Object segregation in infancy

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AovnrucES
IN
lrurnrucv RrseARcH
Vorunnr 11
Co-editors
Carolvn
Rovee-Collier
Departmeit of Psychology
Rutgers University
New Brunswick,
New
Jersey
Lewis P. Lipsitt
Department of Psychology
and
Child Study Center
Brown University
Providence. Rhode
lsland
Ablex Publishing Corporation
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'7.
8.
Anthony and G.ahan
criteria were that probes elicit a nonzero blink response
on at least two triiils in each
dull condition and on at least one ffial in each interesting condition.
5. Observers were
technicians unawate
of ihe
hypothesis
being tested, the
interest va.lue
of the visual foregrounds, and, at the time of ratings, the modality of the probe to be
delivered. Fixation was mted as off the slide,
on the slide
but off center,
or on slide
center,
6. A pilot study of four infanFmother pai$ showed that ary matemal response to probes
has negligible effects on infant blinking. Under conditions similar to those of the
main experiment, only the moiher received blink-eliciring acoustic plobes while the
infant observed
interesting
slides. Three
infantblinks, averaging three
digitized umts,
were recorded-
on the 40 trials of 43 that elicited measurable
maternal resDonse
aver-
aging
950 units.
L. D. SilversteiD and F, K. Groham,
Prychiphysiolo|y 15, 3't't (1918).
F.K, Gtahafi, in The
Orienting
Refex in Hunair, H. D. Kimmel, E. H. van Olst.
J. F
Orlebeke,
Eds. (Erlbaum,
Hiltsdate, N.J.,
1979),
p. l3?; F, K. Graham
and R. K, Ctif-
Lon, Psychol,
Btrll.65, 305 (1966);
J. I, Lacey
and B. C. Lacey,
in physiological
Cor-
relates
of Emotion, P Black, Ed. (Academic
Press, New york, 19?0),
p. 295; A.
Schell and J. Catania, Prychophysiology 12, t41 (1915).
The scoring program discarded trials if EMG activity was present at the time of probe
onset (indicating a bliok in progiess) or if peak latency exceeded
450 msec. Onset
and
peak
latetrcy windows
wete increased
beyond previously
described
values
[F.
K.
Graham, B. D. Stroch B. L. Zf,.rgIer, rn Aspects of the Deyelopment of Cothpetence,
W.
A. Collins, Ed, (Erlbaum,
Hillsdale,
N.J., 1981),
vol. 14, p. 1l ro
accommodate
the
longer latency of the visual blint reflex (meaD
onset
latencies; acounstic,
60 msec;
visual, 180 msec). For details of scoring prograrn,
see F. K. Graham,
L. E. putnam,
and
L. A, Leavitt U E P. Psychol: Human
Percept. Perfonn..i, 161
(19?5)l
or B. D.
Shock lthesis,
University of Wisconsin
(1981)].
B, J. Athony and
F. K. Graham,
unpublished data,
S. A. Hackley, M. A. thesis, University
of Wisconsin
(1981)
J. A. Deutsch
and.D. Deutsch,
PslcroL Reu 70, 80 (1963);
D. A. Notma\ ibid. 75,
522 (1968)t
R. M. Shiffrin and W. Schneide\
ibid, 84, t27 (1977)i
M.I. posner
and
D. R. Snyder,
in Infotmetion
Processing and CoBnitiotL R. L, Solso, Ed. (Erlbaum,
HiUsdale, N.J., 1975), p, 55.
Supponed by the W T. crant Foundarion,
NIH granr
HDOI490, NIMH fellowship
MHO1798 (to B.J.A.), and
Research Scientisr
Award K3MH21?62 (ro FK-c.). We
thank B. L. Zeigler for assistance
in programming
and circuir
design and K, M, Levin
and C. H. Wang
for serving
as observers.
9.
10.
11.
12.
13.
OBJECT
SEGREGATION IN INFANCY-
Amy Needham
DUK.E UM\ERSITY
Rende Baillargeon
Lisa Kaufinan
UMVERSITY OF ILLINOIS
r. TNTRODUCTTON .............2
IL THREE TYPES OF OBJECT KNOWIEDCE. . . , . . . . . . . . . . . . . . . , . . . . . . . . 2
IU, INFANTS'
USE OF CONFIGURAL
KNOWLEDGE. . . ..,,.,,,.4
A. PriorFindings.. ....... ... . ..
. .
4
B, GeneralMerhod. . . . . . . . . . . . . . . . . 7
C. Experiments
with Partly Occluded Displays
. . . . . . . . . . . , . . . 10
D. Experiments
with Adjacent
Displays. . . . ....... 16
E. Future Directions ....... ... ....23
IV. INFANTS' USE OF EXPERIENTIAL KNOWLEDGE, . , , , . . , , 25
A. Expeliments
with
Adjacent Displays.
, .......25
B. Future
Directions . . . . . . . . . . . . . . 31
V. INFANTS' USE OF PHYSICAL KNOWLEDGE . . , . . . . . . . . , . . 32
A. Experiment Involving Infants' Intuirions
About Support . . , . . . . . . . . . . . . 32
B, Experiment
Involving Infants' Intuitions About Impenerrabiliry . . . . . . . . . 35
C. Future Directions . . . . . . . . . . . . . . . 3'l
VI, CONCLUDING
REMARKS.
. ....,.,. 38
VII. REFERENCES ....,.,. 39
iThe research reponed in thi's chapter was supported by granrs ro the firs! author from the
National
lnsLitute
of Child Health
and Human Developmenr
(FIRST
Award HD-32129) and the Duke
University
Research Council, and
by grants
to the
second author from the National Institute
ofchild
Health and
Human Development (HD-21104
and HD-28686). We woutd like to thank
Jeny DeJong,
Cindy Fisher, and Laura Kotovsky for many helpful suggesrions.
Conespondence conceming rhis
chapter
should
be addressed ro Amy Needham,
Department ofPsychology: Experimenral,
Duke Uni-
versity,
Durham,
NC 27?08-0086.

Needhan, Baillaryeon and Kaulnan
I. INTRODUCTION
Object Seercgatio in InJancy
,-@
FiB. L A-F See
textfor description.
Conf.gural
knowledge refers to adults'
expectations about how objects typi-
cally appear: Adults recognize that objects
are
genemlly
regular in shape,
pattern,
color, and
texture. As a rcsult, adults
tend to group
sudaces that
present
the same
featural properties into the same ulits, and surfaces that present
different featural
properties
into separate unitsr Thus,
using configural knowledge
Lo analyze
the
adjacent display in Figure lC would enable us to see
the
box and ball as distinct
objects, because of thc marked
differences in their
feaLures. The same klowledge
applied to the
partly
occluded display in Figure
1D would lead us to group
the
por-
tions of the box visible
to the left and dght of the ball into a single
object, because
of the marked similarities
in their featural
properties.
Physical kJ'owled$e refers
to adults' beliefs about
the lawful ways in which
objects can move and interact,
such as the beliefs
that
objects
cannot remain s[able
without support and
cannot move through space occupied
by other objects. Use of
our
physical
knowledge
would lead
us to see the box and ball in Figure lE as a sin-
gle
unit, because we wouldrealize
that the ball could notretain
its
position
without
being attached to the
box.
Ex.peiential knowledge conesponds
to adults' knowledge
of what specific
objects, or types of objects,
exist in the world.
This knowledge involves represen-
tations of particular
objects as well as more
abstract represenlations of object cat-
egories, Once we had encountered
the display in Figure 1E, our expcriential
knowledge might
lead us to view Figure
1F as a single unit,
namely, the same
box-
and-ball display as in Figure lE, now shown in a diffelent
orientation.
How successful
are
young
infants
at segregating
stationary
adjacent
and
paily
occluded
displays
such as
those
in Figures
1C and lD? How do infants,
segrega-
tion abilities
develop
with age?
This chapter
addresses
these questions.
II. THREE
ryPES OF OBJECT KNOWLEDGE
A
EF

perceptions.
In the following sections,
we review research
conducted
in our laboratories
and
Needhan, Bai\argeon and Kaulman
III. INFANTS'
USE
OF
CONFIGURAL
KNOWLEDGE
Prior
Findings
infants
were shown
two test
displays:
a complete
rod, and
an incomplete
rod com_
posed
of the
two rod segments
that
were visible
above
and
below
the block
in the
habituation
display.
The infants
looked
about
equally
at the two displays,
suggest_
ing that they were uncertain
,,vhether
the rcd segments
visible
in fhe habituation
display belonged
to a single object that extended
behind
the block. The same
ambiguous
percept
was
obseryed
in subsequent
experiments
in which the rod was
replaced
with a triangular
rod figure (Kellman & Spelke,
19g3),
a two_dimen_
Object Segresation ih Infancy
Fie.2. Schenatic druwins of the simitu displays used in (A) and (B) Kellman ahd Spelke ( 1983); (C)
Termine
et qI. (1987); (D) and (E) Schnidt and Spelke (1984); (F) Craton (1993): and (C)
crcto( 1996).
B
A
D
F

Needhan, Bail laryeon and Kaufuan
sional surface
(Termine
et al., 1987)
a cube
or sphere
(e.g.,
Schmidt
& Spelke,
1984),
or, in experimeDts
conducted
with 5-month-old
infants,
a rcctangular
box
with jagged edges
(Cmton,
1993),
or a surface
with a salient
dot patterr (Craton,
1996).
These
results
were taken
to suggest that
infants
aged 4 to 5 months do not
attend to featural information when organizing partly occluded displays,
Investigations
of infants' perception
of adjacent
displays
led to a simiiar
con_
clusion
(e.g.,
Piaget,
i954; Spelke,
Breinlinger,
Jacobson,
& phillips, 1993).
For
example,
Spelke et
al. (1993)
habituated
s-month-old
infants
to a
bell_shaped
dis_
play mad€
ofthin concentric
rings
of foam core
that
were
painted
a uniform
color
and deconted with metallic stars
(see
Figure 3A). Following habituation,
the
infants saw two test events:
a move-together
arrd
a move-apart
event. In both
events, a hand
grasped
the top ofthe display
and
lifted it into the air In the move_
move-togelher
and move-apart
test
events,
as
beforc. The results
suggested
that
the
infants
preferred
the moye-together
over
the move-apart
event, as
though
they
perceived
the display
as composed
of two units
and hence
were
surprised
to see
it
move as one. Because
the difference
between
the responses
of the infants
who
A.
ObjecI Segresation in Infanct
Habituation
Event Test
l\ilove-together
Event
Similar
Condition
B.
Dissimilar
Condition
Fig. 3. Schenatic drawinS of Ihe even$ shown to the infants in the sir,,,ilar (3A) and fie dissimilar
(3
B
) condition in Spelke et al. ( 199 3
).
were shown the similar and the dissimilar
adjacent displays was only marginally
significant,
further expedments were
needed before any frrn conclusion
could be
reached. In the following sections, we report expedments conducted
in our labo-
ratories
that examined
young
infalts' responses to similar and dissimilar
partly
occluded and adjacent
displays. Before describing these
experiments, we first say
a few words about
our experimental
procedurc.
B. General
Method
The method
used in our experiments
was adapted from that devised by Spelke
et al.
(1993).
The infants
participated
i[ a two-phase
procedure
that consisted
of a
familiarization
phase
and a test
phase.
Duri\grhefahiliarization phase,
the
infants
werc
given
the
opportunity to observe a stationary display and form an interpreta-
tion of its composition,
Some displays were
partly occluded displays and
others
adjacent displays;
some displays were
composed of two similar
parts
and
others of
two dissimilar parts.
During the test
phase,
the infants
saw
test events
in which a
gloved
hand took hold of one
part
ofthe display and moved it a short
distance.
For
E

Ne edham, B a i
Ua
ryeon a nd Ka ufuan
half of the infants, the other
part in the display remained
stationary
(move-apart
condition); for the other infants,
the two parts
moved as
a whole (move-together
condition).
The rationale was that
ifthe infants
perceived
the stationary display as
a single unit, they would expect it to move as a whole and
be surpdsed when it did
not. Conve$ely, if the
infants viewed the stationary
display as composed
of more
than one unit, they would expect
the units to move independently
and be surprised
when they
did not, Because infants'
surprise at an event typically manifess itself
by prolonged
attention
to the event
(e.9.,
Bomstein, 1985;
Spelke, 1985b), the
infants were expected to look reliably longer at whichever test eveDt depicted the
motion inconsistent with their interpretation
of the stationary display.
In each experiment, the
infant sat on a
parent's
lap
in front of an apparatus
con-
sisting of a large display box. The parent
was asked to remain calm and neutral,
and to close his or her eyes
during the test trials,
The infant's looking behavior was monitored by two obseryers who watched
the infant through peepholes
in large cloth-covered frames on either side of the
apparatus.
The observers could not see the familiarization
and test
events from
their viewpoints, and the)a
were not told which condition and,/or experiment was
being conducted. Each observer
held a button box linked to a computer and
depressed
the button when the infant attended
to the events. Each
trial was divided
in 100-ms intervals, and the
computer determined in each
interval whether
the two
obseryers agreed on the direction of the infant's gaze. Interobseryer agreement
was calculated
for each
trial on the
basis of the number of intervals
in which the
computer registered agreement,
out of the total lumber of intervals in the trial.
Mean agreement
per trial per infant averaged,92lo
or highet across experiments.
The computer used the primary observer's looking times to determine the end of
the trials (see
below).
At the
back of the apparatus was
an expedmenter who produced
the familiar-
ization and test events.
The actions
of the experimenter
followed
precise,
second-
by-second scripts that were practiced
until they were performed
smoothly and
accumtely. A metronome
helped the experimenter adhere
to the scripts,
The infants received one to three familiarization trials at the start of the exper-
iment. A familiarization
trial typically ended
when the infant either (a) looked
away from the display for 2 consecutive
seconds or (b) looked at the display for a
maximum of 30 cumulative seconds.
Following the familiarization
trial(s), the
infants received two to six test trials
(experiments
with older
subjects typically had
two to four trials, and experiments
with younger
subjects three to six trials). In
each test trial, the event
(move-together
or move-apart
event) uTas repeated contin-
uolsly until the computer signaled
the end of the trial.
A test tial typically ended
when the infant either
(a)
looked away from the
event for 2 consecutiye
seconds or
(b) looked at the event for a maximum of 60 curnulative
seconds. When a trial
ended, an experimelter
lowered a curtain
in front of the apparatus,
During the
intertrial interyal, the test
objects were
quickly
retumed
to their starting
positions,
and the
curtain
\tras then again lifted to begin a new trial,
Objec! Segregation in I IancJ
AII of the experiments we report in the chapter made
use
of a between-subjects
designi The infants
saw either the move-together or the move-apart event across
test trials. We opted for this design rather than for the within-subjects design
adopted
by Spelke
et al.
(1993)
and others
(e.g.,
Kellman & Spelke,
1983;
Temine
el d., 1987) because
we were concemed about contamination
effects with
rcpeated alternating tials. Consider, for example, an infant who is presented
with
a similar adjacent display and
perceives
it as a single
unit. Let us assume that
on
the first test trial, the infani
sees
the move-apai event and on the second test trial
the move-together event. The
infant should
show surprise at the move-apart event
because it violates her interpretation of the display; the infant might also show
sur-
prise at the move-together event, however, because
it leads to an interpretation
(one
unit) inconsistent with that suggested by the move-apart
event
(two units)
presented
on
the
fi$t trial. Because such contamination
effects
would tend
to mask
differences in infants'
responses to the move-together and move-apart
gvents,
sub-
jects
were shown
the
same
event across trials.
Finally, the number of infants tested in each condition ranged from 6 to 18
across experiments. The infants'looking times at the move-apart
and move-
together test
events were typically compared by means ofanalyses of variance and
planned
comparisons. Results are repo ed as statistically reliable if the
p values
associated with them were equal to or smaller
than .05.
Similar Condition
Move-together Condition
Move-apart
Condition
FiB.4. Schena c dnwing ol the events shown to the infants in the sinilar condition in Needhan
et
at. (1997).

Needhan, Bai aryeon ond Kaufman
Dissimilar
Condition
Move-together
Condition
Move-apart
Condition
FiB. 5- Schenatic drawing of the eyents
shown to the infants in the dissimitar condition in Neeanam
el
at. (1997).
C. Experiments
With Parfly
Occluded Disptays
Our research
on young
infants' configural
knowledge
began
with an investiga-
tion of 3.5- ro 4.5-month-old infants, responses
to partly occluded
displays
(Needham,
Kaufman,
& Baillargeon,
1997).
The infants
were randomly
assigned
to either the
similar (see
Figure
4) or the dissimilar
condition (see
Figure
5). The
infants in the similar condition first received
three
familiarization
trials during
which they saw a stationary partly occluded
display consisting
of two rectangular
boxes
standing
behind the
left and
right edges
ofa tall blue
screen;
the boxes were
made ofred cardboard
and were
decorated
with small white
dots. The
infants
in the
dissimilar
condition
saw the
same red
box behind the
left edge
of the screen
and an
irregularly
shaped
box covered
with light green
fabric behind
the right edge of the
screen. After the familiarization trials, the infants saw a test
event in which a gloved
hand tookholdofthe rightbox and
pulled
it a
short distance
toward
the screen,
For
half of the infants
in each condition,
the
box to the
left of the
screen
moved with
therightbox (move-together
condifion);
for the otherinfants,
the left
box remained
stationary
tfuoughout
the event
(move-apan
condition).
The featural
similarity
(similar versus
dissimilar)
and motion (move-together
versus
move-apart)
factors
were thus completely
crossed
to form four different
experimental
groups.
Object Segregation in InJanct
Our reasoning
was as follows: If young
infants wele completely
insensitive
to
the featural properties
of displays, as
prior results with similar partly occluded dis-
plays
suggested
(e.9.,
cmton, 1993, 1995; Kellman & Spelke, 1983; Schmidt &
Spelke, 1984;
Temine et al., 1987),
there should be no difference between the
responses ofthe infants
in the similar and
the dissimilar conditions. The infants in
both conditions should look equally at the move-together and the move-apart
events.
However, if young infants possessed
some
limited ability to group surfaces on
the basis of their featural
properties,
as suggested by the results of Spelke
et al.
(1993),
then different
pattems
of looking might
be
obtained
in the similar and the
dissimilar
cdnditions. For example, the infants
in the similar
condition might look
equally at the move-apaft
and the move-together
events, suggesting an ambiguous
perception
of the similar display; in contrast, the infants in the dissimilar
condition
might look reliably longer at the move-together
than at [he move-aPart event,
revealing an unambiguous
perception of the dissimilar
display as composed
of
separate units.
To our surprise,
we found that reliably different
looking
pattems were
produced
by the
younger
(3 months, 16
days to 4 months, T days) and
the older
(4 months,
8 days
to 4 months, 14 days) infants in the
expedment, Let us first consider
the
results obtained with the
younger,
4-month-old infants.
The infants in the similar
condition tended to look equally
at the move-apart and move-together
events, su8-
gesting
that they (a) were uncertain whother
the two red boxes belonged
to the
same orto different units
and hence
(b)
couldnotpledict whether
the boxes should
move
together or apart. This finding was of course consistent with prior findings
conceming 4-month-old infants' responses to similar partly occluded disPlays
(e.g.,
Kellman & Spelke, 1983; Schmidt
& Spelke,
1984; Termine
et al., 1987).
Interestingly, the looking
patlem
observed in the dissimilar
condition was reliably
different
from that obtained in the similar
condition. The infants looked reliably
longer at the
move-together
than at the move-apart
event, suggesting
that
they
(a)
perceived
the red
and
green
boxes
as separate units
and thus
(b) expected the red
box
to remain stationary when the
green
box
was moved and
were
surprised in the
move-together
event wheD this expectation was
violated.
Let us now tum to the results obtained
with the older,
4,5-month-old
infants,
The infants in the dissimilar condition
performed
in the same manner as the 4-
month-old
infants. They looked reliably longer at the move-together than at the
move-apart event, suggesting
that they had an
unambiguous
percePtion
of the dis-
similar display as involving two distinct
units. Unlike the younger
infants,
how-
ever, the older infants
in the similar condition showed
a reliable
preference
for the
move-apart over
the
move-together
event, suggesting
that they
(a) viewed
the
sim-
ilar display as composed of a single
red box that extended behind
the
screen
and
therefore
(b) expected
this box to move as a whole and were surprised when it did
not,
II

Needhaa Baillaryeon
and Kaufnan
Because
the
rcsults
obtained
with the
4.5-month_old
infants
in the similar
con-
dition were
unexpected,
two additional groups
of4.s_month_olds
were
tested.
One
Together,
the results
presented
in this section
suggest
that,
contrary
to earlier
claims, young infants attend
to featural information when organizing
partly
occluded
displays.
Both the
4- and
the 4.5-month-olds
in the
dissimilar
cinOition
were led
by the
featural
differences
between
the
rectangular
red box and
the irreg_
ularly shaped green
box to view them as
distinct
objects.
Furthermore,
the 4.5-
month-old
infants
in the
similar condition
and
its replication perceived
the
identi-
cal boxes
on either
side ofthe screen
as belonging
to the
sami object.
These
find_
a
Unambiguous Ambiguous
l:.e. !:.Schinatf drcwih8 oJ the dissitnirat dispkys used in (A) Needhan et ar. ( t gsT); and
(B) ond
(C) Schnidt et al. (1986).
Object SegrcEation in Inlancy
ings provide evidence that, by 4 months of age, infants possess
configural
knowledge
and organize
at least some displays in accordance with thet featural
propertres.
The present
results also raise a number of questions
about apparent inconsis-
tencies
in young
infants' rcsponses to displays. Three such inconsistencies will be
noted
here. First, consider
the
results obtained in the dissimilar condition.
These
data are at odds with flndings
(see
Figure
6) reported by Schmidt et al. (1986).
In
this experiment,5-month-old infants were habituated to one oftwo dissimilarpartly
occluded displays involving nonsense forms visible on either side of a narrow
screen. In one display, the forms were different in shape but unifom in color. In the
other display, the forms differed in both shape and color and were also nonplanar
and misaligned. Following habituation, the
screen
wasremoved, and the
infants saw
complete and incomplete versions of the habituation displays. The infants in both
conditions
tended to look equally at the test displays they were shown, suggesting
that their perceptions
of the habituation displays were indeterminate.
How can one reconcile
these ambiguous
responses
with the positive
results
obtained in the present
research with the dissimilar display?
Could this discrep-
ancy reflect differences in the configural knowledge needed to correctly inter-
pret the various displays? A more likely explanation,
we believe, is that the
discrepancy
stems
from differences
in earlier steps in the
segregation
process.
In
order to bring thet configural knowledge to bear on a display, infants must
engage
in at least three
processes,
They must
(a)
encode or represent the featural
properties
of each
surface
in the display;
(b) compare the featural
properties
of
the different surfaces in the display; and Rnally (c) interpret the information
yielded by the first two processes
in light of their configural
knowledge. It
seems
plausible
that, the morc complex the surfaces used in a display, the less
likely infants aje to succeed in representing and comparing the surfaces.
According to this account, the subjects of Schmidt et al. (1986) would thus
have failed to segregate the displays they were shown, not because they lacked
the configural
knowledge
necessary to correctly
interpret
the
features of lhe dis-
plays,
but because the displays themselves were too complex to be adequateiy
encoded
and compared;
the segregation
process
was therefore stalled before it
rcached the interpretation stage, rcsulting in an ambiguous
percept.
Although it
is not entirely clear what would make a display more ot less complex for
infants,
our intuition is that many distinct
factors, including the shape,
pattem,
and spatial arrangement of the surfaces in the display, are likely to contdbute
to
its complexity.
The second inconsistency raised by the
present
data
(see
Figure 7) concems
the
responses
ofthe 4.5-month-olds
in the
similar condition
and
its replication, on the
one hand, and
of the 5-month-olds iD the experiments by Craton
( 1993,
1996) and
Spelke et al. (1993,
similar condition), on the other. Recall that the infants
in these
experiments had ambiguous
percepts
of (a) similar partly occluded displays
involving a rectangular box with
jagged
edges
(Craton,
1993) or a surface with a

Needham. Baiqargeon and Kaufnan
UnambiguousAmbiguous
a
e
o
FiB.7. Schenatic dnwing ofthe sihilar displays used in (A) Needham
et at. (1997)-red boxes:
(B)
Needhah et al. (1997)-lieht gteen boxes; (C) Crubn ( t993); (D) Crcton u996); and (E) Spetke et
at. ( 1993).
large dot pattern
(Craton,
1996), and (b) a similar adjacent
display composed
of
multiple concentric rings
(Spelke
et al., 1993),
Here again we suspect that the
most
likely explanation for these discrepant lesults has to do with the complexity of the
displays used
in the experiments.
The similar displays
in the
present
experiments
involved
simple shapes with smooth edges and amuted pattem.
The displays
used
by Craton and by Spelke
et al., on the
other hand,
presented
edges
or pattems
that,
though rcgular and symmetrical,
were nevertheless composed
of multiple salient
elements. This added
complexity could
have made it more difficult for the
infants
object Se,regation in lnfancJ
Unambiguous Ambiguous
,Mfu
"q
Fis. 8. Schenatic drawinC of the displays used in (A) ond (B) Needhan et at (t997); (C) 4 d (D)
Kellnan and
Spelke
(1983);
(E) T.t,hine
et al. (1987);
and (F) and (G) Schndt ond Spelke
(1984)
to encode
and compare the surfaces
in lhe disPlays,
thereby halting the segregation
process
and leading to ambiguous
percepts,r
The third inconsistency
raised by the
present
rcsults
(see
Figure 8) has to do
with the different
responses of the
4-month-old infants in the similar and dissimi-
lar conditions, Wby were these
young
infants able to form an
unambiguous inter-
pretation
of the dissimilar but not the similar display? This discrepancy was
unlikely
to reflect the relative
complexity of the
two displays, because
the
similar
display could not plausibly be consh'ued
as
being
more complex
than the dissim-
ilar display. Indeed, the same
is true ofmost of the similar displays
used Lo explore
4-month-old infants'
perception of partly occluded
disPlays. The
rod and triangle
used by Kellman and Spelke
(1983),
the surface
used by Termine
et al. (1987),
and
the cube and sphere used by Schmidt
and Spelke
(1984) were
all simple forms
with smooth, fluid contou6.
tOne
quesrion
tharis raised by the
present
analysis is why the
5-month-old
infants tested by Spelke
et al. (1993)
had
an anbiguous
perception
of the similarbut not
the dissimilar display, since bolh dis-
plays
were composed
of muftiple concentric nngs (see
Figure 3). One intrig0ing
possibility is that,
because
of lhe marked differences
in tbe shape and color of the two halves oflhe dissimilardisPlay, the
infanrs encoded
the display in terms of(hese
overall differences,
paying
little attention
to lhe rings that
made up each half.

l6 Needhan, Baillargeon and Kaulman
If the 4-month-old infants could encode and compare the surfaces in both the
similar and the
dissimilar displays, why did they succeed in organizing
the second
but not the fusi display?
At least
two explanations were
possible.
The first was
that
the infants possessed the configural knowledge necessary to interpret the featural
information
in the dissimilar but
not the similar display.
That is, the infants had an
expectation that dissimilar surfaces belong to distinct objects, but had not yet
Ieamed that
similar surfaces typically
belong to the same object. The
second expla-
nation was that, although the infants in the similar and the dissimilar conditions
were equally
capable of encoding, comparing,
and interpreting the
featural infor-
mation before them,
only the infants in the
dissimilar condition attempted to do so.
Perhaps the featural differences between the red and the green box were suffr-
ciendy salient to attract the infants' attention and activate their interpretation pro-
cess, leading to al unambiguous
percept.
The featural
similarities of the two red
boxes, on the other hand, failed to engage
the infants' interpretation
process.
The
infants did not concem themselves during the familiarization trials with the issue
of whether the red boxes formed one or two units; as a result, the infants had no
interpretation that could be condrmed or disconfirmed by the move-apad or the
mov;.together
test event, and
they therefore
perceived
neitherevent
as
surprising.2
One
way
to decide between
the two explanations
just described was to examine
young
infants'
responses to adjacent
as opposed to paily occluded
displays. Evi-
dence that young infants performed better with dissimilar than they did with sim-
ilar adjacent displays
would support
the explanation that young infants
possess
configural expectatioDs only about dissimilar surfaces. On the other hand,
evi-
dence that young infants succeeded with both t)?es of adjacent displays
would
give weight to the notion that
young
infants have acquired
configural knowledge
about both similar and dissimilar surfaces, but do not at fimt make use of this
knowledge
in all contexts in which it is relevart. The results of our research
with
adjacent displays
are
described
in the next section.
D. Experiments With Adjacent Displays
Our first experiment on young infants'
perception
of adjacent
displays com-
pared
4- and 4.5-month-old
infants' responses to a similar
and a dissimilar adjacent
display
(Needham,
199?). The infants
assigned to the similar condition
(see
Figure
9) first received
a
familiarization
trial during which they saw a stationary adjacent
display
composed of two arched boxes,
each with a flat vertical edge
(right
edge for
the left box, left edge
for the right box) that allowed full contactbetween the boxes;
2Ir migbt be proposed
that another explanation
for 4-honth-olds'consistenl failure ro organize
similar partty occluded
displays could be a reluctance
to posit a hidden connection between
the sur-
faces
visible
on either sideoftbe occluder.
However, data obtained
with moving as opposed to staiion-
ary
partly occluded displays make it clear
that 4- and even 2-month-old infants
will readily
posit
such
hidden connections
(€.g.,
Johnson & Nafrez, in press;
Kellman, Cleitman,
& Spelke, 1987; Kellnan &
Spelke,
1983; Slater
et al., 1990).
Object Segrcgation in Infancj
Similar Condition
Move{ogether Condition
Move-apart Condition
[ffi.ffi4 iffiq(z'-
Fig. 9. Schematic druwing of the etents sho\an
to the infants in the
sinilar condition
in Needham
0997).
each box was made of red cardboard and was decorated
with three thin black lines
in a curyed symmetric
pattem
centered at the boundary between
the boxes. The
infants in the dissimilar condition
(see
Figure 10)
saw ihe same display exceptthat
the left box was rectangularin shape. After the familiarization
trial; the infants saw
a test event in which a gloved
hand
took hold of the dght box and
pulled
it a short
distance
to the right. For half of the infants iD
each condition, the left box moved
with the right box (move-together
condition);
for the other infants, the left box
remained stationary tbjoughout
the event
(move-apart
condition).
We reasoned that if the infants were able to encode,
compare, and interpret the
featural information in the simila.r ard the dissimilar display,
then two predictions
followed. First, the infants in the similar condition should expect the two arched
boxes to move together, and they should be surprised in the move-apart event
when only the right box moved.
Second,
the infants in the dissimilar
condition
should expect
the
rectangular
box to remain stationary
wherl the arched
box was
pulled,
and
they should
be surprised
in the move-together
event when
both boxes
moved.
Opposite
pattems
of looking were
thus
predicted
for the infalts in the
sim-
ilar and the dissimilar conditions.
Preliminary analyses ofthe data
revealed no significant difference between the
looking times of the 4- and 4.5-month-old infants; the data
were therefore
col-

l8 Needhaa Baillargeoh and Kaufnan
Dissimilar Condition
Move{ogether Condition
Move-apart
Condition
FiB. I0 Schenutic
druwing of the events shown to the infants in the dissinila. condition in Needham
(t97a).
lapsed in subsequent
analyses. Reliably different patterns of looking were
obseryed in tlle two corditions. The infants in the similar condition looked reliably
longer at the move-apart
than they did at the move-together event, suggesting that
they (a) were led by the featural similarities between the two arched boxes to
group them into a single
unit ard thus (b) expected them to move together and
were surprised when they did not. In conhast, the
infants in the dissimilar condi-
tion showed a reliable preference
for the move-together
over the move-apart event,
as though they (a) were led by the featural differences between the arched and the
rectangular box to group
them into distinct units and hence
(b) expected the boxes
to move independantly and were surprised when they did not. Together, these
results indicate
that 4- and 4.5-month-old infants are
able
to organize both
similar
and dissimilar adjacent displays in accordance
with their featuml
properties;
we
retum at the end of the section to the implications of this finding for accounts of 4-
month-old infants'
pe$istent
failure to orgadze similar
partly occluded displays.
In addition
to the experimentjust descdbed, we conducted another experimeDt
examining 4.5-montl-old infants' perceptior of a dissimilar adjacent
display
(Needham
& Baillargeon, in press-a).
This display was first used
in experiments
on the rcle of physical
knowledge in 8-month-old infants' object segregation
(Needham
& Baillargeon, 1997; see Section V). Our initial intent had been to
Object Sesrceation in lnlancy
Move-together Condition
Move-apart
Condition
Fig. 11. Schenatic drawing of the events shoh'n to the infan$ in the cylinder-down condition in
Needhan and Baillaryeon (in prcss-a).
extend this research to younger
infants. As will soon become clear, however,
our
results led us
in entirely
ncw
directions.
The infants first received a familiarization trial in which they saw a statjonary
adjacent display consisting of a yellow, zigzag-edged,
cylinder
on the left and a
blue,
rectangular box on the right (see
Figure l1). Next, the infants watched
a test
event in which a gloved
hand took hold of the cylinder and
pulled it a short dis-
tance to the left. For half of the infants, the cylinder and box moved
together
as
one
unit (move{ogether
eveDt); for the other infants, the cylinder moved apart
from
the box, which remained stationary throughout the event
(move-apart
event).
We reasoned that if the infants attended to the featural information in the dis-
play-as did the 4.5-month-olds in our previous
experimelts with dissimilar
partly
occluded
(Needham
et al., 1997) and adjacent
(Needham,
1997) displays-
then they should expect the box to
rcmain stationary when
the
cylinder was
pulled,
and they should
be surpdsed when the box and cylinder moved as a whole.
The
infants
were
thus expected to look reliably longer at the move-together than at the
move-apzul
event.
Conftary to this expectation, however, the infants looked about
equally
at the
two test evenLs. These negative
results
suggested
that the infants
(a)
were unce ain
whether the cylinder and box constituted one or two units
and
hence
(b) could nol

A
Needhan, Baillnryeon and
Kaufnan
c
FiB. 12. Schenatic &awing
ofthe displays
used in(12A) Needham
and Bai aryeon
(
in
press-ul;
ano
(t2R),
(t2c), and
(t2D)
Needhatn
(in
presr.
determine
whether the cylinder should
move with or without the box. In subse-
quent experiments
(Needham,
in press),6.5- and 7.5-month-o1d
infants were
tested
using the same
procedure.
Only the 7,5-month-old
infants showed a reliable
preference for the move-together over the move-apart event, suggesting that they
perceived
the cylinder
and box as two distinct objects; like rhe
4.5-month-olds, the
6.5-month-olds tended to look equally
at the two test events.
The results
obtained with the 4.5- and 6.5-month-old
infants in these experi-
ments were inconsistent
with our previous
Rndings
with dissimilar displays:
Recall that 4- and 4.5-month-old
infants
conectly seglegated a partly occluded
display composed
of a rectalgular
red box and an irregularly
shaped
grcen
box
(Needham
et
al., 1997), and
an adjacent display
composed
of an arched and a rect-
angular rcd box (Needham,
1997). The most likely explanation
for these discrcp-
antresults, webelieved,
was the one invoked
in the last
section to account
for other
Object Sesresation in Infancy
inconsistencies in 4.5-month-old infants' responses to displays. That is, we sus-
pected
that the
4,5- and 6.5-month-olds in the
present
experiments were unable to
segregate
the cylinder-and-box display because they had difficulty encoding
and
comparing the surfaces in the display. As can
be seen in Figure
l2A, the cylinder
and box created
a relatively
complex set of surfaces. The cylinder
was
made of a
section of clothes dryer
vent
hose
that was stuffed and had its ends
bent slightly
forward,
giving dse to a complex zi9zag-edged,
curved shape. In addition, the
boundary between the cylinder and box did
not
simply
consist of two surfaces
par-
allel to the infants' line ofsight; instead,
one of the box's come$ faced the infants,
and the right end of the cylinder lay behind and
was
partly
occluded by the box's
left rear wall. These factors could have made
it diffrcult for the infants to complete
their analysis of the
display's
spatial
and featural
properties, thereby
stalling the
segregation
process
before
it reached the interpretation stage
and resulting in an
indeteminate percept
of the display,
To explore whether the complexity of. the cylinder-and-box
display had con-
tributed to the 4.5- and 6.5-month-olds' ambiguous
response, additio[al infants at
each age were tested with a "simplifled" version of the display (Needham,
in
press).
In this simplified display
(see
Figure 12B), (a) a straight
rather than a
curved cylinder
was used;
(b)
the box was
positioled
so
that one ofits sides rather
than one of its comers faced
the infant; (c) the connection belween
the cylinder
and box was
parallel
to the infants'line ofsight; and finally (d)
the front ofthe cyl-
inder and box were aligned, Both the 4.5- and
the 6.5-month-old
infants tested
with the simplified display looked reliably longer
at the move-together
than
at the
move-apaJt event, suggesling
that they viewed the simplified
version
of the cylin-
der-and-box display as composed of two distinct units.
To confirm
the
discrepancy
between infants'
perception
of the original and the
simplified cylinder-and-box display, an additional
experiment was conducted
compadng 4.5-month-old infants' responses
to the same two displays, but now
partly
occluded. The infants
saw either the original or the simplified
display with
a tall, naffow screen occluding
the boundary
between the cylinder and
box (see
Figures 12C
and 12D).
The results indicated that the infants who saw the
original
display tended to look equally at
the test events, whereas the infants
who saw the
simplified display showed
a reliable
preference
for the move-together
over the
move-apart event.
These
results were identical to those
obtained
with the adjacent
displays and confirmed that t}Ie infants
(a)
had an indeterminate
perception
of the
original
cylinder-and-box display, but (b) had an unambiguous
perception
of the
simplified display as composed oftwo units,
Together, the results ofthe experiments
presented
in this section
point to three
conclusions. First, the results
provide further evidence for the conclusion, first
suggested by our experiments with partly occluded displays
(Needham
et 11.,
1997), that even
young infants
possess
configural knowledge,
The 4- and 4.5-
month-old infants in the
present
experiments
who were tested with tire similar or
the dissimilar
adjacent display
(Needham,
1997), and with the adjacent or the

Needham, Baillargeon and Kaufnan
Unambiguous Ambiguous
Fig. 13. Schematic druwing of the displats used ih (a) Needhan et al. (1997)-red bores; (b)
Needhanet al. (
1997
)-liqht Brcen
botes; (c) Needhanet al. (1997); (d) and
(e)
Needhan
(1997);
A
and (d Needhan
(in press); (h) Spelke
eI al. (1993); (i) Cruton
(1993); (j) Craton
(1996): (k) and (l)
Schnidt et al. (1986);
(m) Needham and BaiUargeon
(in prcss-a);
and (n) Needhad
(in press).
ffi
ffirffi
ffi
d
g
Object Seeresation in Infancy
partly
occluded simplified cylinder-and-box display
(Needham,
in press)
all gave
clear evidence that they expected
similar but not dissimilar surfaces to belong !o
the same unit.
Second, the
present
results
lend stlong support
to the notion, first advanced
in
the previous section, that whether infants aged 4.5 months and older form an
ambiguous or an unambiguous
percept
of a display critically depends on the dis-
play's complexity. In the present
experiments, 4,5- and 6.5-month-old infants
were found to be unable to parse
the odginal cylinder-and-box
display;
however,
4.5-month-old
infants readily succeeded in scgregatilg
a simplified ve$ion of the
same display prcsented in either an adjacent or a partly occluded format
(Needham,
in press).
These findings underscore
the context sensitivity
of young
infants' segregation ability. Even relatively modest changes in displays can have a
dramatic
effect on infants'success at segregating the displays. Figure 13 sumrna-
rizes the displays
known to result in unambiguous interpretations
at 4.5 months of
age
(Figure
13A) and in ambiguous responses at
4.5 or 5 months
(Figure l3B).
Finally, the results
obtained with the 4-month-old infants
tested with the
similar
adjacent display
(Needham,
1997) bear
on the two explanations
proposed
in the
last section for young
infarts' inability to organize similar as opposed to dissimilar
pardy occluded displays
(e.g.,
Kellman & Spelke, 1983; Needham
et a1,, 1997;
Schmidt & Spelke, 1984; Termine et al., 1987). The
present
results
argue
against
the first of these explanations, which was that young infants lack configural expec-
tations
about similar surfaces. This leaves open the second explanation, which was
that 4-month-old infants have difficu y with similar pardy occluded displays
because the featural
similarity of the surfaces on either
side of the occluder
is not
sufficiently salient to attact the infants' attention and engage their interprctation
process.
The
finding that4-month-old infants
perform
better with similar
adjacent
as opposed to partly occluded displays could
be taken to support this explanation.
Infants
might be more likely to notice featural similarities in surfaces that are sep-
arated by a boundary than in surfaces
that are
sepamted
by an occluder.
Sinilar
adjacent displays would thus be
better
designed to highlight the featural
similarity
of surfaces and thus activate
young infants' interpretation
process,
Ieading to
unamorguous
percepts.
E. Future Directions
The rcsearch reported
in the last two sections ildicates that, contrary to earlier
claims
(e.g.,
Kellman & Spelke, 1983; Spelke, 1982, 1985a, 1991),
young
infants
possess
configural
knowledge and expect similar but not dissimilar surfaces
to
belong to the same units.
Interestingly, this characterization of young infants'
approach
to three-dimensional displays is very much in line with reports
over the
past two decades
of sophisticated
perceptual
organization in young infants'
responses to two-dimensional
displays (e.g., Bomstein & Krinsky, 1985;
Colombo, Laurie, Martelli, & Hartig, 1984; Ghim, 1990; Ghim & Eimas, 1988;

Needham, Baillatgeon and Kaufnon
Giffen & Haith, 1984; Milewski, 1979;
Quinn,
Burke, & Rush, 1993;
Quinn &
Eimas, 1986; Salapatek,
1975; Slater 1989; Treiber
& Wilcox, 1980). For
exam-
ple, Quinn
et
al. (1993)
familiarized 3-month-old
infants with a display
consisting
of a 4 x 4 square
grid of light and dark elements
affanged in either rows (row dis-
play) or columns
(column
display). Next, the infants
saw test displays consisting
of horizontal or vertical stripes. The authors reasoned
that if the infants grouped
the
elements in the
familiarization
displays
on the basis of their lightness,
then the row
display shoutd
be viewed as resembling
the horizontal-stripe
display, and the
col-
umn display the
vertical-stripe display. The results
indicated that the infants
who
were familiarized with the row display looked reliably longer at the vertical- than
at the hodzontal-stripe display, whereas
the infants who were familiarized with the
column display
showed the reverse
looking pattern.
These
results suggested that
the infants were sensitive to the lightness of the elements
in the familiarization dis-
plays and used this information to organize the elements into Iarger units.
The conclusion
that
young
infants
are caPable ofsophisticated
percePtual
orga-
nization
when
presented
not only with two- but also with three-dimensional dis-
plays
opens
many new directions
for future research.
One such direction
concems
the
nature ofinfants' configural knowledge.
We need lo specify
precisely
what fea-
tural and spatial information infants attend to when judging whether surfacas are
sirnilar or dissimilar, and how this information changes
with age. In particular,
do
infants consider
the aligDment, shape,
pattem,
color, and texture
of surfaces from
the start, or do they come to use
these
variables
one by one over a
pedod
ofweeks
or months
(as
has been found in other areas of infants'
knowledge about objects;
see Baillargeon,
1994, 1995)?
If the latter, in what sequence do infants ideDtify
the
variables, and
what mechanisms
are responsible for their identification?
Another
direction for futurc
research concerns
the odgins ratherthan
the devel-
opment
of young infants' configural knowledge.
The evidence
presented
in the
previous
sections
suggests that, by 4 months of age, infants already
possess co!-
figural expectations
about objects. There is reason to believe
that
younger
infants
lack such
expectations,
and rely exclusively
on spatial continuity/disco inuity
information
to organize displays, Using a variety of exPerimental
procedures,
Spelke
and her colleagues
(e.g.,
Kestenbaum, Termine,
& Spelke, 1987; Pruther
&
Spelke,
1982; Spelke
et a1., 1993) have
examined 3-month-olds'
perceptions
of
stationary
adjacent
displays. The results
of these expelimelts have consistently
shown that
infants this age view spatially clntinuous
surfaces, whether similar
or
dissimilar,
as belonging
to the same units.r
To date, no investigation
has focused
JIn a number
of experiments, Spelke
and hercolleagues examined 5-montb-old
infants' responses
to adjacent
displays using a reaching method
(e.8.,
Hofsten & spelke, 1985;
sPelke, Hofsten, & Kes-
renbaum, 1989).
Interestingly, the results obtained
with this method
were more similar to those found
witb the3- thanwith th€ s-month-oldsin Spelke's
preferentiallooking
experiments
(e g, Keslenbaum
er al., 198?;
Prather & Spelke, 1982;
Spelke et al,, 1993), These
results mightbe laken
as
further
evi-
dence of the ofFno@d discrepancy belween
action and nonaclion assessments
of infants'
perceptual
and cognitive abilities
(e.9.,
Baillargeon, 1993;spelke,
1994).
Object Seercsation in Infancj
on 3-month-olds' lesponses
to stationary
partly occluded
displays. However,
experiments by Slaier
and his collaborato$
(e.9.,
Slater
et al., 1990, 1994) have
repeatedly found that newbom
infants view surfaces
visible on either side of an
occluder,
even when similar
andmoving in perfect
synchjony,
as belonging to seP-
arate units. Together,
these data tentatively
suggest
that infants begin by grouping
spatially
continuous surfaces into the same
unit and spatially discontinuous sur-
faces into distinct units. Between
3 and
4 months of age,
perhaPs
as a result of
marked improvements
in their visual abilities (e.9., Banks, 1983; Yonas &
Granrud, 1984),
infants would become
awarc of the limitations
of their initial rule
and would begin to consider featural information when organizing displays.
Experiments
are needed to test these speculations
and more
generally
to determine
the origins
of object segregation
in infancy.
IV INFANTS'USE OF EXPEFIIENTIAL
KNOWLEDGE
We argued earlier
(see
section II) that
adults bring to bear
not only their configural
but also their expedential knowledge
when segregating
displays. Do infants,like
adults, use information
gained
in prior experiences
with objects
to organize dis-
plays containing the same or similar objects? A preliminary experiment by
Schwartz
(1982) suggested
that the answer
to this question
might be positive.
Schwartz
found that 5-month-old
infants who
were habituated
to a slide depicting
a partly
occluded human
face looked reliably
longer
during test at an incomplete
than at a complete
version of the
face.
These data suggested
that infants use their
knowledge
about familiar
objects such as faces
when organizing
stationary
Partly
occluded displays.
The research described
in this section
was designed to extend
this result
in two directions, FiNt, it focused on infants'
segregation of adjacent
rather
than
partly
occluded
displays. Second,
it examined infants'
use of expenen-
tial knowledge
gained
in very brief encounters with objects
as opposed to knowl-
edge
(such
as
knowledge
of faces)
acquired through long-standing
and extensive
interactions
with objects.
A. Experiments
With Adiacent Displays
The point of departure of our research on young infants' use of experiential
knowledge was the finding obtained with the 4.5-month-old
infants who were
tested with the original cylinder-and-box
display (Needham
& Baillargeon, in
press-a).
Recall that these infants
tended to look equally at the move-together
and
the move-apart
tesl events, as though they
were unsure whether
the cylinder and
box formed one or two units. Our interpretation of this finding was that,
because
ofthe complexity of the
display, the infants had
difflculty representing
orencoding
the surfaces in the display; as a result,
the segregation
process
was stalled before
the interpretation
stage,
preventing
the infants from applying their configural

Needhrn, BaiUarqeon and Kaufuan
Move-together Condition
Move-apart
Condition
FiB 14. Schematic d.awin| of the erents shown to thc ihr)nts faniliarized with onD the bot in
Needham and Baillareeon (in prcss-a).
knowledge. In a subsequent experiment
(Needham,
in-press), 4.5-month-old
infants were tested with a simplified venion of the cylinder-and-box display, one
designed
to be easier
to encode; the infants in this experiment looked
reliably
longer at the move-together than
at
the move-apart event, suggesting
that they
(a)
were now able to represent and compare the cylinder and box and
(b) interpreted
the
featural differences between
them as
signaling
two distinct
objects.
In the present
rcsea-rch
(Needham
& Baillargeon, in press-a),
we explored an
altemative way of facilitatiDg
young
infants' segregation of the
original
cylinder-
and-box display. Instead ofrealigning the cylinder and box so that they
presented
a simpler arralgement of surfaces, we adopted a differcnt approach: We exposed
the infants the box alone before presenting them with the entire display. We rea-
soned that if tho infants
(a)
recognized the box when shown the cylinder-aud-box
display and @) were able to use this information to segregate
the display, then they
should
view it as comprising two distinct objects, the familiar box and
the unfa-
miliar cylinder Giving the infants a prior exposure to the box thus provided them
with an altemative means of segregating the display. Instead ofrepresenting, com-
paring,
and interpreting the featural information in the display, lo determine how
many objects it contained, the
infants
could
focus from the start on what familiar
and unfamiliar objects
were present
in the display. In other words,
the infants
Object SegreSation in InfancY 21
could make use of their experiential rather than their configural knowledge to
organize the display.
As in our initial experiment with the cylinder-and-box
display,
4.5-month-old
infants
received one
familiarization
tdal followed by six test
trials in which they
saw either the move-apart or the move-together test event. The only difference
between
the two experiments
had
to do with the familiarization
trial Whereas
the
infants in the frIst experiment
were shown
the entire
cyliDder-and-bor
display, for
a period
of 10 to 30 s (mean
looking time was 1?.5
s), the infants
in the present
expedment were
shown only the box,
for a
period of 5 s
(see
Figure 14). An exper-
imenter
held the box above the
appamtus
floor and tilted
it altemately
to the right
and to the left until the computer signaled that the infant had cumulated 5 s of
looking at the box.
The design of this experiment
was thus
predicated
on two assumptions. The
fiIst was that
the infants
would be able
to recognize
the box after seeing
it for only
a brief (5-s)
exposure,
and
the second
was that the
infants
would rcadily make
use
of this information to organize
the cylinder-and-box display. How plausible were
these
assumptions?
Young infants'visual
recognition
memory is rernarkably
robust.
By 5 months
of age, infants
are capable
of recognizing
previously experienced
stimuli on the
basis
of modest
amounts of familiarization
and
over appleciable
retention
inter-
vals
(e.g.,
Cornell,
1979;Fagan,l973;
Madin, 1975). To i ustrale,
Fagan
(1973)
showed
5-month-old
infants
a photograph
of a face for 2 min. After delays of 3
hours
and 1,2,7, alrd 14 days,
the infants
were
presented
with the familiar face
paired
with a novel
face.
At all interyals,
the infants
looked
reliably longer
at the
novel
than at the familiar
face. In another experiment,
Fagan
(1974)
examined how
much
familiarization
time 5-month-olds
required
to recognize
various stimuli on
immediate
as opposed
to delayed tests.
He found
that, whereas
20 to
30 s of famil-
iarization time were needed
for faces,
and l7 s
for abstract
pattems composed of
identical
elements,
as little as
4 s were necessary
for stimuli
varying along several
dimensions.
Given these and other
similar nndings
(e g , Comell, 1979; Fagan,
191'1'iLask.y,1980;
Rose,
1980, l98l), it seemed
Probable
that a 5-s familiariza-
tion time would be sufficient
to enable the infants
to recognize
the box in the test
rials.
If the infants
did recognize
the
box
in the test
trials, how
likely were they to use
this inform;tion to segregate
the cylinder-and-box
display?
Recent
research sug-
gests
that infants'pdor
encounte$
with objects
do affect
their subsequent
perceP-
tions of the objects
(e.g.,
Granrud,
Haake,
& Yonas,
1985;
Yonas, Pettersen, &
Granrud, 1982).
For example,
Cmruud
et al. (i9E5) gave
7-month-old
infants
a
. large and
a small novel
object to
play
with for a 10-min familiarization
phase. Dur-
ing the test
phase,
the infalts were
prcsented
with two objects:
the large object,
and a version
of the smatl
objectenlarged
to be ofthe same
size as the large
object
The two objects were
positioned at the same distance
from the infants,
who were
allowed to reach for them. Under monocular
viewing conditions,
the infants

Needhan, Baillargeon and Kau,fnan
Move-together
Condition
Move-apart
Condition
Fig. 15. Schematic druh,ine of the eyents shown to the infants familiafized wih only the cylinder in
Needhan and Baillaryeon (in press-a).
reached reliably more toward
the
previously
small
object, suggesting that they
per-
ceived it to be nearer thaD the
large object. These and
other data indicated that
the
infants brought to bear their knowledge
of the objects'
sizes-acquired during the
familiarization
phase-when estimating
thet distances.
Given all ofthis evidence,
it seemed reasonable to expect
that the infants
might
be able to use their
briefexperience with the
box to segregate the cylinder-and-box
display. The results supported
this expectation: The
infants looked reliably
longer
at the move-together than at the
move-apart event, suggesting
that they
(a)
recog-
nized
the box when
it stood next to the cylinder;
(b) inferred,
based on this infor-
mation, that the box and cylinder were separate
units; and hence
(c) expected the
two to move independently and were surprised in the move-together event that
they did not. These results contrasted
\r,ith those ofour iDitial experiment and indi-
cated that being familiarized with the box alone, rather than with the entire dis-
play, made it possible for the infants to achieve an unambiguous interprctation of
the display.
Th€ results of this last experiment
suggested that, by 4.5
months ofage, infants
are able
to use lheir prior knowledge
of an object to segregate
an adjacent display
containing
the object. In an attempt to confirm
this finding, we conducted another
experiment that
was identical to the
last except that the
infants werc exposed to the
Object Se?regation in InJancy
cylinder rather
than the box during the familiarization
trial (see
Figure 15). Unlike
the infants who were familiarized with the box alone, the infants familiarized with
the cylinder alone tended
to looked
equally at the move-iogether and the move-
apart events, suggesting
that their
perception
of the cylinder-and-box display was
indeterminate.
How could one explain
the discrepant responses of the infants exposed to the
box alone and to the
cylinder alone? One hypothesis
was suggested by the evi-
dence, mentioned
earlier, that more complex stimuli
typically
require more tine to
encode
than simpler stimuli (e.g.,
Fagan,
19'14, 19'77; Rose
& Slater 1983). It
seemed
possible
that, because
the cylinder was
more complex than the box, a 5-s
exposure
to the cylirder did not
givethe
infants
suffrcient time to familiarize them-
selves
with it; as a result, they failed
to recognize it when shown
the cylinder-and-
box display and hence had
no experiential knowledge
they could use to parse
the
display.
This analysis
predicted
that infants'
performance
should improve following a
longer exposure to the cylinder. Our
next experiment sought to test this
Prediction:
The infants were exposed to the
cylinder for 15 rather than
for 5 cumulative sec-
onds dudng the familiarization
trial. The infants now looked
reliably longer at the
move-together
than at the move-apart event, suggesting
that they (a) viewed
the
cylinder aad box as dislilct units
and hence
(b)
expected them to move separately
and were surprised
when
this expectation
was
violated.
These results,like
those obtained with the infants
who were
familiarized with
the box alone, indicate that 4.5-month-o1d infants
bdng to bear
prior experiences
with objects when segregating
displays involving the objects.
Such a finding sug-
gests
that
young infants
possess
a valuable
tool for organizing their daily world.
According
to the
present
results, an infant
who recognizes a rattle standing
next
to
an unfamiliar
cup,
shoe, or whistle should be able to detemine
the boundaries of
these
novel objects because she already knows the boundaries of the rattle.
Thrcugh this sort of "experiential
bootstrapping,"
young infants' exPeriential
knowledge could considerably facilitate
their
parsing
of scenes
containing famil-
iar and novel objects.
Our final experiment began
to explore the conditions
under which young
infants are
able
to use their experiential knowledge
for segregation
purposes.
Of
particular
interest were effects of context and delay. Could infants
make use
of a
prior
experience
with an object to organize a display
containing the object, even if
they viewed the display in a different context
than the object,
and after a delay?
To
retum to our prcvious
example,
would an infant be able
to use her knowledge of a
rattle to segregate it from a cup, even ifshe saw
the
lattle-cup
disPlay in a different
setting and
at a later time than the rattle?
The infants in this experiment were familiarized
with the box in their own
homes about 24 hours before they were scheduled
to be tested
in ihe lab; an
exper-
imenter drove to the infants'homes
and
showed
them the box for 2 min. Sessions
in the lab began directly with the test rials; the infants saw either the move-

Needhan, Baillaryeon and Kaufnan
together or the move-apart event, as before, for six test trials. Reliably different
looking pattems
were found in tle first tbree and last tbree test trials. Dudng the
fiIst block of trials, the infdnts tended to look equally at the move-together and the
move-apart events; during the second block, the infants looked reliably longer at
the move-together
than at the move-apart
event.
These
results
suggested
that,
although the
infants
did not
immediately
recognize the
box upon seeing
it, they did
so after a few trials (since
the infants had not seen the box for 24 hou$, and they
were encountering it in an entirely novel setting, this finding did not seem very
surprising), After recognition emerged, the infants
(a) infeffed that the box and
cylinder were distinct objects and therefore (b) expected them to move indepen-
dently and were surprised when they did not.
The results summarized in tlis section
point to three conclusions. The fiISt is
that, by 4.5 months ofage,
infants
bring
to bear their experiential knowledge when
segregating displays. The infants who, when shown the cylinder-and-box display,
recognized the box or cylinder they had seen
during the familiarization
trial
readily segregated the display
into two distinct units. These results, together with
those rcported in sectiol III, indicate that 4.5-month-old infants can use two ofthe
same types of object knowledge as adults-rconfrgural and experiential knowl-
edge-when organizing displays.
The second
conclusion
suggested by the
present
results
is that
young
infalts'
use of experiential knowledge does more than simply
provide
them with an alter-
natiye route
to segregating
displays:
It also
makes
it possible
for them to deal suc-
cessfully with displays they could not otherwise segregate. We saw earlier
(Section
IiI) that, when faced with complex displays,
young infants
are often
unable to complete thet analysis of the displays' featuml
properties;
the segrega-
tion process
thus becomes stalled before infants can bring their configural knowl-
edge to bear. Fortunately,
young infants' ability to use experiential knowledge
gives
them another
way
to approach complex displays. Recall that the 4.5-month-
old infants in our initial expedment with the
cylinder-and-box display were
unable
to encode and compare the complex arrangement of surfaces in the display and
hence
could not form a clear interpretation of the display.
However,
after
being
exposed to the box alone
(for 5 s) or to the cylinder alone
(for 15 s), the infants
readily succeeded in parsing
the display.Instead ofaconfigural approach-encod-
ing, comparing, and interpreting the featurcs of the
cylinder aDd
box-the infants
were able to adopt an easier, experiential approach and segregate the display into
a familiar and an unfamiliar object,
The third conclusion that can be drawn from the present
data concems the
remarkable robustness of young infants' ability to use experiential knowledge.
The results indicate that, to be of help to infants, a
prior experience
with an
object
does not have to be extensive, nor does it have to occur in the same setting as or
immediately
preceding
infants' exposure to the test display containing the object.
All that matters for a prior experience to be effective is that infants be able to
encode sufficient information about the object to recognize it when they next
Object Sesresation in InJancy
encounter
it in the test display. In the
present
experiments,
exposures ofas little as
5 s
for the box and l5 s for the cylinder
were sufficient to ensure recognition-and
hence a successful segregation
performance-when the test trials immediately
followed the familiarizatior tdal, Although it is doubtful whether such bdef expo-
surcs would have been
adequate when the test
trials were delayed by 24 hours
(e.g.,
Comell, 19?9),
as in our last
experiment, even then a comparatively brief 2-
min exposure
proved
adequate
for the task. Such a finding,
incidentally, confirms
Fagan's
(1973) rcsults: Recall
that 5-month-old infants
who were familiarized
with a face for 2 min were
able to rccognize
the
face
after delays of I day or more.
B. Future Directions
The finding that
young
infants are capable of using their experiential knowl-
edge for segregation
purposes
suggests several
directions for futute
reseatch-
One
such direction
is whether infants' interprctation of a display would be affected by
pdor exposure to an object similar but not identical
to an object in the display. To
illusbate, would 4.5-month-old infants
corectly parse
the cylinder-and-box dls-
play after being familiaized with a box that shared
some, but not all, of the fea-
tures of the test box (e.g.,
a familiarization box of the same size
and shape as the
test box, but of a different
color and
pattern)?
Preliminary evidence
collected
in
Needham's laboratory
suggests that
young
infants are very selective in their use of
expedential knowledge
and benefit
from a prior exposure to an object that is
highly sirnilar, but not moderately or weakly
similar, to the test object.
Such evidence, if valid,
could lead to investigations of prior exposures involv-
ing not single objects, as has been
the case so far, but rather object
categories.
Let
us assume, for example,
that infants arc found not to benefit
from being exposed
to a tall red, green,
or purple
box before seeing
the tall blue box in the
cylinder-
and-box display. Would infants nevertheless be helped
by being exposed to all
three familiarization boxes
before seeing the display?
Could infants, in other
words, categorize the familiarization boxes
on the basis of their common
percep-
tual features,
and then use this same category information
to parse
the cylinder-
and-box
display? Recent evidence indicates
that such
perceptual categodzation
responses fall well within the range of young
infants' ability (e.9.,
Eimas & Quinn,
1994;
Quinn & Eimas, 1996). In one experiment,
for example, Eimas and
Quinn
(1994)
familiarized 3- and
4-month-old infants with color photographs
of horses.
During the test
trials, the infants saw
photographs ofnovel horses
paired
with cats,
zebras, or gimffes.
The infants reliably
prefe[ed the
cats,
zebras,
and
giraffes
over
the novel horses, suggesting
that they had formed
a categorical representation for
hones dudng the familiarization
trials. It will be interesting
to find out whether
young
infants' use ofexperiential
knowledge
in segregation tasks is limited to rep-
resentations of particulal
objects or whether it extends,
under some conditions at
least, to representations of entire object categories.
3l

32 Needhan, Baillaryeon and KauJnan
V. INFANTS'USE OF PHYSICAL KNOWLEDGE
We proposed earlier
(see
Section
III) that adults use
physical
as well as configural
and experiential knowledge when segregating
displays. Recent evidence indicates
that
young
infants
possess
many intuitions
about objects' displacements and inter-
actions with other objects
(e.9.,
see Baillargeon, 1993, 1994, 1995; Baillargeon
et
a1., 1995; Spelke, 1994; a.nd Spelke et a1.,
1992,
for rccent reviews). The two
experiments described in this section began to examine whether young infants,
like adults, bring to bear their physical knowledge when organizing displays. The
first experiment focused on infants' undefftanding of suppon, and the second
experiment, on infants' understanding of impenetrability.
A. Experiment Involving Infants' Intuitions
About Support
There is growing
evidence that
young
infants
possess
intuitions about support
relations between
objects
(Baillargeon,
Needham, & DeVos, 1992; Baillargeon,
Raschke, & Needham, 1997; Needham
& Baillargeon, 1993; see Baillargeon,
1994, 1995,
and Baillargeon et al., 1995, for reviews). One
experiment, for exam-
ple, examined whether 4.5- and 5,5-month-old
infants realize that objects are
Cylinder-up
Condition
Move-together Condition
Move-apart Condition
FiB. 16. Schenntic druwing of the events sho\en to the infants
N ee dha n and B aiUaryeon
(
t 997
). in rhe cylinder-up condition in
Object SesrcEation in lnfancY 33
unstable
when released
against vertical surfaces
(Baillargeon et al , 199?). The
infants saw
a possible
ard an impossible
event
in which a gloved hand
placed
a
small
box against
the center of a vertical surface
and
then released
it. Beneath
the
box was a
platform;
the only difference
between the
two test
events had
to do with
the height
of this platfom. In the
possible event, the
platform was tall enough to
support
the box. In the impossible
event,
the Platform
was
much shorter
and
did
not contact
the box; the box simply
lay against
the vertical
surface,
well above the
platform. The results
revealed
that the 5.5- and even some
of the 4 s-month-old
infants looked
reliably longer
at the impossible
than at the
possible event, suggest-
ing that the infants
(a) rccognized
that the box could not remain stable
when
released
against the vertical
surface
above the shoft
platform and thus (b) were
surprised
in the impossible event
that the
box did not
fall.
In light of this and other
(Baillargeon
et al , 1992;
Needham & Baillargeon,
1993)
demonstmtions
that young infants possess knowledge
about support,
it
seemed
plausible to ask whether
they would make use
of this knowledge
when
parsing displays. The point of departure
for this rcsearch
was the flnding
(Needham,
in press),
reported
earlier that 7 s-month-old
infants
who are
shown
the cylinder-and-box
display
perceive this
disPlay to be
composed
of two distinct
parts.
The quesiion
investigated
in our first experiment
(Needham
& Baillargeon,
in press-b) was how infants
would respond
if the cylinder
was raised
above
the
apparaLus
floor so
that it made
contact with the upper rather
than
the lower portion
of the box.
Would infants,
based on
their knowledge
of suppot, (a)
realize
that the
suspended
cylinder could
not remain
stable
if it were merely
resting
against
the
box, and hence
(b) conclude
that the cylinder
must
be attached
to the box?
Subjects
werc 8-month-old
infants.
The infants
were
randomly
assigned to one
of two conditions:
the cylinder-down
or the cylinder-up
condition
(see
Figures
1 I
and
16).
The infants in the cylinder-down
condition saw
the same
familiarization
display and
move-together or move-apart
test events
as the
?.s-month-olds
in our
earlier experiment
(Needham,
in press). The infants
in the cylinder-up
condition
received similar familiarization
and test trials except
that the cylinde! was sus-
pended
above
the apparatus floor and contacted
the
upper
portion of the box.
We fully expected that,
like the 7.5-month-olds
before them (Needham'
in
press),
the infants
in the cylinder-down
condition would (a)
be
led by the featural
differences
between the
cylinder atd box to view
them as distinct
units and hence
(b) would expect
them to move independently
and be surprised
in the move-
together
event when they
did not, The question
of intercst
concerned the
perfor-
mance
of the infants
in the cylinder-up
condition.
Our
reasoning
was that if these
infants
(a)
underctood,
based
on their knowledge
ofsupporl,
that the
cylinder must
be
attached to the box and
(b)
allowed the interpretation
dictated by their
physical
knowledge
(one
unit) to supersede
that suggested by their
configural knowledge
(two units),
then they should expect
the cylinder
and box to move
together and be
surprised in the move-apart
event when
they did nol.

Needhan, Bailkryeon and Kaufnan
The infants in the
cylinder-down
condition
looked
reliably longer
at
the move-
together
than at the
move-apart
event, suggesting
that
they viewed
the cylinder-
and-box display
as composed
of two separate
units.
This result confirmed
our ear-
lier result
\zr'ith 7.5-month-olds (Needham,
in press)
and
suggests
that infants
this
age,
unlike 4,5- and 6.5-month-old
infants
(Needham,
in press;
Needham
& Bail-
largeon, in press-a),
are readily
able
to encode, compare,
and
interpret
the featural
differences
between the cylinder
and
box,
In contuast
to the
infants in the cylinder-down
condition,
the
infants in the
cyl_
inder-up
condition looked reliably longer
at the move-apart
than
at the move_
together event,
suggesting
tlat they
perceived
the
cylinder and
box as
constituting
a single unit. This finding
indicates that,
by 8 months
of age,
infants b.ing
to bear
their physical
knowledge-in this case, their knowledge
of suppon-in making
decisions
about the composition
of displays.
The
present
rcsult also
suggests
that,
when confronted
with conflicting
interpretations
of a display, one
based on their
configural
knowledge and
the other on their physical
knowledge,
8-month-olds
allow the second
interpretation to override the first. The infants in the cylinder-up
condition
judged that the cylinder and
box formed
a single unit even though
the
two presented the same marked featuml dissimilarities as in the cylinder-down
condition,
where
the cylinder
and box were seen as
distinct units,
Blade-beside
Condition
Movetogether
Condition
Move-apart
Condition
FiB. 17. Schematic druwing of the events shown
Needhan aftd Batllaryeon
(1997). to the infants in the blade-beside condtion m
.Object Segregation in Infancy
Blade-between Condition
Move{ooether Condition
Move-apart
Condition
Fig. 18. Schematic druwin| of the events thown to the itants in the blade-between condition in
Needhan
and Baillatgeo (1997).
B. Experiment
Involving Infants'
Intuitions About lmpenetrability
The experiment
described in the last sectiol indicated
that 8-month-old
infants,
like adults, bring
to bear their
knowledge of supPort
when organizing
a disPlay. To
confirm
and extend this finding,
a second experiment
was
coDducted that tested
whether infants'
organization of a display would also be affected by their irltui-
tions about
impenetability (Needham
& Baillargeon,
1997).
There is a large
body of evidence
demonstrating that
even very
young infants
recognize that oneobject
cannot
pass through the space occupied
by another objecl
(e.g.,
Baillargeon,
1987, 1991; Baillargeon
& Devos, 1991;
Baillargeon, Grabeq
DeVos, & Black, i990; Baillargeon,
Spelke, & Wasserman,
1985; Spelke
et al.,
1992). For example, Spelke
et al. (1992)
habituated
2 5-month-old
infants to an
event in which a ball rolled from left to dght along a platform and disapPeared
behild a screen.
Next, the screen was
temoved to reveal the ball resting
at the end
of the
platform.
Following habituation,
the infants
saw a possible and an impossi-
ble test
event. These events
were
similar
to the
habituation
event excePt that a tall,
thin box stood behind
and
protruded
above
the screen. At the end of the possible
event, the screen
was removed to reveal the
ball resting against
the box. At the end
of the impossible
event, the screen was removed
to reveal
the ball resting at the end

Needhan, Baillaryeon and KaufuM
oftheplatform, as in the habituation
event.
The infants lookedreliably
longer at the
impossible
than at the
possible
event,
suggesting
that they
(a)
understood
that the
ball could not roll through the space
occupied
by the
box and hence
(b) were sur-
prised
in the impossible
event when the
ball was revealed
on the far side of the
box,
In light of the above results,
il seemed reasonable
to ask whether
8-month-old
infants could
bring to bear their
intuitions about impenehability
when
organizing
displays. A secondary
goal of the research
was to obtain further evidence
that
infants
this age are able to organize adjacent
displays in accordance with their fea-
tural properties.
The infants firut received a familiarization trial in which they saw a stationary,
adjaceot
display consisting
of two identical
yellow octagons decorated
with blue
dots and
stripes
(see
Figures
17 and 18). At the
start of each test trial,
a large, thin
metallic blade encased
in a wooden
frame stood to the right of the octagons,
A
gloved
hand lifted the
blade, tumed it 90 deg
(so
thar only irs wooden frame was
visible), and lifted and lowered it repeatedly either
to the side of the octagons
(blade-beside
coldition) or between
the octagons
(blade-between
condition).
Next, the hand
removed the
blade from the
apparatus. Upon reentering
the appa-
ratus, the hand took hold of the right octagon and
pulled
it a short distance to the
right. For half of the infants
in each
blade condition, the
left octagon moved with
the right octagon
when it was pulled (move-together
condition).
For the other
infants,
the right octagon
moved apart
from the left octagon,
which remained sta-
tionary
(move-apafi
condition).
We reasoned that if the infants in the blade-beside condition
were led by the
featural
similarity of the
octagons to view them
as a single unit, then
they should
expect the octagons
to move
jointty and be surpdsed in the
move-apart event when
this
expectation
was violated.
In addition,
if the infants in the
blade-between con-
dition (a)
realized
that, because
the blade could
be inserted between the
octagons,
the two could not constitute
a single
unit, and (b) gave
more importance to the
interpretation
suggested
by their knowledge of impenetability (two units) than to
the interpretation
suggested by thejJ
configural knowledge
(one
unit), then they
should expect the
octagons to move
independently and be surprised in the move-
together
event when they
did not.
The infants in the blade-beside condition
looked
reliably longer at the move-
apart
than at the move-together
event,
suggesting that they
perceived
the octagons
as one cohesive
unit. This result, together with the results of the
ptevious
experi-
ment (Needham
& Baillargeon,
1997),
provide clear
evidence that 8-month-old
infants
possess
configural
knowledge
and expect similar
sudaces
(such
as those
of
the
octagons) to belong
to the
same unit and dissimilar
sudaces
(such
as those
of
the cylinder
andbox) to
belong to distinctunits.
Suchaconclusion
is ofcourse con-
sistent with the findings reported earlier
on 4- and 4.5-month-olds' perception
of
similar and dissimilar
adjacent displays
(Needham,
1997;
see Section III).
In contrast
to the infants
in the blade-beside condition,
the
infants in the blade-
between condition
looked reliably longer at the move-together
than at the
move-
Object Sesrcsation in Infancy
apart event, suggesting
that they viewed the octagons
as two separate units.
This
finding
extends the results of our previous experiment
(Needhajn
& Baillargeon,
1997) and indicates that 8-month-old
infants bring to bear their intuitions about
impenetrability
as well as support when segrcgating displays. Finally,
the
prcsent
result also confirms our previous
conclusion that, when
faced with two conflicting
interpretations of a display, one suggested by their conflguml knowledge and the
other by thel physical
knowledge, 8-month-old
infants allow
the latter to oveffide
the former. After the blade was removed from the apparatus,
the infants in the
blade-between condition saw exactly
the same display-and hence exactly the
same
featural information-as the infants in the blade-beside
condition. Nevethe-
less, the infants in the blade-between
condition
judged
that the octagons consti-
tuted two rather than one unit.
C. Future
Directions
The results of the two expedments
presented
in this section indicate that, at 8
months of age,
infants' organization of a novel
display is affected not only by the
featwal properties of the surfaces in the display, but also by their physical proper-
ties. In particular, infants attend to suppof and impenetrability information that
specifies whether adjacent surfaces are likely to be attached or not, The results also
indicate that when
the
featural
and
physical properties
of a display
point lo oppo-
site
interpretations, infants select the
physical
over
the configural interpreLation.
These findings suggest
several interesting
questions
for future investigations.
One such
question
is whether infants
younger
than 8 months of age are also able
to use their
physical
knowledge
to segregate
displays. In designing
experiments
to
address this issue, special
care will be needed to ensure
that the experiments call
for physical
knowledge that is already available
to infants. Consider, in panicular,
the case of support. Recent
evideDce has brought to light a clear
developmenlal
sequence in young
infants' unde$tanding of suppofi
(see
Baillargeon, 1994, 1995,
and Baillargeon et al., 1995, for reviews). This evidence
comes from a series of
experiments in which infants
aged 3 to 6.5 months were
presented
with suppod
problems
involving
a box and a platform.
The results indicated
that, by 3 months
of age, infants expect the box to fall if it loses
contact with the platform and to
remain stable otherwise. At this stage, any contact between the box and
the plat-
form is deemed sufficient
to ensure the box's stability. At least two developments
take
place
between 3 and 6.5 months of age. First, infants
come to realize that the
type ofcontact between the box and the
platfom must
be taken into account when
judging the box's stability. Infants initially assume
that the box will remain stable
ifreleased on the top of the
platform,
against the side of the
pladorm, or, when
a
hollow platform is used, under the top of the platform. However, by 4.5 to 5.5
months of age (females precede
males
by a few weeks in this development),
infants
are able to distinguish
between these differcnt types of contact and recog-
nize that only the first ensures stability, The second development
is that infants

Needhan,
RaiLLaeeon
and Kauf an
begin to appreciate that the amount of contact between the box and the
pladorm
affects the box's stability. Initially, infants believe that the box will be stable even
if only a small portion (e,9.,
1570) of its bottom surface rests on the
platform.
By
6,5 months of age,
however, infants
expect the
box to fall unless a significant
por-
tion of its bottom surface lies on the
platform.
Given the developmental sequencejust described, it should be clearthat testing
infants aged less than
4.5
to 5.5 months
with the cylinder-up display described ear-
Iier (Needham
& Baillargeon,
199?)
would be
pointless.
Lacking the
knowledge
that objects are unstable when rcleased against other objects, the infants would
have no basis to infer that the cylinder and box must be attached. Infants aged 5.5
months or oldet however, could meaningfully be tested with the cylinder-up dis-
play.
Infants
aged
6.5 months
and
older
could also be tested with a version
of the
display
in which the dght end of the cylinder rested on the top rather than against
the side of the box; rccall that by 6.5 months infants realize that the amount of con-
tact between
an
object
and
its suppot affects the object's stability.
Such experiments would be very useful, for at least three rcasons.
Fi$t, the
experiments would help determine whether infants less than 8 months of age can
bring to bear their physical
knowledge
when segregating displays, Second, the
experiments would help establish whether infants make use of thei physical
knowledge
(a) as soon as it is acquired or (b) only after some time (required per-
haps
for consolidation or genenlization).
Finally, it would be interesting
to find
out how youlg infants construe support displays involving
the cylinder
and box in
light of their inability to parse
these two objects
on configural
grounds
alone.
Recall that both4.5- and
6.5-month-olds
failed to parse
the original cylinder-down
display; only infants aged ?.5 months and older succeeded in segregating the
dis-
play (Needham,
in press;
Needham & Baillargeon, in press-a;
see section IIIC).
Positive data obtained at 5.5 or 6.5 months with support displays involving the
original cylinder and box would suggest
that,
like experiential
knowledge
(see
Section
M), physical
knowledge car help infants arrive at an unambiguous inter-
pretation
ofa display that is so complex as to overwhelm their capacity forfeatural
analysis.
VI. CONCLUDING REMARKS
We have
presented
evidence that young infants' organization of stationary adja-
cent
and
partly occluded displays is affected
by their configural, experiential, and
physical
klowledge. Such findings indicate
that, from a very young
age, infants'
perception
of objects and their boundaries is a complex
process
that depends on
the integmtion of multiple types of information. This characterization of the
young
perceiver
as one who readily makes use of all tie knowledge tiat she acquires
about objects-what objects typically look like, what
padicular
objects or catego-
ries
of objects exist in the
world, how objects
generally
move and
interact-runs
Object Segre\ation in Infa cy
counter
to more traditional accounts of young
infants as limited or modular
pro-
cessors whose
perceptual
organization
is dominated almost entirely by innate
principles (e.g.,
Kellman & Shipley,
l99l; Spelke, 1991;
Wertheimer, 1958).
A full account of the role of object knowledge
in young
infants' object segre-
gation
will requirc the weaving
together of thrce distinct
research efforts. First, we
need to understand the separate developments
of infants' conngural,
experiential,
and
physical
knowledge-how these and
perhaps
other types of knowledge are
acquired, represented, and
used at different ages. Second,
we need to gain afuller
grasp
of the
various
factors
that
limit infants'ability to use their object
krowledge.
For example, rccall that young
infants
can interpret featural
information only if
they first succeed in representing
it (see
Section III). Finally,
we need to investi
gate the processes
by which infants learn to integiate and resolve conflicts
between their different types of object
knowledgg.
We saw in the last section that
when faced with conflicting interpretations
of a display, one suggested
by their
knowledge of support or impenetrability, and the other by their configural know!
edge, 8-month-old
infants opt in favor of the
physical
interpretation
(Needham
&
Baillargeon, 1997). How is this outcome accomplished?
Do infants leam in the
couNe ofobserving and manipulating
objects that support
or impenetrability con-
straints are a much better
predictor
of a display's organization
than are the dis-
play's
featural
properties?
Seeking
the answers to these various
questions
will not only make it possible
to
begin elaborating
an account of object segregation in infancy; it will also reinforce
the recent discovery of many fundamental
continuities between
infants' and
adults'minds (e.9.,
Baillargeon, 1995; Spelke,
1994). TraditionaUy,
researche$
were often interested in early perception because they thought of infants as
extemely simple "preparations"
in which one
could study, without the
complicat-
ing effects of knowledge, experience, or memory, the nature of pure perceptual
processes,
As the evidence reported in this chapter makes clear, however, such a
view must now become
an illusion of the past;
even
young infants are strongly
biased to take advantage of all of the knowledge
that they
possess
about objects as
they attempt to make sense of their visual world.
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