimenter revealed the display's composition bymoving it. At the beginning of this trial, the experimenterheld the display at about the infant's eye level and movedit to attract the infant's attention. In order to make thecomposition of the display as clear as possible, theexperimenter used one hand to reveal the composition ofthe one-object display and two hands to reveal thecomposition of the two-object display. Once the infantwas looking at the display and had looked at it for severalseconds, the experimenter moved the display slowlytoward the infant and placed it on the platform withinthe infant's reach. The experimenter used two hands topresent the two-object display within the infant's reach,but used either one or two hands to present the one-objectdisplay within the infant's reach (see Figure 1). Themotion of the display and the display composition wereclearly visible to the infant throughout the event, but oncethe display was placed on the table and released by theexperimenter, its composition was no longer apparent.

Hidden motion event In the hidden motion event, thecomposition of the display was not visually available

during the trial until the infant contacted the display. Atthe beginning of the trial, the display was in theexperimenter's lap as she sat at the table. She placedthe screen on the presentation table and then put thedisplay in place behind the screen. This procedureallowed the experimenter to put the display on the tablewithout revealing its composition to the infant. Theexperimenter then raised the screen and drew theinfant's attention to the display. The experimentermonitored the infant's looking behavior and replacedthe screen after she estimated that the infant had seenthe display for about 5 to 10 seconds. Next, theexperimenter used the screen to hide the display as shemoved it toward the infant and deposited it on the tablewithin the infant's reach, and finally removed the screenmaking the display available for the infant's grasp.

Procedure

The session took place in a brightly lit room, with theinfant and experimenter sitting across the table fromeach other.To prepare the infant for the hidden motion event, the

session began with a familiarization trial in which amulti-colored chiming toy was presented to the infantusing the screen to hide its motion in the same way thatwould happen in one of the test trials.1

Following the familiarization trial, three groups ofinfants received either the one-object or the two-objecttest display in the visible motion event. One groupreceived the two-object display presented using twohands, the second received the one-object displaypresented using two hands, and the third received theone-object display presented using one hand (seeFigure 1).Two of the three groups of infants (the first and third

group listed above) went on to receive one of thedisplays via the hidden motion event, and the composi-tion of the display was surreptitiously altered betweenthese two trials. Thus, if an infant saw the one-objectdisplay in the visible motion trial, he saw the two-objectdisplay in the hidden motion trial (and vice versa). Thefourth group of infants received a hidden motion trialwithout first receiving a visible motion trial.On all trials, the infant was allowed to play with the

display for approximately one minute or until he or shedropped the display, whichever came first. The displaywas then removed from the infant's grasp (and view).

1Not all infants received a test trial involving hidden motion, but all

infants received this familiarization trial.



Additional trials in the session were conducted withother objects for a different study.The parent was seated beside and slightly behind the

infant and was asked to refrain from interacting with theinfant at the critical points in the procedure, althoughsome interaction between parent and infant was allowed(e.g. some infants engaged in a game of picking up theobjects, inspecting them, and handing them to theparent ± the parent was encouraged to accept the objectas he or she would in a normal play situation and thengive the object to the experimenter. Because we did notcode the infants' actions beyond their initial inspectionof the object(s), this kind of interaction between parentand infant seemed unproblematic and seemed to makethe session more enjoyable for the infants.)

Data coding

The videotapes were coded on a Panasonic CT-2084yvideo monitor and Sony SVO-2000 VHS video recorderthat allowed both forward and reverse frame-by-frameanalysis of the sessions. Although the coders were awareof the general objectives of the study, they were notaware of any specific predictions for different conditions(and in fact there was no prior research that wouldsuggest exactly how the infants in the different condi-tions would behave differently). Three measures weredeveloped to characterize the infants' actions on thedisplay.

Measures The one or two hands measure concernedwhether the infant used one or both hands in his or herinitial grasp of the display. Usually this was easy tojudge, as most infants' first grasp of the display was withthe hand(s) that was (were) active prior to their initialcontact with the display. However, there were twoinfants for whom both hands were active prior to theirinitial contact with the display, but only one hand wasused in the initial grasp of the display, and these graspswere coded as one-handed grasps. Although thecategorization of one-handed and two-handed graspswas made according to the coders' judgment, additionalmeasures we made on the time between grasps led us toconclude that grasps coded as two-handed took placewithin 0.02±0.5 second of each other, while graspscoded as one-handed were separated by at least severalseconds.The grasp placement measure concerned where on the

display the infant placed his or her grasp. A grasp wasdefined as palm contact with the display accompanied

by one or more fingers wrapping around the display,and typically led to the infant's pick-up of the display.Four categories were used to describe the graspplacement: a center grasp (CG) was a grasp thatspanned the boundary between the two parts that madeup the display. The remaining three categories wereestablished by dividing each of the two portions of thedisplay into thirds (the outer third or outer end (OE),the middle third (M) and the inner third or inner end(IE)), and the grasps were categorized according towhich of these regions they occurred within. Grasps thatwere not entirely within one category were categorizedon the basis of which category most of the hand waswithin. There were no grasps that were uncategorizablebecause they were evenly split between two categories.Reliability was assessed for these measures by having

two coders independently code a subset of the infants inthe study. Fourteen infants were coded for thesepurposes, nine of whom contributed two trials to theanalyses (both a visible motion and a hidden motiontrial) and five of whom contributed only one trial (onlythe hidden motion trial), resulting in a total of 23 trials.There was 100% agreement between the two coders onwhether the initial grasp involved one or two hands, andthere was 91% agreement (they agreed on 21 of the 23grasps) on the judgment of grasp placement. In cases ofdisagreement, the judgment of the more experiencedcoder (CR) was used.The reactions to the display measure focused on the

infants' behavior once they noticed the consequences oftheir actions on the display. This measure was developedto assess infants' reaction to the change in thecomposition of the display that was made between thetwo trials for the infants who received both trials, and togather additional information about what the infantswho only received one trial expected the composition ofthe display to be. Some infants had no observablereaction to the display as their actions on the displaymade clear that the display was composed of a singlepiece or of two separate pieces, but other infants hadvery noticeable reactions immediately following theirinitial contact with the display. These reactions includedpausing in the process of picking up the object (whilelooking at the object and sometimes then looking at theexperimenter or parent and smiling), trying to pull theone-object display apart, trying to put the two-objectdisplay back together, laughing while looking at thedisplay, and scrutinizing the inside center ends of theobjects (in the two-object display) immediately afterpick-up. Because the surprise measures merely offeredcorroborative evidence, we did not collect reliabilitydata for these observations.

224 Amy Needham


Results

Visible motion event

One or two hands Of the 20 infants who grasped theone-object display (ten saw the display presented withone hand and ten saw it presented with two hands), 16used one hand in their initial grasp of the display andfour used two hands. Eight out of the ten infants in eachgroup used one hand to grasp the display. In contrast, ofthe ten infants who received the two-object display, eightused two hands in their initial grasp of the display andtwo used one hand.These data were analyzed by means of a 3� 2 �2 test

of independence, with Display=presentation group (one-object display, one-handed presentation; one-objectdisplay, two-handed presentation; and two-object dis-play, two-handed presentation) and Number of hands inthe infant's grasp as between-subjects factors. Thisanalysis yielded �2(2)� 10.01, p< 0.005, indicatingsignificantly different patterns of one- and two-handedgrasps when the infants grasped the one-object and thetwo-object displays. Because the two groups of infantsgrasping the one-object display had the same numbers ofone- and two-handed grasps, and the infants graspingthe two-object display showed the reverse pattern, theresults of this analysis must indicate that the infantsgrasping the one-object display show a reliably differentpattern of one- versus two-handed reaches in theirgrasps than the infants grasping the two-object display.

Grasp placement Graphs of grasp placement for eachgroup are shown in Figure 3. Because our examinationof the placements of the grasps led us to hypothesizethat the infants who had seen the display demonstratedas two separate pieces tended to avoid the point ofseparation between the two objects with their grasps, wedecided to conduct an analysis that would determinewhether the separation between the grasps of the infantswho had seen the display demonstrated as two separateobjects was greater than the grasps of infants who sawthe display demonstrated as a single object.To conduct this analysis, we first needed a measure of

separation between grasps. For one-handed grasps, thenumber of categories between a grasp and its nearestneighbor (not including other grasps within the samecategory and not including two-handed grasps) wascalculated. For two-handed grasps, the number ofcategories between the two grasps was calculated. Thesedistances were analyzed by means of a 3� 2 analysis ofvariance (ANOVA) with Display (one-object presentedwith one hand, one-object presented with two hands, ortwo-object display) and Hands in the Grasp (one or two)

Figure 3 Graphs showing the numbers of grasps terminating ineach of the seven coded locations on the display for the12.5-month-old infants during the visible motion events. Alongthe horizontal axis of each graph are each of the seven graspcategories organized as they were on the display itself. Thus,each graph can be seen as a frequency count of grasps, with thehorizontal axis of the graph recreating the horizontal extent of thedisplay.



as between-subjects variables. This analysis yielded asignificant effect of Display (F(2, 24)� 18.06, p< 0.001),indicating that there was more separation betweengrasps for the infants who grasped the two-objectdisplay (M� 3.6, SD� 0.84) than for the infants whograsped the one-object display, regardless of whether theexperimenter presented the one-object display with onehand (M� 0.5, SD� 1.08) or with two hands (M� 0.7,SD� 1.63). There was also a significant effect of Handsin the Grasp (F(1, 24)� 55.54, p< 0.001), indicating thatthe infants' two-handed grasps (M� 3.5, SD� 1.09)tended to have greater separation between them than theaverage separation between one-handed grasps(M� 0.33, SD� 0.97).There was also a significant interaction between

Display and Hands in the Grasp (F(2, 24)� 7.09,p< 0.005), indicating that the infants who grasped theone-object display produced two-handed grasps(M� 3.0, SD� 1.4) with greater separation betweenthem than the average separation between their one-handed grasps (M� 0.0, SD� 0.0) (F(1, 24)� 16.14,p< 0.0005), while the infants who grasped the two-object display produced one-handed (M� 3.0, SD� 0)and two-handed (M� 3.75, SD� 0.89) grasps that wereabout the same in separation (F(1, 24)� 2.86, p> 0.05).The primary difference driving these effects seemed to bethat the average separation between the infants' one-handed grasps was significantly greater when theygrasped the two-object display (M� 3.0, SD� 0.0) thanwhen they grasped the one-object display (M� 0.0,SD� 0.0) (F(1, 26)� 12.03, p< 0.0005).These results indicate that the infants who grasped the

two-object display produced grasps with greater averageseparation than the infants who grasped the one-objectdisplay, and that, while two-handed grasps may tend tobe more separated than one-handed grasps (as shown inthe results from the one-object display), infants'apparent attempts to avoid the center separation pointin the two-object display counteract this tendency.

Reactions to display None of the ten infants whograsped the one-object display showed any of thereactions to the displays we were making note of(pausing during pick-up, laughing, trying to pull apart(for one-object display) or put back together (for two-object display), or scrutinizing the inside center ends (fortwo-object display)). However, five of the ten infantswho saw the two-object display produced one of thesereactions: two laughed, two scrutinized the inside centerends after pick-up, and one tried to put the objects backtogether after picking them up. These observationsprovide corroborating evidence that, even in thecondition in which the composition of the display was

revealed to the infants immediately prior to their graspof it, they had some expectation (presumably based onfeatural information) that the display perhaps shouldconsist of a single object.

Hidden motion event

One or two hands Of the ten infants who hadexperienced the one-object display in the visible motionevent in the preceding trial, seven used one hand tograsp the display and three used two hands to grasp thedisplay; of the ten infants who had experience with thetwo-object display in the preceding trial, one used onehand to grasp the display and nine used two hands; andof the ten infants who received no prior experience withthe display, nine used one hand to grasp the display andone used two hands.These data were analyzed by means of a 3� 2 �2 test

of independence with Prior experience (one-object dis-play, two-object display, or none) and Number of handsin the infant's grasp as between-subjects variables. Thisanalysis yielded a �2(2) of 14.13, p< 0.005, a resultprobably driven by the similarity in the actions of theinfants who had either no prior experience or experiencewith the one-object display, and the difference betweenthese two groups and the group who had experiencewith the two-object display.Because only six of the 20 infants changed the number

of hands they used to grasp the display between the twotrials, these results suggest that the infants wereexpecting the same display on the second trial that theyhad experienced on the first trial and that they used thisprior experience to prepare their actions on the displayin the hidden motion trial.

Grasp placement Graphs of grasp placement for eachgroup are shown in Figure 4. The infants' graspplacement in the hidden motion event closely paralleledtheir grasp placement in the visible motion event. Theseparation between grasps was calculated and analyzedas in the visible motion trial, with Prior experience (one-object display, two-object display, or none) and Handsin the Grasp (one or two) as between-subjects variables.This analysis yielded a significant effect of Priorexperience (F(2, 24)� 9.11, p< 0.005), indicating thatthere was more separation between grasps for theinfants who had prior experience with the two-objectdisplay (M� 3.4, SD� 0.70) than for both the infantswho had prior experience with the one-object display(M� 1.0, SD� 1.49) and the infants who had no priorexperience with the display (M� 0.70, SD� 1.25).There was also a significant interaction between Priorexperience and Hands in the Grasp (F(2, 24)� 7.0,

226 Amy Needham


p< 0.005), indicating that the infants who had priorexperience with the one-object display produced two-handed grasps (M� 3.0) with greater separation be-tween them than the average separation between theirone-handed grasps (M� 0.14) (F(1, 24)� 12.14,p< 0.001), while the infants who had experience withthe two-object display produced one-handed (M� 3.0)and two-handed (M� 3.44) grasps that were about thesame in separation (F(1, 16)� 1.37, p> 0.05). No othereffects were significant.These results indicate that, much like the results in the

visible motion event, the infants who grasped the two-object display produced grasps with greater averageseparation than the infants who grasped the one-objectdisplay. Thus, while the infants who grasped the one-object display produced two-handed grasps that weremore separated than their one-handed grasps, theaverage separation of one- and two-handed grasps ofthe two-object display were about equal.

Reactions to display If the infants were forming arepresentation of the object in the visible motion trialthat influenced their actions in the hidden motion trial,one would expect to find some `surprise' reactions as theinfants realized that the display had changed between thetwo trials. Analysis of the same behaviors that were codedfor in the visible motion trial indicated that ten of the 20babies in these two groups displayed one of thesebehaviors, resulting in 12 behaviors that could beinterpreted as surprise: five infants paused as they pickedup the display, three infants who saw the two-objectdisplay scrutinized the inside center ends of the displayimmediately after pick-up, three infants who saw the one-object display tried to pull the display apart, and oneinfant who saw the two-object display tried to put thepieces of the display back together. These observationsprovide corroborating evidence that infants formed arepresentation of the display's composition during thevisible motion trial that they used to prepare their actionson the display in the following trial (the hidden motiontrial).Examination of the reactions of the infants who did not

take part in the visible motion trial prior to the hiddenmotion trial reveals that two of the five infants whograsped the two-object display responded with one of thebehaviors listed above: one of them paused during thepick-up of the display and one laughed as he saw thedisplay move apart. In contrast, none of the five infantswho grasped the one-object display responded with any ofthe behaviors we coded for. These observations supportthe conclusion that the infants who saw the display breakapart, and not the infants who saw the display move as asingle piece, were surprised by what they saw.

Figure 4 Graphs showing the numbers of grasps terminating ineach of the seven coded locations on the display for the 12.5-month-old infants during the hidden motion events. Along thehorizontal axis of each graph are each of the seven graspcategories organized as they were on the display itself. Thus,each graph can be seen as a frequency count of grasps, with thehorizontal axis of the graph recreating the horizontal extent of thedisplay.



Discussion

The infants reached for the display in different ways,grasped the display in different places, and reacted to thedisplay differently depending upon whether theythought they were reaching for one or two objects.The infants who were shown that the display wascomposed of two separate pieces tended to reach withtwo hands and avoid the boundary at the center of thedisplay, both in the visible motion trial and in thefollowing hidden motion trial. This boundary-avoidingstrategy led to an increase in the number of graspsplaced toward the outer edges of the display relative toother groups. However, the infants who were shown thatthe display was composed of a single piece tended toreach with one hand and to distribute their grasps evenlyover the length of the display, including across theboundary of the two portions of the display, both in thevisible motion trial and in the following hidden motiontrial. The infants who did not receive a visible motiontrial prior to grasping the display in the hidden motiontrial showed a pattern of action most similar to theinfants who had prior experience with the one-objectdisplay, suggesting that the shape, color and pattern ofthe display may have led the infants to perceive thedisplay as composed of a single object.Some infants produced behaviors following their

actions on the display that could be interpreted assurprise reactions. Three of the groups of infants in thisstudy displayed these surprise behaviors: the infants whograsped the two-object display in the visible motiontrial, the infants who we tried to surprise by switchingthe displays between their visible motion trial and theirhidden motion trial (they exhibited the surprise beha-viors during the hidden motion trial), and the infantswho had no prior experience with the display andgrasped the two-object display in the hidden motiontrial. The only groups of infants who did not show anyof these behaviors were those who grasped the one-object display in the visible motion trial and those whograsped the one-object display in the hidden motion trial(without having any prior experience with the display).One issue that is important to address is the

development of this phenomenon. Existing research oninfants' use of information visible during the time of thegrasp to adjust the grasp to the properties of the objectindicate that by 9.5 months of age infants are quiteadept at these kinds of actions. So, one possibility is that9.5-month-old infants would show the same patterns ofaction as the 12.5-month-olds showed in Experiment 1.Another possibility is that 9.5-month-old infants wouldnot show this organized pattern of response, eitherbecause they are motorically unable to execute this kind

of action, they have not yet learned the appropriate wayof grasping two adjacent objects in this context, orbecause keeping in mind their prior experience regardingthe composition of the display depletes their cognitiveresources sufficiently that they cannot produce a moresophisticated reach than the one that basically gets theirhand to the object. To investigate the developmentalcourse of this behavior, 9.5-month-old infants weretested in Experiment 2.

Experiment 2

Method

Participants

Participants were 30 healthy, full-term infants (17female, 13 male) ranging in age from 9 months 1 dayto 10 months 13 days (M� 9 months 20 days; SD� 13days). Two groups of ten infants first received a visiblemotion trial: one saw the two-object display presentedwith two hands (M� 9 months 18 days; SD� 16 days),and the other saw the one-object display presented withone hand (M� 9 months 26 days; SD� 12 days). Thesegroups went on to receive a hidden motion trialfeaturing whichever display they had not seen in thevisible motion trial. The third group of ten infantsreceived a hidden motion trial without first receiving anyprior experience with the display (M� 9 months 17days; SD� 9 days). Six additional infants were testedbut excluded from the final sample, four because ofexperimenter error and two because of fussiness.

Apparatus, events, procedure and data coding

The apparatus, events, procedure and data coding usedin Experiment 2 were identical to those used inExperiment 1, with the following exceptions. The designof this study was slightly different, with only two groupsrun in the visible motion event: one that received theone-object display presented with one hand and one thatreceived the two-object display presented with twohands.Reliability was assessed for hands in the grasp and

grasp placement by having two coders independentlycode a subset of the infants in the study. Eleven infantswere coded for these purposes, six of whom contributedtwo trials to the analyses (both a visible motion and ahidden motion trial) and five of whom contributed onlyone trial (only the hidden motion trial), resulting in atotal of 17 trials. There was 94% agreement (16 out ofthe 17 grasps) between the two coders on whether the

228 Amy Needham


initial grasp involved one or two hands, and there was82% agreement (they agreed on 14 of the 17 grasps) onthe judgment of grasp placement. In cases of disagree-ment, the judgment of the more experienced coder (CR)was used.

Results

Visible motion event

One or two hands Of the ten infants who grasped theone-object display, eight used a one-handed grasp andtwo used a two-handed grasp; of the ten infants whograsped the two-object display, all ten used a one-handed grasp. This pattern of grasping was analyzed asin Experiment 1, yielding �2(1)� 2.22, p> 0.05. Thispattern of results is much different from that observedfor the 12.5-month-old infants, who tended to use onehand to grasp the one-object display and two hands tograsp the two-object display.

Grasp placement Graphs of grasp placement for eachgroup are shown in Figure 5. The placement of graspswas measured and analyzed as in Experiment 1 withDisplay (one-object display or two-object display) as abetween-subjects factor. Hands in the Grasp was notincluded as a factor because of the relatively smallnumber of two-handed grasps. This analysis producedno significant effects, indicating that the infants whograsped the one-object display produced roughly thesame distribution of grasps (M� 1.0, SD� 0.67) as theinfants who grasped the two-object display (M� 0.6,SD� 0.97) (F(1, 18)� 1.16, p> 0.05).

Reactions to display None of the 9.5-month-old infantsreacted to their pick-up of the display after the visiblemotion event with any of the behaviors we wereattributing to surprise.

Hidden motion event

One or two hands Of the ten infants who hadexperienced the one-object display on the previous trial,nine grasped the display with a one-handed grasp, andof the ten infants who had experienced the two-objectdisplay on the previous trial, nine grasped the displaywith a one-handed grasp (the two groups had the samenumber of one-handed graspers, which would of courselead to a �2 of zero).

Grasp placement Graphs of grasp placement for eachgroup are shown in Figure 6. The placement of graspswas measured and analyzed as in Experiment 1 with

Prior experience (one-object display, two-object display,or none) as a between-subjects factor. Hands in theGrasp was not included as a factor because of therelatively small number of two-handed grasps. Thisanalysis produced no significant effects, indicating thatthe infants who had prior experience with the one-objectdisplay (M� 0.2, SD� 0.63), the two-object display(M� 0.5, SD� 0.97), and with no prior experience(M� 0.0; SD� 0.0) produced roughly the same distri-butions of grasps (F(1, 27)� 1.41, p> 0.05).

Reactions to display Four of the 20 9.5-month-old infantsfor whom the composition of the display was switched

Figure 5 Graphs showing the numbers of grasps terminating ineach of the seven coded locations on the display for the9.5-month-old infants during the visible motion events. Alongthe horizontal axis of each graph are each of the seven graspcategories organized as they were on the display itself. Thus,each graph can be seen as a frequency count of grasps, with thehorizontal axis of the graph recreating the horizontal extent of thedisplay.



between the visible motion and hidden motion trialsreacted to their pick-up of the display after the hiddenmotion event with one of the behaviors we were attributingto surprise: three infants paused during pick-up, and oneinfant laughed when he saw the composition of the display.Of the ten infants who received no prior experience

with the display before the hidden motion event, sixreceived the two-object display. Three of these sixinfants displayed one of the behaviors we were inter-preting as indicative of surprise: scrutinizing the insidecenter ends of the objects after picking them up.

Discussion

Unlike the 12.5-month-old infants, the 9.5-month-oldinfants did not produce a distinctive pattern of action onthe display that could be traced back to whether theyexpected the display to consist of a single piece or twoseparate pieces. The 9.5-month-old infants tended tograsp the display with one hand regardless of thedemonstrated composition (and if they had no priorexperience with the display), and in all conditions theinfants tended to distri-bute their grasps evenly along thesurface of the display.Some of the 9.5-month-old infants did react to a

change in the composition of the display and to the two-object display (if they had no prior experience) with oneof the behaviors we thought could be indicative ofsurprise. Although tentative on their own, theseobservations are consistent with several visually basedstudies indicating that by 9.5 months of age infants canform interpretations of displays using features such asshape, color and pattern (Needham, & Kaufman, 1997;Needham, 1998; Needham & Baillargeon, 1998).

General discussion

The results of the present research show that at 12.5months of age, but not at 9.5 months, infants (a) usedone hand to grasp the one-object display and two handsto grasp the two-object display, and (b) often graspedthe display at its center when they thought it wascomposed of a single unit but avoided the center whenthey had been shown that this was a point of separation(i.e. an object boundary) within the display.These results are consistent with the main relevant

findings in the literature: (a) that infants do producedifferent actions on a display depending upon theirinterpretation of the display's composition (Hofsten &Spelke, 1985; Spelke et al., 1989; Stulac & Vishton, 1997;Vishton et al., 1998), (b) that infants use experientialand featural information to form interpretations of a

Figure 6 Graphs showing the numbers of grasps terminating ineach of the seven coded locations on the display for the9.5-month-old infants during the hidden motion events. Alongthe horizontal axis of each graph are each of the seven graspcategories organized as they were on the display itself. Thus,each graph can be seen as a frequency count of grasps, with thehorizontal axis of the graph recreating the horizontal extent of thedisplay.

230 Amy Needham


display's composition (Needham & Kaufman, 1997;Needham & Baillargeon, 1998), and (c) that infants usetheir prior experiences with a display to guide theiractions on that display (Clifton et al., 1991). In addition,these results suggest that infants may begin to make useof their knowledge about display composition whenacting on a display such as the one used in this researchbetween 9.5 and 12.5 months of age.To what should we attribute the developments

described in this research? At least three explanationsare possible. First, the younger, but not the older,infants may have difficulty keeping the demonstratedcomposition of the display in mind and using thatinformation to override the apparent composition of thedisplay during the reach and grasp of the display (e.g.the infants who saw the two-object display at thebeginning of the trial had to disregard the appearance ofthe display once it was on the table, at which point itlooked very much like a single object, to produce graspsthat were tailored to the display's composition).Second, using two hands to simultaneously grasp two

adjacent objects may pose special motoric challenges,and the differences seen between our younger and olderinfants is produced by advancements in infants' inter-limb coordination (Diamond, 1991; Corbetta & Thelen,1996; Gadacz & Corbetta, 1998). Evidence in favor ofthis interpretation could be seen when looking beyondthe 9.5-month-old infants' initial grasp of the two-objectdisplay. Although it was the case that most of theseinfants produced one-handed grasps as their initial graspof the display, in 20 of these 25 trials, before the end ofthe trial the infant did end up grasping the part of thedisplay not initially grasped (in four of these trials, theinfant used the same hand to grasp both parts of thetwo-object display over the course of the trial). So, thesedata suggest that the 9.5-month-old infants wereinterested in exploring the whole display, but they wereperhaps less efficient than the 12.5-month-olds in theirinitial acquisition of the display. Related to thispossibility is that the older infants' motivation toacquire the entire display as quickly as possible mayhave been greater than the younger infants'.Finally, the older infants may have had a better ability

to anticipate the effects of their actions on the move-ments of the display. For example, the older infants mayhave anticipated that grasping the display across itspoint of separation would not lead to the retrieval ofeither portion of the display (this was, in fact, theoutcome we observed for infants who grasped the two-object display at its center). In contrast, grasping a longobject at its center, rather than at its very end, couldmake lifting the object easier. Thus, it may be the casethat infants' planning and their physical knowledge

influence their grasp placement in this context, and thatat 9.5 months of age their ability to anticipate the likelyoutcomes of their actions is not well developed(McCarty et al., in press).To help determine which of these possibilities best

characterizes the abilities of the 9.5-month-old infants,the following experiment could be done. The infantscould be presented with the two-object display with asmall (but perceptible) gap separating the two pieces ofthe display. If the pattern of results we observed for the9.5-month-old infants in Experiment 2 was caused bytheir difficulty in keeping the composition of the displayin mind as they planned their actions on the display,they should produce simultaneous reaches with both oftheir hands for this display. However, if the observedpattern of results in Experiment 2 was a result of (a) thelack of motoric ability to coordinate reaches with twohands for two objects or (b) the lack of an under-standing of the physical constraints that render someactions on a particular display more successful thanothers, this manipulation should lead to no reliablechange in the data.The development of more effective grasping strategies

described in the present research could be seen as anextension of the many ways in which infants tailor theiractions on objects to the characteristics of the objects.As described in the introduction, many studies haveshown that, between 5 and 9 months of age, infantsbegin to reliably and systematically produce a variety ofmanual anticipations of visible object characteristics,such as orientation (Hofsten & Fazel-Zandy, 1984;Lockman et al., 1984), size (Hofsten & Ronnqvist, 1989;Clifton et al., 1991) and shape (Newell et al., 1989). Thelater-occurring development found in the presentresearch could be later for any of the three reasonsoutlined above for why the 9.5-month-old infants'responses are different from the 12.5-month-old infants'responses (and=or the reasons described in the introduc-tion for why we tested infants older than those in theClifton et al. (1991) study). Specifically, the develop-ments seen in the present research could occur later thanwhat has been found in previous studies because (a)there is an increased cognitive load involved withkeeping the composition of the display in mind andusing that information to override the appearance of thedisplay during the reach and grasp of the display (thiscould explain why infants develop the abilities describedin the present studies later than prior investigations ofinfants' reaching for fully visible objects and for objectshidden by complete darkness), (b) it is especiallychallenging to coordinate two hands to simultaneouslygrasp two objects (Corbetta & Thelen, 1996; Gadacz &Corbetta, 1998), and (c) infants must integrate their



physical knowledge into their action system in order todetermine the way of acting on the display that is mostlikely to lead to the retrieval of the display.In any of these three cases, cognitive resource

limitations could be involved in this development. AsBushnell (1985) has suggested, early reaching may besomewhat cognitively demanding. It may take infantsseveral months of practice in reaching before theiractions are automated sufficiently to add refinements totheir basic reaches that are not absolutely necessary tothe (eventual) retrieval of the display. Tasks that carrywith them a heavy cognitive load may push the displayof more sophisticated reaching behavior within thatcontext somewhat later in development.Exploring these possibilities and others promises to

reveal much about the development of action in the firstyear of life and the ways in which infants' actionssometimes do and sometimes do not reflect theirknowledge.

Acknowledgements

This research was supported by grants from theNational Institute of Child Health and Human Devel-opment (FIRST grant HD-32129) and from the DukeUniversity Research Council to the author. I would liketo thank Deborah Schkolne, Nancy Mazor and Eliza-beth Crigler for their many contributions to thisresearch and the pilot studies that preceded it; CynthiaRamirez for her extensive help with coding and datacollection; Patrick Curran for statistical advice; CarolEckerman for equipment loans that were especiallyhelpful in the early stages of the study; Erika Holz,Avani Modi, Michele Heysse, Jennifer Lansford, LisaKarlisch, Jordy Kaufman and Susan Garland-Bengurfor their help with the data collection; the undergraduatestudents working in the Infant Perception Laboratory atDuke University for their help with the data coding;Henry Walke for constructing the apparatus andstimuli; and the parents and infants who gave their timeand effort to make the research possible.

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Received: 15 October 1997

Accepted: 20 October 1998



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