Developmental Science 13:1 (2010), pp 143–150
DOI: 10.1111/j.1467-7687.2009.00873.x
PAPER
Infants’ individuation of agents and inert objects
Luca Surian and Stefania Caldi
Department of Cognitive Sciences, University of Trento, Italy
Abstract
Using the violation-of-expectancy method, we investigated 10-month-old infants’ ability to rely on dynamic features in object
individuation processes. Infants were first familiarized to events in which two different objects repeatedly appeared and
disappeared, one at a time from behind a screen; at test, the screen was removed, revealing either one or two objects. In
Experiment 1, one self-moving non-rigid agent and one inert object were involved in each trial, while in Experiment 2 two
different agents were presented. Infants preferred to look at one-object outcomes in Experiment 1, but they did not show any
preference for one- or two-object outcomes in Experiment 2. The results suggest that infants can use dynamic information to
detect agents in complex individuation tasks before they can rely on shape or surface features. We propose that the sortals AGENT
and INERT OBJECT appear in development before 12 months without a substantial contribution of linguistic experience. These
findings may motivate a revision of current theories on the development of kind-based individuation and object files.
Introduction
To ensure continuity in our relations with objects in the
visual world, we exploit a set of processes that allow us to
individuate objects and represent their identity over time
and space. The perceptual input we receive is unstable as
objects become occluded and change in their orientation
and luminance. Nevertheless, we experience a world that
is occupied by whole objects characterized by stable
properties and whose identity is established with little
effort. To achieve this, adults individuate objects by using
several sources of information and distinct perceptual
and cognitive processes. They rely on spatiotemporal
information and knowledge of core principles
constraining the motion of solid, physical objects
(Spelke, Kenstenbaum, Simons & Wein, 1995).
Adults also rely on visual properties such as colour,
shape and texture. Suppose that you insert a red marble
into a bag and then extract a green one: it is very likely
that you will infer that the red one is still inside, rather
than that it has changed colour. Moreover, we rely on
perceptual properties to establish the category
membership of a given object and this provides a
criterion for its individuation. We often trace the
numerical identity of living beings and artefacts by
relying on their sortal concepts. Sortal concepts provide
criteria for individuation and identity and are typically
lexicalized by count nouns (Hirsch, 1982; Macnamara,
1987).
A crucial question in cognitive science concerns the
ontogenesis and architecture of object individuation
processes. To make progress on this front, it is crucial
to assess whether and how human infants and nonhuman species can rely on these different sources of
information to individuate objects (Leslie, Xu, Tremoulet
& Scholl, 1998; Marcus, 2001; Santos, Sulkowski,
Spaepen & Hauser, 2002; Spelke et al., 1995; Wilcox &
Baillargeon, 1998a, 1998b; Xu & Carey, 1996; Xu, Carey
& Welch, 1999; for a review see Xu, 2007).
In a seminal study, Xu and Carey (1996) familiarized
infants to events in which two different objects (i.e. a cup
and a bottle) emerged, alternately, from behind a screen
and returned behind it. Then the screen was removed
revealing either both objects (the ‘expected outcome’) or
just one (the ‘unexpected outcome’). Infants aged
12 months looked longer at the unexpected outcome,
suggesting that they used property information (such as
shape) and possibly even sortals (truck, duck) to
represent two numerically distinct objects; in contrast,
10-month-olds failed to show looking times that were
consistent with correct numerical expectations. Tenmonth-old infants succeeded only when relevant
spatiotemporal evidence was provided to them by
having the two objects shown, simultaneously, at the
beginning of the trials. Xu and Carey (1996) interpreted
these findings as showing that, before 12 months, infants
can employ only the very general sortal physical object.
More specific sortals, such as dog and ball, are
acquired later, are affected by linguistic experience and,
in fact, may depend on lexical development (Xu, 2002).
Converging evidence that specific kind representations
emerge toward the end of the first year was also obtained
Address for correspondence: Luca Surian, Department of Cognitive Sciences, University of Trento, Corso Bettini, 31, 38068 Rovereto (Trento), Italy;
e-mail: [email protected]
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and
350 Main Street, Malden, MA 02148, USA.
144 Luca Surian and Stefania Caldi
using manual searches as a dependent variable (Van de
Walle, Carey & Prevor, 2000).
A further series of studies challenged some of the
claims put forward by Xu and Carey (1996). When
infants are tested using a simplified task, with single
rather than repeated object occlusions, even 9-montholds succeed in individuating a ball and a box despite the
absence of disambiguating spatiotemporal information,
as long as the objects differ in shape, colour and texture
(Wilcox & Baillargeon, 1998a; Xu & Baker, 2005). Using
a procedure that does not include the removal of the
occluding screen, Wilcox (1999) found that, at 4 months,
infants were able to rely on shape differences, at
7 months on size, and at 11 months on colour
differences. However, in this procedure it is not clear
whether subjects perceive a scenario involving two
objects or rather a single object that changes some of
its features (Xu & Baker, 2005; see also Mareschal &
Johnson, 2003). In sum, a number of studies have
shown that infants younger than 4 months can use
spatiotemporal information for tracking two numerically
distinct objects (Spelke & Van de Walle, 1993; Spelke et
al., 1995; Wynn, 1992) and, by at least 9 months, that
they can also rely on surface information in simple
individuation tasks without repeated object occlusions
(Wilcox & Baillargeon, 1998a; Xu & Baker, 2005).
One account of the development of individuation skills
emphasizes performance factors involving differences in
task complexity across different procedures. Young
infants succeed in simple tasks, but fail in more
complex tasks involving multiple occlusions (Wilcox &
Baillargeon, 1998a, 1998b). However, three types of
evidence suggest that this cannot be the whole story.
First, 10-month-olds succeed on some complex
individuation tasks, for example when the presence of a
human-like face discriminates between the two objects
involved in the test events (Bonatti, Frot, Zangl &
Mehler, 2002; Bonatti, Frot & Mehler, 2005). Second,
infants use shape cues earlier than other surface features
such as size and colour (Kaldy & Leslie, 2003; Wilcox,
1999). Third, 12-month-olds succeed in individuating
two objects in complex tasks when the objects belong to
different basic-level kinds, but they fail when objects,
although differing in shape, belong to the same basiclevel kind (Xu, Carey & Quint, 2004).
The pattern of early successes and late failures across a
wide range of tasks suggests that task complexity alone is
unlikely to explain all of the available developmental
evidence. One possibility is that, at about 12 months,
infants are able to individuate objects not only by
features, such squared or red, but also by kind (Xu et al.,
2004) and some sortals may be used for individuation
purposes even before 12 months. Bonatti et al. (2002,
2005) provided evidence suggesting that human being
may be one such early sortal.
The aim of the two experiments reported here was to
investigate whether infants younger than 12 months can
rely on the general sortals agent or animate
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd.
being in a complex object individuation task. Previous
investigations have shown that categorizations based on
global categories precede categorizations based on more
specific, basic-level concepts (Mandler & McDonough,
1993); it has been hypothesized that such categorizations
are guided by dynamic information extracted from
motion pattern, rather than by morphological features
or colour (Caramazza & Shelton, 1998; Leslie, 1994;
Mandler, 1992; Premack, 1990). However, no previous
study has investigated the effect of dynamic information
on infants’ object individuation processes. We predicted
that preverbal infants might be able to individuate
two objects when some crucial dynamic information is
available, even in the absence of relevant spatiotemporal
cues.
Experiment 1
Infants were tested on a complex object individuation
task that followed closely the procedure used in Xu and
Carey’s (1996) property ⁄ kind condition. Infants were
shown events in which two different objects emerged
from behind a screen and then returned behind it,
alternately for a fixed number of times. One of the two
objects (an animal) moved autonomously and nonrigidly and reacted at a distance to the contingent
motions of a hand. The other object, a simple artefact,
exhibited the opposite dynamic features (rigid and
passive motion) while it was grasped and dropped on
the floor by a hand. Following familiarization, the screen
was removed revealing either one of the two objects, or
both.
Method
Participants
Thirty-two full-term infants, 16 females and 16 males
(9 months 24 days to 10 months 24 days, M = 10 months
9 days) participated in the study and were randomly
assigned to either the agency (n = 16, M = 10 months
8 days) or the baseline condition (n = 16, M = 10 months
10 days). Six additional infants were excluded due to
fussiness (three), experimenter error (two) or parental
interference (one). Infants were recruited by obtaining
their birth records from the birth register of the city of
Padua. Their parents were contacted by telephone and
were given a certificate of attendance.
Apparatus
Infants sat on their parent’s lap facing a 19 inch
computer monitor placed at eye level at about 60 cm
from their head. A Macintosh personal computer was
used to control stimulus presentation. A piece of black
cardboard was placed around the computer monitor and
Individuation of agents and inert objects 145
curtains were hung behind the monitor, and on the walls,
to conceal distracting stimuli. The room was dimly lit
and parents were instructed to be silent and avoid any
interference with their babies. The entire session was
videotaped using two cameras, one focused on the
infant’s face and the other focused on the monitor.
Materials and procedure
We used computer animations, rather than events staged
with real objects as in most of the previous studies on this
topic, in order to facilitate the presentation of dynamic
features and avoid undesired variation in agents’ motion
across different trials. Infants were presented with
computer animations involving pairs of simple objects.
In the introductory trials, these animations involved a blue
bear (3.5 cm · 3.5 cm) and a red truck (5.5 cm · 3.0 cm).
Two additional pairs of objects were used in the test trials:
a red cup (3.0 cm · 2.8 cm) and a green caterpillar
(4.5 cm · 1.1 cm), a blue rectangle (4.5 cm · 3.5 cm)
and a black and yellow bee (5.0 cm · 3.5 cm). Four
coloured screens (10.5 cm · 5.5 cm) were used to stage the
object occlusion events.
Infants were tested in either the agency or the baseline
condition. Both conditions involved four introductory
trials, followed by four test trials. At the beginning of
each trial, the infant’s attention was drawn to the
monitor by squeezing a noisy toy. When infants looked
at the monitor, one experimenter started the animation
program, while the second experimenter, who was blind
to the type of test event, timed infants’ looking times as
the screen was removed. A given trial ended when
infants looked away for more than 2 consecutive
seconds or until 60 s had elapsed. All sessions were
videotaped and later coded independently by two
experimenters. Inter-rater reliability was high (mean
Pearson’s r = .93).
Introductory trials
In the introductory trials, a screen was lowered onto an
empty stage and then removed showing either a bear or a
truck, or both. When the trial ended, the
experimenter started the movie again which continued
showing a hand that pushed the object(s) away. Two
orders of outcomes (1, 2, 2, 1 and 2, 1, 1, 2) were
used and were counterbalanced across subjects.
Introductory trials provided no information on how
many objects would be involved in the events; they
simply introduced the child to displays involving either
one or two objects.
Test trials – agency condition
In the first two test trials the same pair of objects, e.g. a
red cup and a green caterpillar, was involved. Each test
trial consisted of a familiarization phase followed by a
test phase. During the familiarization phase, a screen was
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd.
lowered onto an empty stage. Then, a hand brought out
one of the two objects, e.g. the cup, from behind the left
side of the screen, raised it and then dropped it onto the
floor. Finally, the hand put the cup back behind the
screen. After 2 seconds, another hand on the opposite
side of the monitor moved toward the screen. Before the
hand had reached the screen, the second object, e.g. a
caterpillar, came out from behind the screen without
making any contact with hand. The caterpillar displayed
a number of dynamic features that are typical of animal
motion: it moved in non-rigid fashion and reacted
contingently at a distance to the movements of another
object (a human hand). The caterpillar emerged from
behind the screen, stretching and arching itself three
times and in this way approached the hand moving at a
speed of about 1.4 cm ⁄ s. First the hand withdrew from
the approaching caterpillar and then there was a reversal
of actions: it started to chase the caterpillar, which in
turn moved back behind the screen by arching itself
another three times (see Figure 1).
On each emergence from behind the screen, the objects
involved in the events were visible for about 5 s and
moved at approximately the same speed. However, the
type of motion was drastically different on causal
grounds: one object was passively moved by the human
hand while the other moved autonomously by changing
the relative position of its parts. The first test trial started
with eight familiarization emergences, four for each
object. Then, the screen was removed and the infant saw
either the two-object expected outcome or the one-object
unexpected outcome. At the end of each trial, a hand
cleared the scene of the objects as well as the screen. The
same set of toys was used for the first and the second test
trials. The second test trial started with four
familiarization routines, two for each object, before the
screen was removed. Then, the whole procedure was
repeated for a second pair of test trials with a different
pair of objects (a black and yellow bee and a blue cube).
There were eight familiarization routines in the first trial
and four routines in the second trial of each trial pair.
There were two possible orders of test outcomes (1, 2, 2,
1 or 2, 1, 1, 2) and they were counterbalanced across
subjects. Half of the subjects first saw the one-object
outcome involving an inert object and then the other
one-object outcome involving an agent; the other half
received the reversed order.
Test trials – baseline condition
The baseline condition was identical to the agency
condition except for one difference: in the test trials,
infants were not shown the familiarization phase, but
only the test phase. Therefore no information was
provided to them to form a specific numerical
expectation. The baseline condition provided a
measure of the infants’ intrinsic preference for one or
two objects in the experimental situation used for the
present study.
146 Luca Surian and Stefania Caldi
BASELINE CONDITION
1. Screen introduced
2. Screen raised: infants
see one or two objects
Two-object outcome
One-object outcome
AGENCY CONDITION
1. Screen introduced
2. First object comes out and
goes back behind the screen
3. Second object is brought out
and placed behind the screen
Steps 2 and 3 repeated
4. Screen raised: infants
see one or two objects
Expected outcome
Unexpected outcome
Figure 1 Schematic representation of the events presented in Experiment 1. In the test phase of both conditions, the screen was
removed and infants could see either one or two objects. In the baseline condition, children were not provided with any
disambiguating information on the number of objects involved in the events. In the agency condition, before the test phase, infants
were familiarized with the two objects alternately emerging and disappearing from behind a screen, for a fixed number of times. The
animal moved autonomously and non-rigidly while the artefact was brought out, raised and dropped on the floor by a hand.
Results and discussion
Introductory trials
Infants’ looking times were entered into a three-way
ANOVA with condition (baseline or agency) as a
between-subjects factor and outcome (one- or twoobject) and trial block (first or second) as withinsubjects factors. There were no significant main or
interactions effects. In the first and second introductory
trials, infants showed no preference for looking at
one- (M = 11.87 s, SD = 9.71) or two-object outcomes
(M = 13.70 s, SD = 8.95).
Test trials
A 2 (condition) · 2 (outcome) · 2 (trial block) ANOVA
on looking times revealed a significant interaction
between condition and outcome, F(1, 30) = 13.27,
p < .001, gp2 = .307. This interaction shows that the
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd.
looking time pattern in the agency condition was
significantly different from that of the baseline
condition (see Figure 2). The looking times in the
baseline condition for the one- and two-object
outcomes were Mone-obj. = 10.28 s (SD = 6.43) and
Mtwo-obj. = 14.40 s (SD = 13.02). The looking times for
the two outcomes in the baseline condition did not differ
significantly, t(15) = 1.95, p = .07. In the agency
condition, looking times were as follows: Mone-obj. =
17.35 s (SD = 13.95) and Mtwo-obj. = 10.34 s
(SD = 9.35). They differed significantly in the predicted
direction, t(15) = 3.17, p < .01. Separate t-tests on each
condition and each pair of test trials were performed to
further analyse the effect of outcome. In the baseline
condition there was no significant preference for one- or
two-object outcomes (first trial pair: Mone-obj. = 9.48 s,
SD = 5.70; Mtwo-obj. = 10.18 s, SD = 8.00; second trial
pair: Mone-obj. = 11.07 s, SD = 7.18; Mtwo-obj. = 18.61 s,
SD = 15.76). In contrast, on the second pair of trials
in the agency condition, the predicted preference for
Individuation of agents and inert objects 147
one-object outcomes was significant: Mone-obj. = 19.99 s
(SD = 16.21), Mtwo-obj. = 10.69 s (SD = 10.54), t(15) =
2.67, p = .017, two-tailed. However, in the agency
condition the mean looking times for the one-object
(M = 14.70 s, SD = 11.15) and the two-object displays
(M = 9.98 s; SD = 8.33) did not differ significantly in
the first pair of trials. It is not clear why infants’
preference for one-object outcomes, in the agency
condition, was significant for the second pair of test
trials, but not for the first pair. It is possible that the
main factor responsible for the difference between the
two trial blocks was the different degree of
familiarization infants had with the novel dynamic
features observed in the two objects involved in the test
event and with the human hand that interacted with one
of the two objects. Since infants were not presented with
these interesting novel features in the introductory trials
that preceded the test trials, their attention in the first
pair of trials may have been so occupied in processing
these features that looking times lost sensitivity as a
measure of infants’ numerical expectations.
Non-parametric analyses confirmed that the pattern of
looking times differed significantly in the two conditions.
Thirteen out of 16 infants looked longer at the oneobject outcomes in the agency condition, whereas only
six out of the 16 infants in the baseline condition showed
the same preference (v2 (1) = 6.35, p = .012).
When infants were presented with objects that differed
on a number of dynamic features, they changed their
pattern of preferences compared to that found in
the baseline condition. These findings suggest that
10-month-olds can individuate two objects in complex
occlusion events when relevant dynamic information is
available. These results are in line with the hypothesis
that infants used the sortals agent and inanimate
object.
However, one might argue that individuation was
simply facilitated by the presentation of more salient,
20
One object
Two objects
Mean looking times (s)
18
16
dynamic features rather than shape or surface features,
as in previous studies. If this is true then infants should
be even more facilitated in forming correct numerical
expectations when both objects present such features. By
contrast, if they relied on the sortal agent, they should
not succeed when both objects are agents belonging to
different categories. In Experiment 2, we tested this
possibility by presenting infants with pairs of animals
both displaying autonomous, non-rigid motion.
Experiment 2
Method
Participants
Thirty-two full-term infants, 19 females and 13 males (9
months 10 days to 10 months 15 days, M = 10 months)
participated. Equal numbers of infants were randomly
assigned to either the two-agent condition (M = 10 months
4 days) or the baseline condition (M = 9 months 25 days).
Five additional infants were excluded due to fussiness
(three) or experimenter error (two). Infants were recruited
as in Experiment 1.
Apparatus, procedure and stimuli
The apparatus and the procedure were the same as in
Experiment 1. The same objects used in the introductory
trials of Experiment 1 were used in Experiment 2. The
test trials involved two new pairs of stimuli: one pair
consisted of a brown dog (5 cm · 4 cm) and a green
caterpillar (4.5 cm · 1.5 cm); the second pair consisted
of a pink rabbit (6 cm · 4 cm) and a black and yellow
bee (5.0 cm · 3.5 cm). All objects involved were animals
showing autonomous, non-rigid and reactive motion.
Moreover, they all displayed a different locomotion
(walking, crawling, jumping, flying). All sessions were
videotaped and later coded independently by two
experimenters. The inter-rater reliability was high
(mean Pearson’s r = .99).
Results and discussion
14
Introductory trials
12
10
8
6
Baseline
Agency
Experiment 1
Baseline
Two-agent
Experiment 2
Figure 2 Results of the test trials in Experiments 1 and 2,
showing mean looking times (and standard errors) at the oneand two-object outcomes as a function of condition.
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd.
A 2 (condition) · 2 (outcome) · 2 (trial) ANOVA
showed no preference for looking at one- (M = 12.00 s)
or two-object outcomes (M = 13.89 s). Results revealed a
main effect of trial, F(1, 30) = 6.27, p = .018, gp2 = .173,
and a significant interaction of condition and trial block,
F(1, 30) = 6.99, p = .013, gp2 = .189. Infants looked
longer at the first (M = 17.09 s) than at the second trials
(M = 9.13 s) in the baseline condition, but did not do so
in the two-agent condition (first trial: M = 12.66; second
trial: M = 12.88).
148 Luca Surian and Stefania Caldi
Test trials
A 2 (condition) · 2 (outcome) · 2 (trial) ANOVA
yielded no significant main effect or interaction (see
Figure 2). Looking times in the two-agent condition
did not differ from those found in the baseline
condition, F(1, 30) = .002, p = .962, gp2 < .001.
Infants spent the same time looking at the one- and
two-object outcomes (baseline: Mone-obj. = 12.44 s,
SD = 11.29; Mtwo-obj. = 13.13 s, SD = 7.70; two-agent
condition: Mone-obj. = 9.57 s, SD = 11.54; Mtwo-obj. =
10.11 s, SD = 12.37). Nine out of 16 infants in the
two-agents condition looked longer at the one-object
outcomes and six out of 16 infants showed the same
preference in the baseline condition (v2 (1) = 1.09, ns).
These results failed to provide any evidence of infants’
ability to form a specific numerical expectation despite
the presence of several dynamic and static features.
General discussion
The results of our investigation showed that preverbal
infants could individuate two objects in repeated
occlusion events when one of them moved
autonomously and non-rigidly in an animal-like
fashion, and the other instead behaved passively, like
an inanimate object. When they appeared from behind
the screen, the agents in this study were already set in
motion so children were not provided with the cue of
self-initiation of motion, an aspect of agents’ motion that
has been the focus of several theoretical discussions and
empirical studies on infants’ understanding of agents and
actions (Csibra, Gergely, Biro, Koos & Brockbank, 1999;
Mandler, 1992, 2004; Premack, 1990; Saxe & Carey,
2006; Saxe, Tzelnic & Carey, 2007; Schlottmann, Surian
& Ray, in press). However, infants were presented with
agents that repeatedly changed their paths without
contacting any object. This is a powerful cue to detect
autonomous motion and infer the presence of an internal
source of energy (Leslie, 1994; Luo & Baillargeon, 2005).
Previous studies have reported that infants fail an
object individuation task when they are presented with
one animal and one artifact (e.g. a camel and a cup);
however, in those experiments infants were provided with
neither spatial cues nor with discriminating dynamic cues
(Xu & Carey, 1996). If considered together with previous
studies that used similar individuation tasks (e.g. Bonatti
et al., 2002; Van de Walle et al., 2000; Xu & Carey, 1996;
Xu et al., 1999, 2004), the present findings suggest a
primacy of dynamic information in object individuation
processes as compared to shape or surface information,
with the exception of human face-like features that
appear to be used successfully by 10-month-olds (Bonatti
et al., 2002, 2005).
Moreover, it was not the presence of attractive
animations and kinetic information per se that
enhanced infants’ performance. In Experiment 2,
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd.
infants were presented with two animals, for example a
bee and a rabbit that displayed different motion patterns,
and no specific numerical expectation was revealed by
their reactions to the test outcomes. The pattern of
successes (Experiment 1) and failures (Experiment 2) is
in line with the proposal that 10-month-olds employed
the sortals agent and inert object. This would suggest
that in individuation processes, as in categorization tasks
(Mandler, 1992; Mandler & McDonough, 1993), infants
start to use the sortal concept agent before using more
specific concepts, such as bee or cup.
One familiar way to phrase this developmental trend
would be that infants go from superordinate sortal
concepts to basic concepts and not vice-versa. However,
it is doubtful that we can easily rely on the distinction
between basic and superordinate levels of categorization
to describe infants’ conceptual development. One
difficulty is that what is claimed to be at the basic level
for adults may be at a different hierarchical level for
infants: agent could be superordinate in the adult
conceptual system, but it could be at the basic level in the
infants’ semantic memory. Moreover, there are many
difficulties in using the subordinate–basic–superordinate
distinction with older children and adults and even more
methodological and theoretical concerns when one tries
to apply it to infants (see Mandler, 2004, for an in-depth
analysis).
Our conclusions on infants’ use of the sortal agent in
our experiments follow the computational analysis
proposed by Xu and Carey (1996; see also Xu, 2007) by
endorsing the role of sortals in explaining infants’
performance on complex object individuation tasks.
However, our findings challenge a crucial developmental
aspect of the Xu and Carey model: we propose that some
sortals that are more specific than the sortal object (e.g.
agent, animate being or person) may appear before
12 months without a substantial contribution of linguistic
experience, while such experience may instead have a
more significant effect on the acquisition of other less
general sortals, such as cat or car.
These findings also have some interesting
implications for the neuropsychological model
proposed by Leslie et al. (1998). In that model, object
individuations are based on a specialized attentional
mechanism called the object indexing system. As soon
as an object is located in space, an indexed object file is
opened and this is how objects are mentally
individuated (Kaldy & Leslie, 2003). First, object files
include only spatial information, but soon other object
features are attached and bound to these. Files are fed
with input from both the ‘what’ and the ‘where’ visual
systems (Goodale & Milner, 2004; Mareschal &
Johnson, 2003) and last for at least 8 seconds (Noles,
Scholl & Mitroff, 2005). The model posits that the
‘what’ system is active very early in development,
allowing very young infants to categorize objects by
shape and colour. However, the ‘what’ pathway is not
fully connected with the object indexing system until
Individuation of agents and inert objects 149
the end of the first year of life. Only the maturation of
these connections, at the end of the first year, allows
infants to bind features from the ‘what’ system to the
object files.
The results of our study may stimulate a revision of the
object indexing model because, as it was originally
proposed, such a model cannot account for them. Our
results suggest that infants may start to bind dynamic
features to object files before they can successfully bind
shape or surface features. Therefore the model needs to
overcome the simple dichotomy between spatiotemporal
information, treated by the ‘where’ system, and other
features, treated by the ‘what’ system (Giovannini,
Jacomuzzi, Bruno, Semenza & Surian, in press). It
needs to pay closer attention to some relevant
distinctions among different types of features that are
dealt with by the ‘what’ system (Kaldy & Leslie, 2003).
Also, the object indexing model will have to show why
partially immature connections between the what and
where systems would let dynamic feature be bound to
object files before shape or surface features.
Overall, our findings provide indirect support for
several current proposals that have emphasized infants’
early ability to attend to motion cues in order to
understand objects’ behaviour and causal relations
(Csibra et al., 1999; Johnson, 2003; Luo & Baillargeon,
2005; Leslie & Keeble, 1987; Mandler, 2004;
Schlottmann & Surian, 1999; Schlottmann et al., in
press; Simion, Regolin & Bulf, 2008; Surian, Caldi &
Sperber, 2007). Future studies could extend and clarify
these findings by testing the necessity and sufficiency of
dynamic and contingency features to infants’ object
individuation processes. Previous studies have shown
that the presence of contingency cues is sufficient to elicit
‘gaze’ following in 12-month-old infants even when
infants are presented with an object that lacks any
human or animal feature. This finding suggests that
contingency cues are sufficient for infants to detect
agents (Johnson, 2003; Johnson, Slaughter & Carey,
1998). Dynamic information is often viewed as
specifying motion in general (e.g. Vuong & Tarr, 2004),
but in the present context we should focus on some
specific aspects of motion, namely those that
reveal distinctive mechanical properties of the agents,
such as the presence of internal sources of energy that
cause a body to move (Leslie, 1994; Luo & Baillargeon,
2005).
In our study, all agents presented non-rigid motion
and changed direction independently, thus showing clear
evidence of their capacity to move autonomously. We
believe that this information was sufficient for infants to
individuate some objects using the sortal agent, but we
are aware that our findings fall short of demonstrating
this. This is because all agents also interacted at a
distance with a human hand. Therefore, these findings do
not tell us whether, in the infant’s cognitive system,
dynamic and contingency cues were independently
sufficient to individuate agents. To settle the issue, one
2009 The Authors. Journal compilation 2009 Blackwell Publishing Ltd.
needs to manipulate systematically the presence of
contingency cues and dynamic information. It would
also be important to test the relative contribution of
different types of dynamic features. Finally, further
studies should try to investigate whether the primacy of
dynamic and contingency cues also emerges in different
object individuation tasks such as those used by Wilcox
and Baillargeon (1998a).
Acknowledgements
This research was supported by a PRIN grant from the
Italian Ministry of Education. We are grateful to Gergely
Csibra, Bradford Mahon, Michael Siegal, and three
anonymous reviewers for their useful comments and
suggestions. We also wish to thank Stefano Andriolo and
the staff at the Dipartimento di Psicologia dello Sviluppo
e della Socializzazione (University of Padua, Italy), the
parents and the infants involved in this study for their
kind help.
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Received: 12 July 2007
Accepted: 18 November 2008