infants’ use of featural information in the segregation … · the segregation of stationary...

INFANTS’ USE OF FEATURAL INFORMATION IN THE SEGREGATION OF STATIONARY OBJECTS

Amy Needham Duke University

Infants’ use of featural information (e.g. shape, color, pattern) to segregate stationary displays was

investigated in three main experiments. The first experiment showed that 7.5-month-old infants, but

not younger infants, were able to form a clear interpretation of a display consisting of a curved yellow

cylinder and an adjacent tilted blue box as composed of two separate units. Subsequent experiments

determined that infants as young as 4.5 months of age could segregate into two units a simplified ver-

sion of this display consisting of a straight yellow cylinder and a straight blue box (whether the display

was fully visible or boundary-occluded). These results indicate that infants as young as 4.5 months of

age can use object features (at least simple ones) to determine the locations of object boundaries. The

results are discussed in terms of the processes underlying object segregation in infancy, and why com-

plex features could be difficult for younger, but not older infants to perceive.

perceptual development cognitive development stationary objects object features object complexity

adjacent objects partly occluded objects

INTRODUCTION

Imagine what an infant’s first glimpse inside the family’s refrigerator would be like: a kalei- doscope of shapes and colors consisting of large white jugs of milk; a group of bottles and jars filled with pink salad dressing, purple jam, and brown mustard; stacked packages of sliced meats and cheeses; shiny foil bundles next to a plastic-wrapped glass bowl filled with orange and red melon balls. Separating

this jumble of shapes and colors into a collection of discrete objects is a task that adults find so effortless, we hardly even consider it a “task.” This process of transforming a compli- cated and disorganized collection of surfaces into discrete objects is a process called object

segregation.

The question of how object segregation takes place is an important one for theories of visual and cognitive development; without the ability to see objects and their boundaries

l Amy Needham, Department of Psychology: Experimental, Duke University, Durham, NC, 27708-0086; e-mail:

[email protected].

INFANT BEHAVIOR & DEVELOPMENT 21 (l), 1998, pp. 47-76 ISSN 0163-6383

Copyright 0 1998 ABLEX Publishing Corporation All rights of reproduction in any form reserved.

48 INFANT BEHAVIOR & DEVELOPMENT Vol. 21, No. 1, 1998

accurately, infants’ learning about the physical

world would certainly be impeded (Baillar-

geon, 1995; Kellman, 1996; Marr, 1982; Man- dler, 1992; Spelke, Breinlinger, Macomber, & Jacobson, 1992). There are many cues within a display that could reveal the locations of object boundaries, and researchers have been

interested in determining when infants begin to use each of these cues.

Classes of information that have been investigated include spatial information, such

as whether or not there is a visible separation between the objects (Kestenbaum, Termine, &

Spelke, 1987; Spelke, Hofsten, & Kesten-

baum, 1989); physical information, such as common or relative motion of object parts or information about the support relations between objects (Kellman, Gleitman &

Spelke, 1987; Kellman & Spelke, 1983; Kell- man, Spelke, & Short, 1986; Needham & Bail- largeon, 1997; Slater, Morison, Somers,

Mattock, Brown, & Taylor, 1990; Spelke et

al., 1989); and featurul information, such as the shapes, colors, and patterns of object surfaces (Kellman & Spelke, 1983; Needham &

Baillargeon, 1997; Schmidt & Spelke, 1984; Spelke, Breinlinger, Jacobson & Phillips, 1993).

One question regarding these sources of information concerns how the information is

used to locate object boundaries. According to a model recently proposed by Needham, Bail-

largeon, and Kaufman (1997), infants’ suc- cessful use of featural information in a display to determine the location of an object boundary depends on three more basic abilities. First, infants must have the basic visual capacities necessary to detect the information at all. If infants’ acuity is not sufficient to allow them to notice the difference between two small patterns or if their color perception is not accurate enough to allow them to notice a difference between two colors, this information clearly can not be used to find a boundary where the surfaces’ pattern or color changes. Next, the information must be processed, that is, infants must encode, represent, and compare the information present in all of the sur-

faces in the display. For example, infants must compare the shapes, colors, and patterns on

either side of an occluding screen to determine how similar or dissimilar the surfaces are on each of these dimensions.

Finally, configural knowledge (i.e., the knowledge that surfaces with different features belong to separate units, but objects with

similar features belong to the same unit) must be used to form an interpretation of the display

as consisting of one or two units. For example, configural knowledge would be used to form

an interpretation of a display as composed of

two separate objects behind a screen based on output from the comparison process indicating

that the surfaces were quite different in shape, color, and pattern. Clearly, using configural knowledge in conjunction with featural information is essential for forming interpretations of displays and is a crucial part of the segrega-

tion process according to Needham et al. (1997). Thus, infants’ failure to use featural information to segregate a display could be the

result of a failure at the detection, processing,

or interpretation levels, each of which is necessary for producing a veridical interpretation of a display.

Concerning spatial and motion information, studies are in agreement that, early in the first year of life (at 3 and 4 months of age, respectively), infants can use the spatial layout

of the objects and the motion of the objects in a display to group their surfaces into units

(Kellman & Spelke, 1983; Kellman et al., 1986; Kestenbaum et al., 1987; Slater, Mat- tock, & Brown, 1990; Spelke et al., 1989). Thus, by 3 or 4 months of age, infants must possess the basic knowledge that spatially separate surfaces belong to different objects and that surfaces that move together typically belong to the same object (see Spelke’s principles of cohesion and boundedness in Spelke, 1991).

Researchers have also investigated at what point in development infants use featural information alone to segregate adjacent surfaces into different units or to group the visible surfaces of a partly occluded object into

Use of Object features 49

the same unit (Craton, 1996; Kellman & Spelke, 1983; Kellman et al., 1986; Kesten-

baum et al., 1987; Needham & Baillargeon, 1997, 1998; Schmidt, 1985; Schmidt & Spelke, 1984). Overall, the results of these

studies suggest that infants’ use of featural information to accomplish these tasks is likely

to emerge sometime between 4.5 and 8

months of age.

The first studies conducted to systemati-

cally investigate this question used displays

composed of a partly occluded object (Kell- man & Spelke, 1983; Kellman et al., 1986;

Schmidt, 1985; Schmidt & Spelke, 1984). For example, in their classic experiments, Kell- man and Spelke (1983) asked whether 4-

month-old infants used the shape, color, and texture of the visible portions of a center

occluded object to see them as connected behind the occluder. During the habituation trials, the infants saw a stationary rod whose

center was occluded by a block. Next, the

block was removed and the infants were shown two test displays: a complete rod, and an incomplete rod composed of the rod seg-

ments that were visible above and below the

block in the habituation display. The infants looked about equally at the two displays, suggesting that they were uncertain whether the

rod segments visible in the habituation display belonged to a single object that extended

behind the block, and were apparently unable to use the similar features of the visible portions of the rod to group them into a single unit.

In a more recent study, Craton (1996) investigated 5.5- and 6.5-month-old infants’ perception of a display similar to that used by Kellman and Spelke (1983). Craton’s display consisted of a yellow rectangle that was sup- ported from behind and positioned so that its center was hidden by a thin blue rectangular screen. During the test events, the screen was pulled to the side to reveal a complete or bro- ken display (this part of the design was similar to the Kellman and Spelke study). The results of this study (and a baseline condition) revealed that 6.5- but not 5.5-month-old

infants expected the visible portions of the

rectangle to be connected behind the occluder.

These findings suggest that infants begin to

use featural information to group together the visible portions of a stationary partly occluded object around 6.5 months of age.

A similar developmental pattern has been uncovered for infants’ use of featural informa-

tion to segregate the surfaces of adjacent objects into different units (Hofsten & Spelke, 1985; Kestenbaum et al., 1987; Needham &

Baillargeon, 1997; Spelke et al., 1993). At 3

months of age, infants have been found to group surfaces into a single unit if they are

touching, regardless of differences in shape and color (Spelke et al., 1993), or irrcolor and

pattern (Kestenbaum et al., 1987). By 4.5 months of age, infants perceive as ambiguous

a display composed of adjacent surfaces of different shape, color, and pattern (Needham & Baillargeon, 1998). In this study, infants

were shown two adjacent objects: a tall, blue

box on the right, and a zig-zag-edged yellow

cylinder touching the box on the left (see Fig- ure 1). After being familiarized with this display, the infants saw a gloved hand take hold

of the cylinder and move it a short distance to the side. Half of the infants saw the box move

with the cylinder when it was pulled (move- together event), and half saw the cylinder

move away from the box, which remained sta-

tionary throughout the event (move-apart event). The authors reasoned that if the infants thought the display was composed of two separate pieces, they should look longer at the

move-together than at the move apart event and that the reverse pattern of results would be obtained if the infants thought the display was

composed of a single unit. The results showed that the 4.5-month-old infants looked about equally at the two test events, indicating that they were uncertain about the connection between the cylinder and box, and were presumably unable to use the markedly different features of the objects to segregate their surfaces into separate units.

However, by 8 months of age, evidence has been found for infants’ segregating into sepa-


rate units adjacent surfaces of different shape,

color, and pattern, and their grouping together into a single unit adjacent surfaces of similar

shape, color, and pattern (Needham & Baillar- geon, 1997). The pattern of results described here suggests that infants may first consider

all adjacent surfaces to be connected, then

begin to use some kinds of featural information sometime after 4.5 months of age, and

finally perceive adjacent surfaces in accor-

dance with their featural properties by 8 months of age.

The first question addressed by the present

research was at what point between 4.5 and 8 months of age infants develop the ability to use the dissimilar features of two adjacent

objects to segregate their surfaces into two separate units. This question was examined in

Experiment 1 by testing two age groups between 4.5 and 8 months of age: 6.5- and 7.5-

month-old infants. The display used in the

present research was used in the studies

described above involving 4.5- and S-month- old infants (Needham & Baillargeon, 1997, 1998). This display consisted of a zig-zag- edged yellow cylinder on the left and a tall

blue box on the right (see Figure 1). As in the prior studies, half of the infants saw the move- together event, in which both objects moved together when the cylinder was pulled, and

half saw the move-apart event, in which the cylinder was pulled away from the stationary

box.

As in the prior studies, the rationale behind this experiment is based on the well-estab- lished finding that infants tend to look longer at events that violate their expectations than at events that confirm their expectations (Born- stein, 1985; Spelke, 1985). If the infants saw the display as consisting of two separate units, they should look reliably longer at the move- together than at the move-apart event, just as the S-month-old infants did in Needham & Baillargeon (1997). In contrast, if the infants saw the display as ambiguous, they would look at the move-apart and move-together events about equally, just as the 4.5-month-old infants did in the prior study (Needham &

Baillargeon, 1998). Because the logic of this methodology rests on the assumption that infants evaluate the test event within the con- text of their interpretation of the display formed during the familiarization trial, each infant was shown only one test event. If each

infant saw both test events, there would be two sources of surprise present in the infants’ responses to trials following the first test trial

that would be impossible to separate: 1) their

surprise (or lack thereof) at the composition of the display revealed during the test event relative to their initial interpretation of the static display, and 2) their surprise at the change in the nature of the objects from one test trial to the next, an occurrence that must be infre- quently observed in the real world.

EXPERIMENT 7

Method

Participants

The participants in this experiment were 36 healthy, full-term infants. Two age groups were included in this experiment: 6.5- and 7.5- month-old infants. Half of the infants were 6.5-month-olds, and ranged in age from 5 months, 26 days to 6 months, 22 days (M = 6 months, 11 days). Half of the infants saw the

move-apart event (M = 6 months, 9 days), and half saw the move-together event (M = 6 months, 12 days). Half of these infants were 7.5-month-olds, who ranged in age from 6 months, 25 days to 7 months, 16 days (M = 7 months, 8 days). Half of the infants saw the move-apart event (M = 7 months, 8 days), and half saw the move-together event (M = 7 months, 7 days). Half of these infants were male and half were female. One additional infant was tested and eliminated due to the inability of the primary observer to follow the infant’s gaze.

The infants in the present experiment and subsequent experiments were identified through public birth records and contacted via

Use of Object Features 51

letter and follow up telephone calls. They were reimbursed for their travel expenses but were not compensated for their participation.

Apparatus

The apparatus consisted of a wooden cubi- cle 200 cm high, 106 cm wide, and 49.5 cm deep. The infant faced an opening 56 cm high and 95 cm wide in the front wall of the apparatus. The floor of the apparatus was covered with pale blue cardboard with a clear Plexiglas cover (this allowed the felt-bottomed objects to move smoothly and silently across the apparatus floor). The side walls were painted white and the back wall was covered with brightly patterned white contact paper.

At the start of the test event, a zig-zag- edged cylinder and a rectangular box stood side by side on the floor of the apparatus. The cylinder was 22 cm long and 10 cm in diame-

ter. It consisted of a section of clothes dryer vent hose that was stuffed with Styrofoam so that it was rigid and formed a modified “C” shape with its ends curved slightly forward. The left end of the cylinder was covered with cardboard; the right end was covered with a thin metal disc. The entire cylinder was

painted bright yellow. The box was 35 cm high, 13 cm wide, and 13 cm deep. It was made of foam core and was covered with bright blue contact paper decorated with small white squares. One of the box’s corners faced the infants. The left rear wall of the box (not visible to the infants) had a magnet inset 3.5 cm from the bottom. The cylinder lay on the floor of the apparatus with its right, metallic end set against the box’s bottom magnet (the magnet made it possible for the box to move with the cylinder when the latter was pulled by the experimenter’s hand). The bottom surfaces of the cylinder and the box were covered with felt so they both slid smoothly and silently across the Plexiglas on the apparatus floor. The front 2.5 cm of the cylinder’s right end protruded from the box’s left corner; this pro- trusion was designed to make clear to the infants that the cylinder and box were adja-

cent. In its starting position, the box was 17.5

cm from the front edge of the apparatus and 3 1.5 cm from the right wall; the cylinder was 28 cm from the front edge of the apparatus and 33.5 cm from the left wall. Together, the cylinder and box subtended about 30 degrees (hori-

zontal) and 27 degrees (vertical) of visual angle from the infants’ viewpoint.

In each test event, the cylinder was pulled to the side by an experimenter’s right hand wearing a 59-cm-long lavender spandex glove. The hand entered the apparatus through an opening 55.5 cm high and 37.5 cm wide in the left wall. This opening was partially hidden by a white muslin curtain; the curtain and the experimenter were positioned in such a way that the infant could not see the experimenter’s face through this opening.

The infants were tested in a brightly lit room. Four clip-on lights (each with a 40-W light bulb) were attached to the back and side

walls of the apparatus to provide additional light. Two wooden frames, each 200 cm high and 69 cm wide and covered with blue cloth, stood at an angle on either side of the apparatus. These frames served to isolate the infants

from the experimental room. At the end of each trial, a curtain consisting of a white mus-

lin-covered frame 57 cm high and 98 cm wide was lowered in front of the opening in the front wall of the apparatus.

Events

Move-together event. At the start of each test trial, the experimenter’s right hand rested on the floor of the apparatus about half-way between the cylinder and the opening in the left wall. After a l-s pause, the hand grasped

the cylinder (1 s) and pulled it 14 cm to the left at the approximate rate of 7 cm/s (2 s). The cylinder and box moved as a single, rigid unit with no slight movements of one object relative to the other. The hand paused for 1 s and then pushed the cylinder and the box back to their starting positions (2 s). The hand then resumed its initial position on the apparatus floor (1 s). Each event cycle thus lasted about


8 s. Cycles were repeated without stop until

the computer signaled that the trial had ended

(see below). When this occurred, a second

experimenter lowered the curtain in front of

the apparatus.

Move-apart event. The move-apart event

was identical to that just described except that

only the cylinder moved: the box remained

stationary throughout the trial (see Figure 1

for a depiction of these events).

Procedure

During the experiment, each infant sat on

his or her parent’s lap in front of the apparatus.

The infant’s head was approximately 63.5 cm

from the box.

The infant’s looking behavior was moni-

tored by two observers who viewed the infant

through peepholes in the cloth-covered frames

on either side of the apparatus. The observers

were not told and could not determine whether

the infants were assigned to the move-apart or

the move-together condition.’ Each observer

held a button box connected to a Gateway

2000 microcomputer and depressed the button

when the infant attended to the events. Each

trial was divided into lOO-ms intervals, and

the computer determined in each interval

whether the two observers agreed on the direc-

tion of the infant’s gaze. Inter-observer agree-

ment was calculated for each trial on the basis

of the number of intervals in which the com-

puter registered agreement, out of the total

number of intervals in the trial. Agreement in

this experiment and in subsequent experi-

ments averaged 92% or more per trial per

infant. The input from the primary (more

experienced) observer was used to determine

the end of the trials

Test Events

Move-apart Event

Move-together Event FIGURE 1

Schematic diagram of the original cylinder-and-box display and the move-apart and move-together

test events seen by the infants in Experiment 1. In Experiment 1, the infants saw the stationary display

during one familiarization trial and then saw either the move-apart or the move-together test event on

three successive test trials.


Each infant first received a familiarization trial to acquaint him or her with the cylinder

and box in their starting positions and to allow the infant to produce an interpretation of the

display as composed of one or two units. The experimenter’s hand did not enter the appara-

tus during this trial, so as not to distract the infant. The trial ended when the infant either

(a) looked away from the cylinder and box for 2 consecutive seconds after having looked at

them for at least 10 cumulative seconds or (b) looked at the cylinder and box for 30 cumulative seconds without looking away for 2 consecutive seconds.

Following the familiarization trial, each

infant saw either the move-apart or the move- together test event on three successive trials. A

between participants design was employed in

this and subsequent experiments reported in this paper. Each test trial ended when the infant (a) looked away from the event for 2 consecutive seconds after having looked at it

for at least 8 cumulative seconds or (b) looked at the event for 60 cumulative seconds without looking away for 2 consecutive seconds.

Each infant in this experiment contributed a

full set of 3 test trials to the analyses.

RESULTS

Preliminary Analyses

Records of two of the infants’ looking times during the familiarization trial were lost due to computer error; the remaining 34 infants’ looking times were analyzed. No reli-

able difference was found between the looking times, during the familiarization trial, of the infants who would see the move-apart (M = 13.8, SD = 5.2) and the move-together (M = 15.9, SD = 5.4) test events, F(1,32) = 1.31,~ > .05.

A preliminary analysis revealed no effect of Sex on the infants’ looking times at the two test events (all F’s c 3.84, p > .05). The data were therefore collapsed over sex for subsequent analyses.

Main Analysis

The looking times of the infants in Experi- ment 1 at the two test events are shown in Fig- ure 2. Inspection of the graphs indicates that the 6.5-month-old infants looked slightly longer at the move-apart than at the move- together events, whereas the 7.5-month-old infants showed the opposite tendency. The infants’ looking times were summed across the three test trials and analyzed by means of a 2 x 2 Analysis of Variance (ANOVA) with Age (6.5- or 7.5-month-old infants) and Event (move-apart or move-together event) as between-participants variables. This analysis produced a significant Age x Event interaction, F(1, 32) = 10.21,~ < .005. Planned comparisons revealed that the 6.5-month-old infants looked about equally at the move-apart (M = 115.9, SD = 36.4) and move-together (M = 90.9, SD = 46.3) events, F( 1,32) = 2.82, p > .05, while the 7.5-month-old infants looked reliably longer at the move-together (M = 146.9, SD = 15.6) than at the move-apart event (M = 104.5, SD = 17.2), F(1, 32) = 8.07, p < .Ol.

There was also a significant main effect of Age, indicating that the 7.5-month-old infants (M = 125.7, SD = 27.0) looked reliably longer overall than the 6.5-month-old infants (M = 103.4, SD = 42.4), F(1, 32) = 4.46, p < .05. This effect was undoubtedly driven by the looking time of the 7.5-month-old infants who saw the move-together event: their average looking time was 146.9 s, which was 31 s longer than that of any other cell. No other effects were significant.

DISCUSSION

The results of Experiment 1, combined with those of previous research, suggest the following developmental progression in infants’ ability to segregate the cylinder-and-box display. At 4.5 and 6.5 months of age, infants are unable to form a clear interpretation of the display; they perceive the display to be ambiguous. In contrast, 7.5- and 8-month-old infants


6.5month-old infants 7.5month-old infants

? 180- B 180- t

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I Together

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Apart Together

FIGURE 2

Mean summed looking times of the 6.5- (left panel) and 7.5-month-old infants (right panel) in Experi-

ment 1. The results showed that the 7.5-, but not the 6.5-month-old infants looked reliably longer at

the move-together than at the move-apart event, an indication that they thought the display was com-

posed of two separate units.

see the cylinder and box as two separate, adjacent objects. A transition occurs around 7 months of age in infants’ ability to segregate the cylinder-and-box display.

To what should we attribute this change in infants’ segregation of the cylinder-and-box display? As outlined in the introduction, at least three explanations were possible. The first was that the infants failed to detect the available information. This explanation seemed unlikely, because most 4.5- and 6.5- month-old infants have the ability to perceive colors that approaches adult levels (Teller & Bornstein, 1987; Werner & Wooten, 1979), and most 6.5-month-old infants have visual acuity that approaches adult levels (Sokol, 1978; see Banks, 1983, for a comprehensive review of both of these literatures). Further- more, the cylinder and box differed in so many ways that even somewhat degraded visual information would probably be sufficient to determine that they were quite different in appearance. The second explanation was that the 7.5-month-old, but not the 6.5-month-old infants possessed configural knowledge, (i.e., expectations about how objects typically

look), that Needham et al. (1997) have sug-

gested is used by infants to interpret the featural information present in a display. The third possibility was that 6.5-month-old (and perhaps the 4.5-month-old) infants detected

the features in the display and possessed the configural knowledge to interpret that information, but were unable to process the featural information sufficiently to use this knowledge to form an interpretation of the display. Before accepting the notion that infants younger than 7.5 months of age lack configural knowledge, the possibility that processing difficulties were responsible for their segregation failures was investigated.

At least two changes in the perceptual- motor system that occur around the 7-month birthday could lead to advancements in infants’ perceptual and representational abilities. First, the developing sensitivity to pictorial depth cues could allow infants to make use of more of the information present in a two- or three-dimensional display to arrive at a veridical interpretation of the display (e.g., Yonas & Granrud, 1984). Even though kinetic information (which infants are sensitive to at birth or

Use of Object Features

soon after; see Slater, Mattock, & Brown, 1990 and Granrud, 1987) and binocular information (which infants typically become sensi-

tive to by about 5 months of age; see Yonas & Granrud, 1984; Fox, Aslin, Shea, & Dumais, 1980) would also specify the depth relations

between the objects in a three-dimensional display, redundant depth information provided by pictorial cues could help resolve any ambi- guities that could exist in the three-dimen-

sional shapes or the depth relations between the objects in the display. In addition, the

onset of self-produced locomotion could bring with it improved spatial representational abilities that would allow infants to form more accurate three-dimensional descriptions of a

display (e.g., Bai & Bertenthal, 1992;

Bertenthal, Campos, & Barrett, 1984).

These observations suggest that if the three-dimensional shapes of the objects or the three-dimensional layout of the objects in

55

space were less complex, 4.5- and 6.5-month-

old infants’ ability to encode, represent, and

compare the features of the objects could be

facilitated, thereby allowing the infants to

form a clear interpretation of the cylinder-and-

box display. This possibility was addressed in

Experiment 2.

EXPERIMENT 2

For this experiment, a display was created that

was identical to the cylinder and box display

used in Experiment 1, with two exceptions:

the cylinder was straightened, and the box was

turned so that one side, instead of a comer,

faced the infant. Because these changes

resulted in a display that looked simpler than

the original display, this new display is called

the “simplified display” (see Figure 3).

Simplified Cylinder-and-Box Display

Move-apart Event

Move-together Event

FIGURE 3 Schematic diagram of the simplified cylinder-and-box display and the move-apart and move-together

test events seen by the infants in Experiment 2. The infants in Experiment 2 saw the simplified cylin-

der-and-box display, and received a single familiarization trial before seeing either the move-apart or

the move-together test event.


Beyond these two basic differences between the original and simplified displays

(i.e., the shape of the cylinder and the orientation of the box), there were two differences in the relative positions of the cylinder and box.

First, the appearance of the boundary was different in the original and the simplified dis-

plays. In the original display, the box occluded part of the cylinder’s end, and this occlusion

created an unusual looking boundary between the objects (see Figure 1). In contrast, the

boundary between the cylinder and the box in the simplified display was parallel to the infants’ line of sight: a boundary aligned with one’s line of sight could be easier to assess

than a boundary at an angle as in the original display.

Secondly, the alignment of the front surfaces of the cylinder and box was different in the original and simplified displays. In the

original display, the cylinder’s ends were

curved forward into a C shape, and the box’s corner faced the infant-this led to pronounced differences in the distances between the infant and the front surface of each object. In the simplified display, in contrast, the front

surfaces of the cylinder and the box were aligned.

One important detail to note is that even

though the shapes of the cylinder and box

were probably easier to encode, represent, and compare in the simplified than in the original

display, the two objects were probably also more difficult to distinguish from each other in the simplified than in the original display. Whereas the front surfaces of the objects in the simplified display were straight and aligned, the front surfaces of the objects in the original display contained considerable depth variations, both within (3 cm for the cylinder,

10 cm for the box) and between (13 cm) the objects. This fact made it highly unlikely that any differences between the cylinder and box in low-level variables (e.g., luminance, disparity) were greater in the simplified than in the original display and could therefore contribute to infants’ success in segregating the simplified but not the original display.

If the 4.5- and 6.5-month-old infants were

unaware that markedly different-looking adjacent surfaces often belong to separate objects,

simplifying the features of the cylinder-and- box display should not affect the infants’ seg-

regation of the display: they should look equally at the move-together and move-apart

events with the simplified display just as they did with the original display.’ However, if the

infants have configural knowledge that would

allow them to see markedly different adjacent objects as separate from each other, but the complex features of the display in Experiment

1 impeded 4.5- and 6.5-month-old infants’ ability to form a clear interpretation of the original cylinder-and-box display, these younger infants (perhaps both the 4.5- and

6.5-month-olds) should be able to segregate the simplified display into two separate units.

If they do see the simplified cylinder-and-box

display as composed of two separate units, the

infants should look reliably longer at the move-together than at the move-apart event, just as the 7.5- and S-month-old infants did in the experiments involving the original cylin-

der-and-box display (Experiment 1 in the

present paper; Needham & Baillargeon, 1997).

Method

Participants

The participants in this experiment were 36

healthy, full-term infants. Twenty-four of the infants (the 4.5-month-olds) ranged in age

from 4 months, 0 days to 5 months, 9 days (M = 4 months, 11 days). Half of the infants

saw the move-apart event (A4 = 4 months, 13 days) and half saw the move-together event

(M = 4 months, 10 days). Half of the infants were male and half were female.

Twelve of the infants (the 6.5-month-olds) ranged in age from 5 months, 29 days to 6

months, 28 days (A4 = 6 months, 11 days). Half of the infants saw the move-apart event (A4 = 6 months, 11 days) and half saw the move-together event (M = 6 months, 11 days).


Half of the infants were male and half were

female.

RESULTS

Three additional infants were tested but not

included in the final sample because of proce-

dural errors.


Apparatus, Events, and Procedure

The apparatus, events, and procedure used

in Experiment 2 were the same as those used

in Experiment 1 with the following excep-

tions.

No reliable difference was found between the looking times, during the familiarization trial, of the infants who would see the move- apart (M = 17.2, SD = 6.0) and the move- together (M = 18.8, SD = 7.3) events,

F( 1,34) = 0.50.

A new cylinder was created that was identi-

cal to the one used in Experiment 1, except

that it was 20 cm long (approximately the

same horizontal extent as the original cylin-

der) and straight from one end to the other

(i.e., the cylinder’s ends were not curved into a

modified “c” shape like they were in the orig-

inal display). The box was positioned so that

its front side faced the infant. The circular end

of the cylinder met the box at its left side,

where the outside edges of the cylinder’s end

were aligned with both the front and bottom

surfaces of the box, creating alignment

between the front surface of the box and the

front surface of the cylinder.

A preliminary analysis revealed no effect of Sex on the infants’ looking time at the two test events (all F’s c 0.32). The data were therefore collapsed over sex for subsequent analyses.

Main Analysis

The looking times of the infants Experi- ment 2 at the test events are shown in Figure 4. Inspection of the graph reveals that the infants looked longer at the move-together than at the move-apart event. Each infant’s looking times for the three test trials were summed and analyzed as in Experiment 1, with Age (6.5 or 4.5-month-old infants) and Event (move-apart or move-together) as between participants factors.

Each infant in this experiment contributed a

full set of 3 test trials to the analyses. This analysis produced a significant main

effect of Event, F( 1,32) = 12.75, p < .Ol, indi-

180

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................................................................

80 _ _~.~.~.~_~.~_~.~.~.~.~.~.~.~.~.~ ................................

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30- ................................................................ ................................................................ ................................................................ ................................................................ ................................................................ ................................................................ ................................ ................................ ................................

Apart

FIGURE 4

Together

Mean summed looking times of the 4.5- and 6.5-month-old infants in Experiment 2. Infants of both

ages looked reliably longer at the move-together than at the move-apart event, indicating that they

perceived the display as consisting of two separate units.


eating that the infants looked reliably longer at

the move-together (M = 148.9 SD = 29.5) than at the move-apart (M = 112.1; SD = 29.5)

event. There was not a significant effect of Age, F( 1, 32) = 0.01, or a significant Age x

Event interaction, F( 1, 32) = 0.18. Further-

more, planned comparisons for the two age

groups indicated that both the 4.5month-old infants (move-together M = 147.7, SD = 33.1;

move-apart M= 114.1, SD = 21.9 ; F(l, 32) =

7.42, p < .05) and the 6.5-month-old infants

(move-together M = 151.1, SD = 23.4; move- apart M = 108.3, SD = 43.2; F(l, 32) = 6.0,~ < .05) looked reliably longer at the move-

together than at the move-apart event. No other effects were significant.

DISCUSSION

Based on prior results (Needham & Baillar- geon, in press-a) and those of Experiment 1,

evidence had been presented for the following developmental progression in infants’ perception of the original cylinder-and-box display: while the 4.5- and 6.5-month-old infants had

an indeterminate perception of the display,

both 7.5- and 8-month-old infants saw the dis-

play as composed of two separate units. The

results of Experiment 2 showed that both the 4.5- and the 6.5-month-old infants saw the simplified cylinder-and-box display as com-

posed of two separate pieces. Thus, while it was not until 7.5 months of age that infants could segregate the original cylinder-and-box display into two separate units, infants as

young as 4.5 months of age (the youngest infants tested) were able to segregate the simplified cylinder-and-box display into separate units. This three-month disparity in the suc-

cessful segregation of the cylinder and box was produced by the rather modest changes

made to the original cylinder-and-box display

to create the simplified display.

The results of this experiment suggest at least two conclusions about young infants’ object segregation. First, this is the first evidence that 4.5- and 6.5-month-old infants can

segregate into two units two adjacent objects with markedly different perceptual features.

Thus, infants as young as 4.5 months of age

apparently can make use of featural information to decide that different-looking adjacent

surfaces belong to separate objects. According to Needham et al. (1997), infants would be

unable to make such a judgment without configural knowledge: the knowledge that differ-

ent-looking surfaces tend to belong to

different objects. This demonstration could also be taken as evidence that infants have

configural knowledge by 4.5 months of age. Secondly, something about the changes made

in the original display to create the simplified display, perhaps simplifying the shapes or the

spatial layout of the objects, facilitated

infants’ segregation of the display. This issue

will be discussed further in the General Dis-

cussion.

EXPERIMENT 2A

Even though the original and simplified displays were very similar to each other and one

could consider the 4.5- and 6.5-month-old

infants’ responses to the original display

(equal looking at the move-apart and move-

together events) to be a control for their responses to the simplified display, an additional control experiment was conducted to

support the conclusion that the infants in Experiment 2 formed an interpretation of the display as composed of two separate units and

their responses to the test events were a result of this interpretation. In this experiment, a group of 4.5- and 6.5-month-old infants saw the test events seen by the infants in Experi- ment 2 without first receiving a familiarization

trial.

The reasoning behind this control can be

understood by re-examining the rationale behind the method used in Experiments 1 and 2. In these experiments, it was hypothesized that infants would (a) form an interpretation of the display as composed of a single unit or of two separate units (if they were capable of


doing so) when the objects were adjacent and stationary during the familiarization trial, (b) evaluate the test event they saw (either the move-apart or the move-together event) in

relation to the interpretation they had formed

in the familiarization trial, and then (c) respond to the test event with lengthened looking if it was inconsistent with their interpreta-

tion or with attenuated looking if the test event was consistent with their interpretation. According to this logic, presenting the infants

with the test event without first presenting

them with a familiarization trial should severely limit the time available to form an interpretation of the stationary display. In

Experiment 2, the infants had as long as 30 s to evaluate the stationary display, but in

Experiment 2A, there were approximately 2 s before the experimenter’s hand moved the object(s). If 2 s was not enough time for the

infants to segregate the display, they would be unable to evaluate the test event they saw in relation to their interpretation of the display because they would have no interpretation of the display.

The possibility remained that infants would

be able to formulate an idea of the composition of the display either within 2 s or in a

post-hoc manner. At the beginning of each

event cycle for both the move-together and move-apart events, the cylinder and box were adjacent and stationary (seen for two consecutive seconds as many as 15 times during a

trial), and it is possible that the infants could evaluate the movement of the cylinder (in the move-apart event) or the cylinder and box (in the move-together event) based on the appearance of the adjacent cylinder-and-box display. Thus, if the infants looked longer at the move-

together than at the move-apart event in the present experiment, it would not necessarily mean that they were responding to the events (in this and the prior experiment) in a superfi- cial manner that did not relate to object segregation. Instead, the infants in the present experiment could have evaluated the likelihood of the object motion they were viewing given the features of the cylinder and the box.

However, if the infants looked equally at the move-together and move-apart events, it

would suggest that the infants in Experiment 2

(a) used the familiarization trial to build an

interpretation of the display, (b) evaluated the

likelihood of the test event based on this inter-

pretation, (c) believed that the simplified cyl-

inder-and-box display was composed of a

separate cylinder and box that should move

independently and therefore (d) did not expect

to see them to move as one in the move-

together event.

Method

Participants

Participants were 20 healthy, full-term

infants. Ten of the infants were between 3

months, 26 days of age and 5 months, 1 day of

age (M = 4 months, 13 days) and ten were

between 6 months, 1 day and 6 months, 27

days (M = 6 months, 19 days). Half of the

infants saw the move-together event and half saw the move-apart event; half of the infants

were male and half were female. One addi-

tional infant was tested and eliminated from

the final analysis due to construction noise

during the experiment that distracted the

infant.


The apparatus, events, and procedure were

identical to that used in Experiment 2 with one exception: the familiarization event was not

included in the procedure. Each infant in this

experiment contributed a full set of 3 test trials

to the analyses.

RESULTS


A preliminary analysis revealed no effect of Sex on the infants’ looking time at the two

test events (all F’s c 2.07, p > .05). The data


were therefore collapsed over sex for subse- they tended to look at both events about

quent analyses. equally. No other effects were significant.

Main Analyses

The looking times of the infants in Experi-

ment 2A at the test events are shown in Figure

5. The infants’ looking times were analyzed as

in Experiment 2. This analysis produced no

significant effects. Specifically, there was not

a significant main effect of Event, F(1, 16) =

0.08, indicating that the infants did not look

reliably longer at the move-together (M =

105.0; SD = 36.3) than at the move-apart (M =

111.0; SD = 52.4) event. There was not a sig-

nificant effect of Age, F( 1, 16) = 0.12, or a

significant Age x Event interaction, F( 1, 16) =

0.4 1. Furthermore, planned comparisons for

the two age groups indicated that neither the

4.5-month-old infants (move-together M = 115.4, SD = 43.9; move-apart M = 107.9, SD = 62.8; F(1, 16) = 0.06) nor the 6.5-month-old

infants (move-together M = 94.7, SD = 27.9;

move-apart M = 114.1, SD = 47.0; F(1, 16) =

0.43) looked reliably longer at the move-

together than at the move-apart event; instead

To determine whether the results of Experi-

ment 2A were reliably different from those of Experiment 2, the data from both experiments were analyzed together by means of a 2 x 2

analysis of variance (ANOVA) with Familiar- ization (Familiarization or No Familiarization) and Event (move-apart or move-together) as

between-participants variables. This analysis produced a significant Familiarization x Event interaction, F( 1,52) = 4.61, p < .05, indicating

the existence of reliably different patterns of results for the infants who had received a familiarization trial before test and those who

had not.

DISCUSS/ON

The infants in Experiment 2 who saw the simplified display during familiarization and test looked reliably longer at the move-together

than at the move-apart event. In contrast, the

infants in Experiment 2A who saw the same display and test events, but did not receive a

Apart Tog&her

FIGURE 5

Mean summed looking times of the 4.5 and 6.5-month-old infants in Experiment 2A. Without a

familiarization trial during which the infants could form an interpretation of the stationary display, the

infants did not respond differentially to the test events.


familiarization trial, looked about equally at

the move-together and move-apart events. These results indicate that the infants in Experiment 2 (a) formed an interpretation of

the simplified display during the familiarization trial, (b) evaluated the test event they saw in comparison to this interpretation, and therefore (c) looked reliably longer at the move- together than at the move-apart event because

they expected the cylinder and box to be separate objects and were surprised to see them move together. Without the familiarization

trial, the infants did not form an interpretation

of the display and did not consider one test event to be more surprising than the other.

Together, the results of Experiments 2 and

2A provide strong evidence that infants as young as 4.5 months of age can (when given sufficient processing time) perceive different- looking adjacent objects as clearly separate from each other. Accounts of object segregation that involve the infant’s use of object knowledge to interpret featural information

(e.g., Needham et al., 1997) would hold that

these results are also evidence that, by 4.5

months of age infants possess configural knowledge (i.e., the knowledge that different- looking surfaces tend to belong to different units and similar-looking surfaces tend to

belong to the same unit).

In addition, these findings indicate that the specific features present in a display strongly

influence infants’ ability to segregate the display. Although the original and simplified displays shared many features (e.g., the sizes, colors, patterns, and textures of the two

objects), the relatively small differences between them resulted in a three-month discrepancy in the age at which infants could segregate the original and simplified displays: while 4.5-month-old infants (the youngest infants tested) were able to segregate the simplified display into two separate units, it was not until 7.5 months of age that infants suc- ceeded in segregating the original display into two units.

These results suggest that the most likely explanation for this three-month lag in infants’

segregation of the two displays concerns

infants’ developing information processing

abilities. As infants develop the perceptual and

cognitive resources necessary for encoding, representing, and comparing more and more complex object features, they can make use of

their configural knowledge to segregate dis-

plays containing more and more complex features .

Because the procedure employed in these

studies included a maximum looking time on

each trial of 60 s, it is possible that removing

the familiarization trial from the procedure

would have produced such an increase in

infants’ looking at the test events that the real differences in looking time between the

infants who saw the move-together than the

move-apart events would have been masked. Evidence against this possibility can be found

by comparing the overall level of looking of the infants in Experiment 2 (M = 130.5), who all received a familiarization trial before see-

ing the test events, with that of the infants in

Experiment 2A (M = lOS), who did not

receive a familiarization trial. The infants who

did not receive a familiarization trial looked

reliably less overall than the infants who did receive a familiarization trial (F( 1,52) = 5.11,

p c .05), indicating that the removal of the familiarization trial did not lead to a ceiling effect in the infants’ responses to the test

events. This point also supports the conclusion

made earlier: that withholding the familiariza-

tion trial from the infants limited too severely the time available for producing an interpretation of the display that would have allowed

them to evaluate the likelihood of the test

events.

EXPERIMENT 3

The results of Experiment 2 indicate that,

when the shapes and spatial orientations of the objects in a display are simple, infants as young as 4.5 months of age can use the featural differences between the objects to group their surfaces into two separate units. In


Experiment 3, confirming evidence for these findings was sought using partly occluded ver-

sions of the original and simplified cylinder- and-box displays. The question considered

here is whether occluding the connection between the cylinder and box in each display would have an impact on 4.5-month-old infants’ ability to segregate the displays into two separate units.

The partly occluded versions of the original and simplified cylinder-and-box displays

were created by placing a thin screen in front

of the boundary between the cylinder and the box in the original and simplified displays (See Figure 6). The test events were again move-apart and move-together events, but the

motion was produced in such a way that the boundary between the cylinder and box remained occluded throughout the entire experiment.

Recall that the 4.5-month-old infants who saw the original display looked about equally

at the move-apart and move-together events, indicating that they were unsure whether the cylinder and box were connected or separate. In contrast, the 4.5-month-old infants who saw

the simplified display looked reliably longer at the move-together than at the move-apart event, suggesting that they saw the display as composed of two separate units that they did

not expect to see moving together. If the infants in the present experiment could suc- cessfully segregate a partly occluded version of the simplified, but not the original display,

further support would be found for the claim that young infants’ segregation is strongly affected by the complex shapes or spatial layout of objects.

Method

Participants

Participants were 24 healthy, full-term infants ranging in age from 3 months, 23 days to 4 months, 12 days (M = 4 months, 5 days). Twelve of the infants saw the original occluded boundary display (M = 4 months, 4

days): half of these infants saw the move-apart event (M = 4 months, 6 days) and half saw the move-together event (M = 4 months, 2 days). Twelve of the infants saw the simplified

occluded boundary display (M = 4 months, 5 days): half of these infants saw the move-apart event (M = 4 months, 6 days) and half saw the move-together event (M = 4 months, 4 days). Fourteen of the infants were male and 10 were female. An additional 6 infants were tested but not included in the final sample: 5 because of procedural error and 1 because of fussiness.

Apparatus

The apparatus used in Experiment 3 was identical to that used in Experiments 1 (for the original occluded boundary display) and 2 (for

the simplified occluded boundary display) with the following exceptions.

To accommodate the motion of the objects

toward and away from the infant, the back wall of the apparatus was moved back slightly, creating an opening that was 53 cm (instead of 49.5 cm) deep.

A rectangular foam-core screen (measuring 35.5 cm tall and 10 cm wide) covered with bright green contact paper was placed in front of the boundary between the cylinder and the box. This screen occluded approximately the right 6 cm of the cylinder and the left 4.5 cm of the box from the infant’s perspective throughout the experiment.

Events

Just as in Experiments 1 and 2, the infants in Experiment 3 saw either a move-apart or a move-together event. These events were highly similar to the events shown to the infants in the first two experiments, with the following exceptions. Because the objective of this experiment was that the connection between the cylinder and box be occluded, it was important that the portions of the objects initially hidden by the screen remained hidden by the screen throughout the event. To ensure that this happened, the cylinder was moved by


Original Occluded Boundary Display

Move-apart Event

Move-together Event

Simplified Occluded Boundary Display

Move-apart Event

Move-together Event

FIGURE 6

Schematic diagram of the test events featuring the original occluded boundary display (top) and the

simplified occluded boundary display (bottom). The infants in Experiment 3 saw either the original or

the simplified occluded boundary display, and received a single familiarization trial before seeing

either the move-apart or the move-together test event.


the hand away from and then toward the infant instead of away from and then toward the box. This movement in depth made it possible to produce a move-apart and a move-together event while keeping the boundary between the cylinder and box hidden by the screen throughout the event.

Procedure

The procedure followed in Experiment 3 was the same as that followed in Experiments 1 and 2. Each infant in this experiment contributed a full set of 3 test trials to the analysis.

RESULTS


The looking times during the familiarization trial were analyzed for the infants in the four experimental groups. This 2 x 2 ANOVA revealed no reliable difference in looking at

160-

60-

Original Occluded Simplified Occluded Boundary Display Boundary Display

the familiarization event for infants who saw

the original occluded boundary display (move-apart M = 16.6, SD = 7.2; move- togetherM=21.5,SD=8.5;F(1,20)= 1.08,~ > .OS) or the simplified occluded boundary display (move-apart M = 18.7, SD = 9.0; move-together M = 26.9, SD = 7.6; F( 1, 20) =

3.07, p > .OS).


test events (all F’s < 1.62, p > .05). The data were therefore collapsed over sex for subse-

quent analyses.

Main Analysis

The looking times of the infants Experi- ment 3 at the test events are shown in Figure 7.

It can be seen that the infants who saw the original occluded boundary display looked about equally at the move-together and move-

apart events, whereas the infants who saw the simplified occluded boundary display looked

Apart Together Apart Together

FIGURE 7 Mean summed looking times of the 4.5-month-old infants in Experiment 3. The infants in Experiment

3 saw the simplified or the original occluded boundary display, and received a single familiarization

trial before seeing the test events. Paralleling the results of the experiments involving the fully visible displays, the infants saw the simplified, but not the original occluded boundary display as composed

of two separate units.


longer at the move-together than at the move- apart event.

The infants’ looking times were analyzed as in Experiment 1, with Display (original occluded boundary or simplified occluded

boundary display) and Event (move-apart or

move-together) as between participants factors. This analysis yielded a significant Dis-

play x Event interaction, F( 1,20) = 11.2, p c

.005. Planned comparisons revealed that this

interaction was produced by different patterns of looking by the infants who saw the original

occluded boundary and the simplified

occluded boundary displays. Specifically, the infants who saw the original occluded bound-

ary display looked about equally at the move- apart (M = 149.1, SD = 37.4) and the move-

together (A4 = 130.2, SD = 28.5) events,

F(1,20) = 0.85, whereas the infants who saw

the simplified occluded boundary display looked reliably longer at the move-together (A4 = 166.8, SD = 32.3) than at the move-apart

(M= 89.0, SD = 42.2) event, F(1,20) = 14.5,~ < .005. No other effects were significant.

DISCUSSION

The infants who saw the original occluded

boundary display looked about equally at the

move-together and move-apart events, but the infants who saw the simplified occluded boundary display looked reliably longer at the

move-together than at the move-apart event. These results suggest that, when a screen occluded the boundary between the cylinder and the box, 4.5-month-old infants saw the

original display as ambiguous and the simplified display as composed of two distinct units.

These results provide further evidence that, for simple objects at least, 4.5-month-old infants can use featural information to segregate dissimilar surfaces into separate units.

Beyond providing confirming evidence for the claims made in Experiment 2, the results of Experiment 3 can be used to make two additional points. First, this is the first published investigation of infants’ perception of a

single set of objects presented as a fully-visible adjacent display (in Experiments 1 and 2)

and as a partly occluded display (Experiment 3). Because no prior study had investigated infants’ perception of adjacent and partly occluded versions of the same display, differ-

ences between results of studies using adja-

cent and partly occluded objects could have been a result of basic differences in the processes underlying the perception of these two kinds of displays or they could have been a

result of the different objects or procedures used. The latter of these two possibilities seems especially likely in light of the results of Experiments 1 and 2 of the present paper, which show that small changes in seemingly

insignificant features of a display can lead to

dramatic differences in how infants perceive the display. Therefore, there was no precedent

for predicting whether the same or different responding would be expected for adjacent

and partly occluded versions of the same display. Thus, the present results provide new evidence that there could be basic similarities in the processes underlying infants’ segregation of adjacent and partly occluded objects.

These results also provide some information about what the crucial difference was

between the original and simplified displays that led to the discrepancy in infants’ segrega-

tion of these two displays (or, more accurately, what the crucial difference was not). Because the infants’ responses to the original and simplified events were not affected by the introduction of a screen that occluded the boundary

between the cylinder and box, we can con- clude that aspects of the appearance of this boundary (such as how visible the boundary was or the amount or kind of visible contact between the cylinder and box) did not produce the differences in the infants’ perception of these two displays.

One might be concerned that these results contradict those of Kellman and Spelke (1983), because they are evidence of 4.5- month-old infants use of featural information to segregate a partly occluded display. But further reflection reveals that there is no contra-


diction, for at least two reasons. First, Kellman and Spelke’s participants were 4 months of age, and the infants in the present study were a bit older than that, so developmental differences could explain the differences in Kellman and Spelke’s results and the present results. A more likely explanation concerns the displays used in the two sets of experiments. Kellman and Spelke did not use a display consisting of stationary objects whose visible portions were dissimilar. In different experiments, they used a stationary object whose visible portions were similar, a moving object whose visible portions were similar, and a moving object whose visible portions were dissimilar, but not a stationary object whose visible portions were dissimilar. As was mentioned in the introduction, infants might be able to use prominent featural differences in stationary displays even if they would not use this information when the motion of the objects in the display led to a contradictory interpretation of the display.

EXPERIMENT 3A

To provide a comparison for the responses of the infants who saw the simplified occluded boundary display in Experiment 3, a group of infants was shown this display with no familiarization trial preceding the test trials. As was the case for Experiment 2A, the rationale behind this control experiment was that without a familiarization trial during which the infants could view the stationary display, they would not have the opportunity to form an interpretation of the display before the composition of the display (as a single unit or as two units) was revealed.

Method

Participants

The participants were 10 infants ranging in age from 3 months, 14 days to 5 months, 4 days (M = 4 months, 10 days). Half of the

infants saw the move-apart event (M = 4

months, 5 days) and half saw the move- together event (M = 4 months, 15 days). Six of the infants were male and 4 were female.


The apparatus, events and procedure for

Experiment 3A were identical to those used in Experiment 3 (for the simplified occluded boundary display) with the following excep-

tion: the infants did not receive a familiarization trial before seeing the test events. Each infant in this experiment contributed a full set of 3 test trials to the analysis.

RESULTS

Preliminary Analysis


test events (all F’s < 0.19). The data were

therefore collapsed over sex for subsequent analyses.

Main Analysis

The looking times of the infants in Experi- ments 3 and 3A at the test events are shown in Figure 8. The looking times were analyzed

with the data from the infants in Experiment 3 who saw the simplified occluded boundary display. Thus, all of the infants in this analysis

saw the same test events; some of the infants were given a familiarization trial before seeing the test events (the infants from Experiment 3) and some infants saw the test events without receiving a familiarization trial first. The analysis was the same as that performed on Exper- iment l’s data, with Familiarization condition (one familiarization trial or no familiarization trial) and Event (move-apart or move-together event) as between participants factors.

This analysis produced a significant Famil- iarization condition x Event interaction, F(1,18) = 10.61, p < .005. Planned compari-


No Familiarization One Familiarization A :: 180 5 1 T

180

150

1

T

E 150- i= ......................................................................

F 120- .:i::::::::::::::::::::::::::::::: ................................. ................................. :::::::::::::::::::;::::::::::::::: 120- ................................. ................................. ...................................................................... * .................................

E :::::::::::::::::::y::::::::::::::. ................................. ................................. .................................

8 ................................. .................................

go- ................................. ................................. d .:::::::::::::::::::::::::::::::::: .................................

:::::::::::::::::::::::::::::::::::: ................................. .....................................................................

3 :::::::::::::::::::::::::i:::::::::: ..................................................................... ..................................................................... :::::::::::::::::::::::::::::::::::: ...................................................

80- .................................................................... 80 _ ::::j:::::::::::::::::::::~::i:: ................................. ................... :::::::::::::::::::::::::::::::::::: ..................................................................... ......................................................................................................

::::::::::::::::j:::::::::::::::::: ...................................................................................................... ..................................................................... :::::::::::::::::::::::::::::::::::: ..................................................................... .................................................................. ..................................................................... .:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: .....................................................................

; 90 _ ::::::::::::::::::::::::::::::::::::i iiiiiiiiifiiiiiiiiiiiiiiiiiiiiiii .................................................................. .................................................................. .................................

......................................................................... 90 - :::::::::::::::i:::::::::::::::: ..................................... E

..:.:.:.:.:.:.:.:.:.:.~:.~:.:.:.:. ..................................................................... ::::::::::::::::::::::::j::::::: .................................... ..................................................................

:::::::::::::::::::i::::::::::::. ...................................................................................................... .................. ..................................................................

:.:.:.:.:.:.:.:_:.:_:.:.:.:.:.:.:.: 0 ;..- .I

:.:.:.:.:.:.:.:.:.:.~:.:.:.:.:.:.:. .................................................................. t

...................................................................................................... 0 .................................

Apart Together Apart Tog&her

FIGURE 8

Mean summed looking times of the 4.5month-old infants in Experiment 3A (left panel) and of the

infants from Experiment 3 who saw the simplified occluded boundary display (right panel). The data in

both panels is from infants who saw the simplified occluded boundary display; the only difference

between them is that the infants in Experiment 3A (left panel) received no familiarization trial and the

infants in Experiment 3 (right panel) received one familiarization trial. Paralleling the results of Experi-

ments 2 and 2A, without a familiarization trial during which the infants could form an interpretation of

the stationary simplified occluded boundary display, the infants do not respond differentially to the test

events involving this display. . .

sons revealed that, while the infants who

received a familiarization trial looked reliably

longer at the move-together (M = 166.8, SD = 32.3) than at the move-apart event (M = 89.0,

SD = 42.2), F(1, 18) = 11.39, p c .005, the

infants who received no familiarization trial

looked about equally at the move-together (M

= 121.4, SD = 47.1) and move-apart events (M

= 155.0, SD = 37.9), F(1, 18) = 1.77, p > .05.

No other effects were significant.

DISCUSSION

The infants in Experiment 3 who saw the sim-

plified occluded boundary display during

familiarization and test looked reliably longer

at the move-together than at the move-apart

event. In contrast, the infants in Experiment

3A who saw the same display and test events,

but did not receive a familiarization trial,

looked equally at the move-together and

move-apart events. These results indicate that

the infants in Experiment 3 (a) formed an

interpretation of the simplified occluded

boundary display during the familiarization

trial, (b) evaluated the test event they saw in

comparison to this interpretation, and there-

fore (c) looked reliably longer at the move-

together than at the move-apart event because

they expected the cylinder and box to be sepa-

rate objects and were surprised to see them

move together. Without the familiarization trial, the infants did not form an interpretation

of the display and did not consider one test event to be more surprising than the other.

Together, the results of Experiments 3 and

3A provide additional evidence that 4.5-

month-old infants use featural information to infer that surfaces with markedly different features belong to separate units.

As was the case when comparing Experi-

ments 2 and 2A, it is unlikely that the lack of a familiarization trial in the present experiment merely created a ceiling effect in the infants’

responses to the test events, masking differ-


ences between the responses of the infants

who saw the move-together and move-apart test events. The overall level of looking of the

infants who saw the simplified occluded

boundary display in Experiment 3 with the

familiarization trial prior to testing (M =

127.9) was quite comparable to that of the infants in Experiment 3A (M = 138.2) who

saw the same display and events without a familiarization trial prior to testing. This point

also supports the conclusion made earlier: that withholding the familiarization trial from the

infants limited too severely the time available

for producing an interpretation of the display

that would have allowed the infants to evaluate the likelihood of the test events.

GENERAL DlSCUSSlON

The experiments presented in this paper

involved two displays: the original cylinder- and-box display and the simplified cylinder-

and-box display (see Figures 1 and 3). The

two displays were quite similar to each other

except that the shapes and spatial layout of the objects in the simplified display were less complex than those in the original display. In

Experiments 1 and 2, it was found that 7.5 month-old infants were the youngest infants

who could segregate the original cylinder-and- box display into two units, but infants as

young as 4.5 months of age could segregate the simplified display into two units. These

findings suggest that 4.5month-old infants expect dissimilar surfaces (even if they are

adjacent and stationary) to belong to separate objects, but their ability to demonstrate this expectation is limited to displays that do not

place heavy demands on their processing resources. By 7.5 months of age, infants’ information processing skills (e.g., their abil-

ity to encode, represent, and compare the three-dimensional shapes and arrangement of the surfaces) have improved to the point that their segregation of the original display is no longer affected. In Experiments 3 and 3A, boundary-occluded versions of the original

and simplified displays were shown to 4.5- month-old infants, and the results showed that there was no difference in the way that the infants interpreted adjacent and partly

occluded versions of the two displays. These results provide further evidence for young

infants’ use of featural information in object

segregation, demonstrating that infants can use the features of partly occluded objects to segregate a display into separate units.

The findings of the present experiments

have a number of implications for the literature on object perception in infancy. First, they provide the first evidence that 4.5-month-old

infants can locate object boundaries using the featural information present in the display,

which in turn suggests that infants this age

have expectations about how objects typically look (i.e., configural knowledge). Secondly, they indicate that the specific features present in the object surfaces have a pronounced effect

on infants’ ability to segregate the display into its component parts. Displays containing complex features that place high demands on infants’ processing capacities are difficult for young infants to segregate, not because they

lack expectations about how objects typically look, but because they cannot adequately

encode, represent, or compare the different portions of the display. According to this view, simple processing difficulties, rather than a

lack of knowledge, could explain young infants’ failure to segregate displays used in previous research (Johnson & Aslin, 1995; Needham et al., 1997).

For example, in a set of experiments con-

ducted by Spelke et al. (1993), 5-month-old infants responded differently to a display composed of two differently shaped and colored portions than they responded to a similar dis-

play that was uniform in color and shape. However, the infants did not seem to expect the former display to be composed of two separate units. According to the present results, one possible interpretation of these findings is based on a detail of the construction of the objects that might usually go unnoticed: they were made of stacked pieces of foam core,


resulting in objects composed of many edges (and many possible boundaries or points of separation). These edges could have produced greater complexity in the display from the infants’ perspective and impeded their forma- tion of a clear interpretation of the display.

Displays that are too complex for infants’ processing capacities could underestimate the extent of infants’ configural knowledge. Con- flicts within the literature regarding when infants first become capable of using featural information could be resolved by comparing

the displays used in the different experiments and determining whether the displays contain

features that could be difficult for young infants to encode, represent, or compare (Needham et al., 1997) and testing to see whether simplifying those features could lead to infants’ success in segregating the displays.

Finally, because the 4.5-month-old infants

perceived the simplified display as two separate units and the original display as ambiguous whether the displays were fully visible or partly occluded, one implication of these results could be that similar processes are responsible for infants’ perception of adjacent and partly occluded objects. These results provide the first evidence that is relevant for this issue, which is important for gaining a complete understanding of the mechanisms

responsible for object perception during infancy and into adulthood.

Which Features Are Used?

The finding that 4.5-month-old infants’ responses to the original and simplified displays were the same in the adjacent and partly occluded versions of the displays suggest that the appearance or visibility of the boundary in the original display was not the primary prob- lematic factor in infants’ segregation of that display. These results along with preliminary results from experiments that have investigated the effects of removing the ridges of the cylinder (by wrapping the cylinder in felt), or straightening the front surface of the box (by essentially slicing the box in half from top to

bottom along the diagonal, creating a triangu- lar box) suggest that the question of which feature is the crucial differentiating feature does not have a simple answer. Instead, it seems that what may be the crucial difference between these two displays (from the infants’

perspective) is a constellation of features that produce a higher or lower level of complexity

(see Johnson & Aslin, 1996 for a similar argument) .

Complexity

Clearly, the concept of stimulus complexity is rich and not easy to define. No definition for “complexity” is given in this paper, although prior investigations of infants’ visual perception have attempted to find a metric for complexity, considering factors such as number of stimulus elements (Greenberg & O’Donnell, 1972; Munsinger & Weir, 1967). However, the number of elements in a display is not the only

dimension on which a display could be judged to be complex; other factors such as whether the display seems organized (simple) or disorganized (complex) are sometimes considered when rating the complexity of a display (Banks, 1983).

Studies of infant habituation and recognition memory have also involved different levels of stimulus complexity, leading to interesting conclusions about the time course and developmental course of stimulus encoding. Using checkerboard patterns as stimuli, studies have shown that younger infants need more time to encode a given display than older

infants do (Martin, 1975). Also, within a given age, infants need more time to encode displays with more elements than displays with fewer elements (Caron & Caron, 1968, 1969). These findings suggest that “number of elements,” even if it does not provide a comprehensive definition of complexity, is certainly an important factor in infants’ stimulus encoding (see Banks, 1983 and Werner & Perlmutter, 1979 for interesting discussions of the complexity issue as it relates to pattern preferences and recognition memory).


In the present research, in what specific ways were the objects in the original display more complex than in the simplified display?

For the curved cylinder, it was not just the curvature that could have been difficult to appre-

hend: the curved shape produced different orientations of the cylinder’s ridges. Only the ridges at the center of the curved cylinder were parallel to the infant’s line of sight, while the other ridges were at an angle. This collection of orientations could seem somewhat disorganized in comparison to the straight

cylinder whose ridges were all parallel to each other. In the case of the box, the corner view of the box seen in the original display contained more lines and angles than the side view seen in the simplified display. Complex- ity captures the differences between the original and simplified displays in an intuitively appealing way, even if the cylinder’s complexity is a result of one set of factors (i.e., disor- ganization and curvature) and the box’s complexity is a result of another (i.e., number

of contour lines and angles).

Clearly, there are many features that could

be involved in our ratings of complexity for a given object display, including the objects’ shapes, patterns, and spatial arrangements. Do each of these featural components contribute to a display’s complexity in the same way? This question might have a different answer for adults and infants; it might even have a different answer for infants of different ages. The present research tells us that the complexity of the objects in a display is likely to affect infants’ ability to process the visual stimuli and therefore affect their ability to segregate the display. Specifying the many ways in which complexity affects the segregation process is an interesting question that deserves further study.

Categorization and Object Segregation

Some results relevant for the interpretation offered for the present research can be found in the literature on infants’ categorization of patterns and objects. In one study, Younger

and Gotlieb (1988) investigated 3-, 5-, and 7- month-old infants’ ability to categorize two-

dimensional dot patterns representing simple,

“good” forms, intermediate forms, or complex forms. Their results revealed that 3-month-old

infants categorized only the simple forms, 5- month-olds categorized the simple and the

intermediate forms, and 7-month-olds categorized patterns of all three levels of complexity. These results support the idea that older infants (i.e., infants around 7 months of age)

are better able to encode, represent and compare more complex visual stimuli than are younger infants (i.e., 3- to 5-month-old infants). Thus, these findings provide evidence in favor of the argument presented for

the results of the present experiments: that the 4.5- and 6.5-month-old infants were unable to

form an interpretation of the original display because its features were too complex for infants this age to encode, represent, or com-

pare, and that simplifying the features of this display facilitated the younger infants’ segregation of the display.

Another more cognitive approach to an

explanation for the present results can be found in the work of Rosch and her colleagues

(Rosch, Mervis, Gray, Johnson, & Boyes- Braem, 1976). In this research, the authors explored a number of ways to consider mem- bers of basic level categories similar to each other. One way they attempted was to examine the similarity of the shapes of different

instances of basic level objects. One important factor they needed to establish for each category was in what orientation the objects in that

category are typically imagined. Interestingly, there was considerable agreement across participants on which viewing orientation was considered canonical: for clothing and fumi-

ture, it was the front view, whereas for vehi- cles and animals, it was the side view.

The fact that there was agreement on the canonical view for a class of objects suggests that there could be a generalized representation for that kind of object (Rosch and her colleagues go on to hypothesize that the basic level is special because it is the level at which

Use of Objecf Features 71

you can imagine an object of a particular

shape while still representing a large amount of information in the image). This basic-level representation is presumably stored in memory and used to compare with new potential instances of that class of objects. This line of research could have relevance for the present experiments because it suggests that infants’ interpretations could be influenced by their comparison of parts of the display with representations of classes of objects that are stored

in memory.

Perhaps the objects in the simplified cylinder-and-box display were matched with the

stored basic level representations for “rectangle” and “cylinder” (even if these are not basic level categories for adults, they could be for infants), whereas the objects in the original display did not match with any stored repre-

sentations. Infants might not have stored representations for cylinders of different curvature, and it might be difficult for infants to encode and represent the specific curvature

of a new cylinder. Even though the box is a simple geometric shape, the unusual viewpoint infants have on the box results in a complex projection of this simple shape. Making use of a stored canonical-view representation of a rectangular solid that could be a “match” for the corner-facing box would necessitate infants’ transforming the corner-facing view into the side-facing view (or vice-versa). This process, that seems akin to mental rotation (Cooper, 1975; Shepard & Metzler, 1971), might deplete the cognitive resources of younger, but not older, infants.

Origins of Configural Knowledge

In the present research, the youngest infants tested were 4.5 months of age; their results showed that they were able to segregate the simplified display into two separate units. This leaves open the question of when young infants first use object features to group object surfaces into units.

Searching for the time at which infants first demonstrate the use of featural information in

segregating objects would begin to shape explanations of the mechanism underlying the

development of infants’ configural knowledge. If feature-based segregation first became evident at or soon after 4 months of age, there would be evidence in favor of a manual experience-based explanation. Research by Rochat (1989) on the development of object manipulation skills between 2 and 5 months of age

showed that there is a change between 3 and 4 months of age in how infants first examine a

novel object. While 3-month-old infants bring

the object to their mouths first for oral exploration, 4-month-old infants bring the object to

their eyes first for visual exploration. Because of this change, 4-month-old infants would obtain more information about how objects look than 3-month-old infants do, and this could lead 4-month-old infants to make gener-

alizations about object appearance that would contribute to their configural knowledge.

If, however, evidence for feature-based segregation is found before infants engage in

spontaneous object manipulation, a manual experience-based explanation would seem less likely than an observational learning-based

explanation. According to an observation- based explanation, infants could learn about how objects typically look as a result of watching other people (e.g. parents or sib- lings) interact with objects during normal

daily activities: father lifts a bottle from the table and begins feeding baby, mother takes a

book off of the shelf and begins reading. Future research will help to decide which of

these kinds of explanations is more likely.

The present research also brings us closer to considering the possibility that the processes underlying infants’ organization of two-dimensional and three-dimensional displays are similar in origin. Over the past twenty years, there have been a number of demonstrations of 3- and 4-month-old infants’ use of gestalt-like principles to organize displays of two-dimensional elements (Atkinson

& Braddick, 1992; Ghim, 1990; Giffen & Haith, 1984; Koffka, 1935; Milewski, 1979; Quinn, Brown, & Streppa, 1997; Quinn,


Burke & Rush, 1993; Quinn & Eimas, 1986; Treiber & Wilcox, 1980; Wertheimer, 1958).

The present results suggest that infants’ use of featural information to segregate displays of three-dimensional objects may develop

according to a similar timetable.

CONCLUDING REMARKS

The experiments presented in this paper provide the first evidence that 4.5-month-old infants use featural information to segregate

stationary adjacent or partly occluded objects into two units. These results indicate that 4.5 month-old infants have the knowledge that

different-looking surfaces belong to different

units. The specific features present in a display (the proposal here is that it is the complexity

of the features that is especially influential)

play an important role in infants’ ability to

segregate the display. In the present research, a display composed of simply shaped objects in

straightforward spatial orientations was segre- gated into two units by infants as young as 4.5

months of age; the youngest infants who could segregate a nearly identical display composed of objects with more complex shapes and spa-

tial orientations were 7.5 months of age.

The present research indicates that more

information is available and used by young infants for the task of carving the three-dimen-

sional world into objects than was previously believed. Although there are clearly limitations in young infants’ ability to organize the three-dimensional world, these limitations seem to be in infants’ processing resources rather than in their understanding of how indi- vidual objects in the world tend to look.

Acknowledgment: This research was sup- ported by grants to the author from the

NICHD (FIRST grant # HD32129) and the Duke University Research Council. I would like to thank RenCe Baillargeon, Laura

Kotovsky, Warren Meek, and David Rubin for

helpful conversations about this research;

Laura Kotovsky and Warren Meek for their

detailed, constructive comments on earlier

drafts of the manuscript; Erika Holz for her

assistance with some of the data analyses;

Elizabeth Abrams, Susan Garland-Bengur,

Erika Holz, Scott Huettel, Jordy Kaufman,

Jennifer Lansford, Deborah Schkolne, and the

undergraduate students working in the Infant

Perception Lab at Duke University for their

help with the data collection; the Infant Cogni-

tion Lab at the University of Illinois for their

help in collecting a portion of the data of

Experiment 1; and the parents and infants who

generously spent their time participating in the

studies.

NOTES

Many of the conditions in experiments reported

in this paper were run simultaneously (there

were always at least 2 simultaneous conditions

involving different displays as well as two dif-

ferent events being run), making it difficult for

observers to determine which display (original,

simplified, etc.) or event (move-apart or move-

together) the infant was seeing. Immediately

following the experimental session for 110 of

the 126 subjects included in this paper, the pri-

mary observer was asked to guess which event

the infant had seen. For 61 out of these 110 ses-

sions, the primary observer correctly guessed

which event the infant had seen. This level of

accuracy (55%) is not different from chance

(50%) p = 0.38, using a normal approximation

of the exact binomial probability. These results

indicate that observers were unable to determine

which event the infant was watching during the

experiment.

It is possible that equal looking to the move-

apart and the move-together events is an indica-

tion of the feature processing failure proposed in

this section instead of the absence of contigural

knowledge. The results of Spelke et al. (1993)

and Kestenbaum et al. (1987) both suggest that

when infants lack configural knowledge they

expect adjacent surfaces to belong to the same

unit and look longer when the surfaces are

shown to be separate than when they are shown

to be connected.


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30 August 1996; Revised 12 May 1997m

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