INFANTS’ USE OF FEATURAL INFORMATION
IN
THE SEGREGATION
OF STATIONARY
OBJECTS
Amy Needham
Duke
University
Infants’ use of featural
investigated
information
(e.g. shape, color, pattern) to segregate
in three main experiments.
The first experiment
not younger infants, were able to form a clear interpretation
cylinder
and an adjacent
determined
tilted blue box as composed
stationary
displays was
showed that 7.5-month-old
infants, but
of a display consisting of a curved yellow
of two separate units. Subsequent
experiments
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
was fully visible or boundary-occluded).
the display
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 complex features could be difficult for younger, but not older infants to perceive.
perceptual
development
cognitive
development
adjacent
stationary
objects
INTRODUCTION
Imagine what an infant’s first glimpse inside
the family’s refrigerator would be like: a kaleidoscope 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
l
Amy Needham,
Department
of Psychology:
objects
partly occluded
object features
object complexity
objects
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 complicated 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
Experimental,
Duke University,
Durham,
NC, 27708-0086;
e-mail:
[email protected].
INFANT
BEHAVIOR
& DEVELOPMENT
21 (l), 1998,
Copyright 0 1998 ABLEX Publishing Corporation
pp. 47-76
ISSN 0163-6383
All rights of reproduction
in any form reserved.
48
INFANT
accurately, infants’ learning about the physical
world would certainly be impeded (Baillargeon, 1995; Kellman, 1996; Marr, 1982; Mandler, 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, & Kestenbaum, 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; Kellman, Spelke, & Short, 1986; Needham & Baillargeon,
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, Baillargeon, and Kaufman (1997), infants’ successful 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-
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
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 segregation 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, Mattock, & 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
the same unit (Craton, 1996; Kellman &
Spelke, 1983; Kellman et al., 1986; Kestenbaum 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 systematically investigate this question used displays
composed of a partly occluded object (Kellman & Spelke, 1983; Kellman et al., 1986;
Schmidt, 1985; Schmidt & Spelke, 1984). For
example, in their classic experiments,
Kellman and Spelke (1983) asked whether 4month-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 segments 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 supported 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 broken 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
49
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 information 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 Figure 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 (movetogether event), and half saw the cylinder
move away from the box, which remained stationary 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-
50
INFANT
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 & Baillargeon, 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 accordance 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 7.5month-old infants. The display used in the
present research was used in the studies
described above involving 4.5- and S-monthold infants (Needham & Baillargeon,
1997,
1998). This display consisted of a zig-zagedged 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 movetogether 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-established finding that infants tend to look longer
at events that violate their expectations than at
events that confirm their expectations (Bornstein, 1985; Spelke, 1985). If the infants saw
the display as consisting of two separate units,
they should look reliably longer at the movetogether 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 &
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
Baillargeon, 1998). Because the logic of this
methodology
rests on the assumption
that
infants evaluate the test event within the context 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 infrequently observed in the real world.
EXPERIMENT7
Method
Participants
The participants in this experiment were 36
healthy, full-term infants. Two age groups
were included in this experiment: 6.5- and 7.5month-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
were
identified
subsequent
experiments
through public birth records and contacted via
Use of Object Features
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 cubicle 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-zagedged 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 diameter. 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 protrusion was designed to make clear to the
infants that the cylinder and box were adja-
51
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 (horizontal) 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 muslin-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
52
INFANT
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 monitored by two observers who viewed the infant
through peepholes in the cloth-covered frames
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
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 direction of the infant’s gaze. Inter-observer agreement was calculated for each trial on the basis
of the number of intervals in which the computer registered agreement, out of the total
number of intervals in the trial. Agreement in
this experiment
and in subsequent
experiments 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
test events seen by the infants in Experiment
during one familiarization
three successive test trials.
display
1. In Experiment
and the move-apart
and move-together
1, the infants saw the stationary display
trial and then saw either the move-apart
or the move-together
test event on
Use of Object
Features
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 apparatus 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 movetogether 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 reliable 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.
53
Main Analysis
The looking times of the infants in Experiment 1 at the two test events are shown in Figure 2. Inspection of the graphs indicates that
the 6.5-month-old
infants
looked slightly
longer at the move-apart than at the movetogether 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
54
INFANT BEHAVIOR & DEVELOPMENT
6.5month-old
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B
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Vol. 21, No. 1, 1998
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I
Apart
Apart
Together
Together
FIGURE 2
Mean summed looking times of the 6.5- (left panel) and 7.5-month-old
ment
1. The results showed that the 7.5-, but not the 6.5-month-old
infants (right panel) in Experiinfants looked reliably
longer at
the move-together than at the move-apart event, an indication that they thought the display was composed 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 6.5month-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). Furthermore, 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 suggested 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 perceptualmotor 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
55
soon after; see Slater, Mattock, & Brown,
1990 and Granrud, 1987) and binocular information (which infants typically become sensitive 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 ambiguities that could exist in the three-dimensional 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
1992;
display
(e.g., Bai & Bertenthal,
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
space were less complex, 4.5- and 6.5-monthold 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-andbox 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
Move-apart
Display
Event
Move-together
Event
FIGURE 3
Schematic
diagram
of the simplified
cylinder-and-box
test events seen by the infants in Experiment
der-and-box
display, and received
the move-together
test event.
display and the move-apart
2. The infants in Experiment
a single familiarization
and move-together
2 saw the simplified
cylin-
trial before seeing either the move-apart
or
56
INFANT
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 displays. 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.
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
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-andbox display should not affect the infants’ segregation 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 cylinder-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).
Use
ofObject Features
Half of the infants
57
were male and half were
RESULTS
female.
Three additional infants were tested but not
included in the final sample because of procedural errors.
Apparatus,
Preliminary
Analyses
No reliable difference was found between
the looking times, during the familiarization
trial, of the infants who would see the moveapart (M = 17.2, SD = 6.0) and the movetogether (M = 18.8, SD = 7.3) events,
F( 1,34) = 0.50.
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.
Events, and Procedure
The apparatus, events, and procedure used
in Experiment 2 were the same as those used
in Experiment
1 with the following exceptions.
A new cylinder was created that was identical to the one used in Experiment 1, except
that it was 20 cm long (approximately
the
same horizontal extent as the original cylinder) 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 original 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.
Each infant in this experiment contributed a
full set of 3 test trials to the analyses.
Main Analysis
The looking times of the infants Experiment 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.
This analysis produced a significant main
effect of Event, F( 1,32) = 12.75, p < .Ol, indi-
180
................................................................
................................................................
................................................................
_~_i~.~_~.~_~.~_~_~.~.~.~ ......................................
................................
................................
................................
................................................................
80
_
_~.~.~.~_~.~_~.~.~.~.~.~.~.~.~.~
................................................................
................................
................................................................
................................
................................
................................................................
................................
_~_~.~_~.~.~_~_~_~_~.~.~.~.~.~.~
:.~:::
_._._ ~_~.~_._~_~_~_~_~
................................
................................
................................
................................
................................
................................
................................
................................................................
................................................................
................................
30-
................................
................................................................
................................................................
................................................................
................................................................
................................
................................
................................
Apart
Together
FIGURE 4
Mean
summed
ages looked
perceived
looking times of the 4.5- and 6.5-month-old
reliably
longer at the move-together
infants in Experiment
than at the move-apart
the display as consisting of two separate units.
event,
2. Infants of both
indicating
that they
58
INFANT
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. Furthermore, 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; moveapart M = 108.3, SD = 43.2; F(l, 32) = 6.0,~ <
.05) looked reliably longer at the movetogether than at the move-apart event. No
other effects were significant.
DISCUSSION
Based on prior results (Needham & Baillargeon, 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 display 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 composed 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 successful 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
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
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 different-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 Discussion.
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
to the original display
infants’ responses
(equal looking at the move-apart and movetogether 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 Experiment 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
Use of Object
Features
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 interpretation 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 movetogether 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 superficial 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.
59
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 interpretation, (c) believed that the simplified cylinder-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 movetogether 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 additional infant was tested and eliminated from
the final analysis due to construction
noise
during the experiment
that distracted
the
infant.
Apparatus,
Events, and Procedure
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
Preliminary
Analyses
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
60
INFANT
were therefore collapsed
quent analyses.
over sex for subse-
Main Analyses
The looking times of the infants in Experiment 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 significant 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 movetogether than at the move-apart event; instead
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
they tended to look at both events about
equally. No other effects were significant.
To determine whether the results of Experiment 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 Familiarization (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
familiarization
looking
times of the 4.5
and 6.5-month-old
infants in Experiment
trial during which the infants could form an interpretation
infants did not respond differentially
to the test events.
2A. Without
a
of the stationary display, the
Use of Object
61
Features
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 movetogether 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 differentlooking 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 differentlooking 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
the relatively
small differences
objects),
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 succeeded 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 displays 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 seeing 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 familiarization 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
62
INFANT
Experiment 3, confirming evidence for these
findings was sought using partly occluded versions of the original and simplified cylinderand-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 successfully 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
infants ranging in age from
to 4 months, 12 days (M =
Twelve of the infants
occluded boundary display
healthy, full-term
3 months, 23 days
4 months, 5 days).
saw the original
(M = 4 months, 4
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
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
Use of Object Features
63
Original Occluded Boundary Display
Move-apart
Event
Move-together
Event
Simplified Occluded Boundary Display
Move-apart
Move-together
Event
Event
FIGURE 6
Schematic
diagram
simplified
occluded
the simplified
of the test events featuring
boundary
occluded
either the move-apart
the original
occluded
boundary
display (bottom). The infants in Experiment
boundary
display,
or the move-together
and received
test event.
display
(top) and the
3 saw either the original
a single familiarization
trial before
or
seeing
INFANT
64
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.
& DEVELOPMENT
Vol. 21, No. 1, 1998
the familiarization
event for infants who saw
the original
occluded
boundary
display
(move-apart
M = 16.6, SD = 7.2; movetogetherM=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).
A preliminary analysis revealed no effect
of Sex on the infants’ looking time at the two
test events (all F’s < 1.62, p > .05). The data
were therefore collapsed over sex for subsequent analyses.
Main Analysis
RESULTS
Preliminary
BEHAVIOR
Analyses
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
The looking times of the infants Experiment 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 moveapart events, whereas the infants who saw the
simplified occluded boundary display looked
Original Occluded
Boundary Display
Simplified Occluded
Boundary Display
160-
60-
Together
Apart
Apart
Together
FIGURE7
Mean summed looking times of the 4.5-month-old
3 saw the simplified
or the original
trial before seeing the test events. Paralleling
displays,
the infants saw the simplified,
of two separate units.
infants in Experiment
occluded boundary
3. The infants
in Experiment
display, and received a single familiarization
the results of the experiments
but not the original
involving
occluded boundary
the fully visible
display as composed
Use of Object Features
longer at the move-together than at the moveapart 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 Display 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 boundary display looked about equally at the moveapart (M = 149.1, SD = 37.4) and the movetogether (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
65
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, differences between results of studies using adjacent 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’ segregation 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 conclude 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 4.5month-old infants use of featural information
to segregate a partly occluded display. But further reflection reveals that there is no contra-
INFANT
66
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
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
infants saw the move-apart
event (M = 4
months, 5 days) and half saw the movetogether event (M = 4 months, 15 days). Six of
the infants were male and 4 were female.
Apparatus,
Events, and Procedure
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 exception: 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
A preliminary analysis revealed no effect
of Sex on the infants’ looking time at the two
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 Experiments 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 Experiment 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 Familiarization
condition
x
Event
interaction,
F(1,18) = 10.61, p < .005. Planned compari-
67
Use of Object Features
No Familiarization
A
::
5
E
i=
F
180
1
150120-
E
8
d
3
80-
;
90 _
E
t
0
One Familiarization
180
T
T
150
......................................................................
1
.:i:::::::::::::::::::::::::::::::
:::::::::::::::::::;:::::::::::::::
......................................................................
120:::::::::::::::::::y::::::::::::::.
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.................................
.................................
.................................
*
go.::::::::::::::::::::::::::::::::::
.....................................................................
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80 _
::::j:::::::::::::::::::::~::i::
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iiiiiiiiifiiiiiiiiiiiiiiiiiiiiiii 90 :::::::::::::::i::::::::::::::::
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:.:.:.:.:.:.:.:.:.:.~:.:.:.:.:.:.:.
:.:.:.:.:.:.:.:_:.:_:.:.:.:.:.:.:.:
.................................
;...I
0
Apart
Together
Apart
Tog&her
FIGURE 8
Mean
summed
looking
infants from Experiment
times of the 4.5month-old
3 who saw the simplified
infants in Experiment
occluded
both panels is from infants who saw the simplified
between
them is that the infants in Experiment
infants in Experiment
boundary
occluded
3 (right panel) received one familiarization
ments 2 and 2A, without
the stationary simplified
a familiarization
occluded
boundary
3A (left panel) received
3A (left panel)
and of the
display (right panel). The data in
display;
the only difference
no familiarization
trial. Paralleling
trial and the
the results of Experi-
trial during which the infants could form an interpretation
boundary . display,
the infants do not respond differentially
.
of
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 simplified 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 therefore (c) looked reliably longer at the movetogether 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 3 and
3A provide additional
evidence
that 4.5month-old infants use featural information to
infer that surfaces with markedly different features belong to separate units.
As was the case when comparing Experiments 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-
INFANT
68
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 cylinderand-box display and the simplified cylinderand-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-andbox 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 ability 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
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
and simplified displays were shown to 4.5month-old infants, and the results showed that
there was no difference in the way that the
and
partly
infants
adjacent
interpreted
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 conducted 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 display 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,
69
Use of Object Features
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 formation of a clear interpretation
of the display.
Displays that are too complex for infants’ processing capacities could underestimate
the
extent of infants’ configural knowledge. Conflicts 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 comof the mechanisms
plete understanding
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 problematic 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 triangular 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).
70
INFANT
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 apprehend: 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. Complexity 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., disorganization
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
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
and Gotlieb (1988) investigated 3-, 5-, and 7month-old infants’ ability to categorize twodimensional dot patterns representing simple,
“good” forms, intermediate forms, or complex
forms. Their results revealed that 3-month-old
infants categorized only the simple forms, 5month-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 compare, 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, & BoyesBraem, 1976). In this research, the authors
explored a number of ways to consider members 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 fumiture, it was the front view, whereas for vehicles 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
71
Use of Objecf Features
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 representations. 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 generalizations 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 observationbased explanation, infants could learn about
how objects typically look as a result of
watching other people (e.g. parents or siblings) 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,
72
INFANT
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
objects
three-dimensional
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 segregated 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 spatial 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-dimensional 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 individual objects in the world tend to look.
This research was supported 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
Acknowledgment:
Kotovsky,
helpful
Laura
Warren
Meek,
conversations
Kotovsky
and David Rubin
about
and Warren
this
Meek
for
research;
for their
BEHAVIOR
& DEVELOPMENT
Vol. 21, No. 1, 1998
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 Cognition 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 different events being run), making it difficult for
observers to determine which display (original,
simplified, etc.) or event (move-apart or movetogether) the infant was seeing. Immediately
following the experimental session for 110 of
the 126 subjects included in this paper, the primary observer was asked to guess which event
the infant had seen. For 61 out of these 110 sessions, 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 moveapart and the move-together events is an indication 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.
Use of Object Features
73
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