A direct comparison of infants' use of featural and spatiotemporal information when individuating objects in a non-occlusion event-monitoring design

A central issue within the field of object individuation concerns the kind of information that infants rely on when they succeed in individuating objects. By means of the violation-of-expectation strategy, the present study reports a comparison of 8.0- and 6.5-month-old infants' use of featural and spatiotemporal information in a new non-occlusion event-monitoring design. Using a mirror setup the memory demands were minimized, because all apparent changes in the unexpected test events took place in full view of the infants. The results indicate that the 8.0- and 6.5-month-old infants individuated objects successfully regardless of whether they were provided with featural or spatiotemporal information. The results are discussed in relation to the relevant literature.     

Object individuation: infants' use of shape, size, pattern, and color.

Recent research indicates that when an event-monitoring paradigm is used, infants as young as 4.5 months of age demonstrate the ability to use featural information to individuate objects involved in occlusion events (Wilcox & Baillargeon, 1998a, Object individuation in infancy: The use of featural information in reasoning about occlusion events. Cognitive Psychology 37, 97-155; Wilcox & Baillargeon, 1998b, Object individuation in young infants: Further evidence with an event monitoring task. Developmental Science 1, 127-142). For example, in one experiment (Wilcox & Baillargeon, 1998b, Object individuation in young infants: Further evidence with an event monitoring task. Developmental Science 1, 127-142) 4.5-month-old infants saw a test event in which a green ball with colored dots disappeared behind one edge of a narrow or wide screen, and a red box with silver thumbtacks appeared at the other edge; the narrow screen was too narrow to hide both objects simultaneously, whereas the wide screen was sufficiently wide to hide both objects at the same time. The infants looked reliably longer at the narrow- than at the wide-screen test event. These and control results suggested that the infants had: (a) used the featural differences between the ball and box to conclude that two objects were involved in the event; (b) judged that both objects could fit simultaneously behind the wide but not the narrow screen; and hence (c) were surprised by the narrow-screen event. The present experiments build on these initial findings by investigating the features to which infants are most sensitive. Four experiments were conducted with infants 4.5-11.5 months of age using the same procedure, except that only one feature was manipulated at a time: shape, size, pattern, or color. The results indicated that 4.5-month-olds use both shape and size features to individuate objects involved in occlusion events. However, it is not until 7.5 months that infants use pattern, and 11.5 months that infants use color, to reason about object identity. It is suggested that these results reflect biases in the kind of information that infants attend to when reasoning about occlusion events. Possible sources of bias are discussed.     

Object individuation in young infants: Further evidence with an event-monitoring paradigm

Recent results indicate that, when tested with an event-monitoring task, 7.5- and 9.5-month-olds give evidence that they can individuate objects in different-objects occlusion events – events in which two distinct objects appear successively on either side of an occluder (Wilcox and Baillargeon, in press). The present research sought to confirm and extend these findings. The experiments examined 7.5- and 4.5-month-olds’ ability to correctly interpret a different-objects (ball-box condition) and a same-object (ball-ball condition) occlusion event. The infants in the ball-box condition saw a test event in which a ball disappeared behind the left edge of a screen; after a pause, a box emerged from behind the screen's right edge. For half of the infants (wide-screen event), the screen was wide and could occlude the ball and box simultaneously; for the other infants (narrow-screen event), the screen was narrow and should not have been able to occlude the ball and box at the same time. The infants in the ball-ball condition saw identical wide- and narrow-screen events except that the ball appeared on both sides of the screen. The infants in the ball-box condition looked reliably longer at the narrow- than at the wide-screen event, whereas those in the ball-ball condition tended to look equally at the events. These results suggest that the ball-box infants (a) were led by the featural differences between the ball and box to view them as distinct objects; (b) judged that the ball and box could both be occluded by the wide but not the narrow screen; and (c) were surprised in the narrow-screen event when this judgment was violated. In contrast, the ball-ball infants (a) assumed, based on the featural similarities of the balls that appeared on either side of the screen, that they were one and the same ball, and (b) realized that the ball could be occluded by either the wide or the narrow screen. These results indicate that, by 4.5 months of age, infants are able to use featural information to correctly interpret different-objects and same-object occlusion events. These findings are discussed in the context of the newly-drawn distinction between event-monitoring and event-mapping paradigms (Wilcox and Baillargeon, in press).     

Infants' reasoning about opaque and transparent occluders in an individuation task

There has been some debate about whether infants 10 months and younger can use featural information to individuate objects. The present research tested the hypothesis that negative results obtained with younger infants reflect limitations in information processing capacities rather than the inability to individuate objects based on featural differences. Infants aged 9.5 months saw one object (i.e. a ball) or two objects (i.e. a box and a ball) emerge successively to opposite sides of an opaque occluder. Infants then saw a single ball either behind a transparent occluder or without an occluder. Only the infants who saw the ball behind the transparent occluder correctly judged that the one-ball display was inconsistent with the box-ball sequence. These results suggest that: (a) infants categorize events involving opaque and transparent occluders as the same kind of physical situation (i.e. occlusion) and (b) support the notion that infants are more likely to give evidence of object individuation when they need to reason about one kind of event (i.e. occlusion) than when they must retrieve and compare categorically distinct events (i.e. occlusion and no-occlusion).     

Object individuation using property/kind information in rhesus macaques (Macaca mulatta)

Around 1 year of age, infants develop the ability to individuate objects in the absence of spatiotemporal information. Some have proposed that this capacity relies on the emergence of language and, in particular, that comprehending an object's label is required to individuate it as a particular kind. One approach to testing this hypothesis is to conduct experiments on pre-linguistic human infants. A second is to test non-linguistic animals. We followed the second approach, exploring whether semi-free-ranging rhesus macaques can individuate objects using property/kind information. To make the results most directly comparable, we adapted a reaching paradigm used to examine property/kind individuation in infants. Results from three experiments demonstrate that, like 12-month-old infants, adult rhesus macaques can use both spatiotemporal and property/kind information to individuate food objects. In a fourth experiment designed to examine which properties are used to individuate food objects, results revealed that rhesus use color, but not shape. These results, together with experiments involving different procedures, provide support for the conclusion that in the absence of linguistic abilities, some non-human primates spontaneously use property/kind information to individuate objects.     

Memory load affects object individuation in 18-month-old infants

Accurate representation of a changing environment requires individuation-the ability to determine how many numerically distinct objects are present in a scene. Much research has characterized early individuation abilities by identifying which object features infants can use to individuate throughout development. However, despite the fact that without memory featural individuation would be impossible, little is known about how memory constrains object individuation. Here, we investigated infants' ability to individuate multiple objects at once and asked whether individuation performance changes as a function of memory load. In three experiments, 18-month-old infants saw one, two, or three objects hidden and always saw the correct number of objects retrieved. On some trials, one or more of these objects surreptitiously switched identity prior to retrieval. We asked whether infants would use this identity mismatch to individuate and, hence, continue searching for the missing object(s). We found that infants were less likely to individuate objects as memory load grew, but that infants individuated more successfully when the featural contrast between the hidden and retrieved objects increased. These results suggest that remembering more objects may result in a loss of representational precision, thereby decreasing the likelihood of successful individuation. We close by discussing possible links between our results and findings from adult working memory.     

Infants' ability to use object kind information for object individuation.

The present studies investigate infants reliance on object kind information in solving the problem of object individuation. Two experiments explored whether adults, 10- and 12-month-old infants could use their knowledge of ducks and cars to individuate an ambiguous array consisting of a toy duck perched on a toy car into two objects. A third experiment investigated whether 10-month-old infants could use their knowledge of cups and shoes to individuate an array consisting of a cup perched on a shoe into two objects. Ten-month-old infants failed to use object kind information alone to resolve the ambiguity with both pairs of objects. In contrast, infants this age succeeded in using spatiotemporal information to segment the array into two objects, i.e. they succeeded if shown that the duck moved independently relative to the car, or the cup relative to the shoe. Twelve-month-old infants, as well as adults, succeeded at object individuation on the basis of object kind information alone. These findings shed light on the developmental course of object individuation and provide converging evidence for the Object-first Hypothesis [Xu, F., Carey, S., 1996; Xu, F., 1997b]. Early on, infants may represent only one concept that provides criteria for individuation, namely physical object; kind concepts such as duck, car, cup, and shoe may be acquired later in the first year of life.     

Seven-month-old infants chunk items in memory

Although working memory has a highly constrained capacity limit of three or four items, both adults and toddlers can increase the total amount of stored information by "chunking" object representations in memory. To examine the developmental origins of chunking, we used a violation-of-expectation procedure to ask whether 7-month-old infants, whose working memory capacity is still maturing, also can chunk items in memory. In Experiment 1, we found that in the absence of chunking cues, infants failed to remember three identical hidden objects. In Experiments 2 and 3, we found that infants successfully remembered three hidden objects when provided with overlapping spatial and featural chunking cues. In Experiment 4, we found that infants did not chunk when provided with either spatial or featural chunking cues alone. Finally, in Experiment 5, we found that infants also failed to chunk when spatial and featural cues specified different chunks (i.e., were pitted against each other). Taken together, these results suggest that chunking is available before working memory capacity has matured but still may undergo important development over the first year of life.     

Object individuation and labelling in 6-month-old infants

The ability to determine how many objects are involved in physical events is fundamental for reasoning about the world that surrounds us. Previous studies suggest that infants can fail to individuate objects in ambiguous occlusion events until their first birthday and that learning words for the objects may play a crucial role in the development of this ability. The present eye-tracking study tested whether the classical object individuation experiments underestimate young infants’ ability to individuate objects and the role word learning plays in this process. Three groups of 6-month-old infants (N = 72) saw two opaque boxes side by side on the eye-tracker screen so that the content of the boxes was not visible. During a familiarization phase, two visually identical objects emerged sequentially from one box and two visually different objects from the other box. For one group of infants the familiarization was silent (Visual Only condition). For a second group of infants the objects were accompanied with nonsense words so that objects’ shape and linguistic labels indicated the same number of objects in the two boxes (Visual & Language condition). For the third group of infants, objects’ shape and linguistic labels were in conflict (Visual vs. Language condition). Following the familiarization, it was revealed that both boxes contained the same number of objects (e.g. one or two). In the Visual Only condition, infants looked longer to the box with incorrect number of objects at test, showing that they could individuate objects using visual cues alone. In the Visual & Language condition infants showed the same looking pattern. However, in the Visual vs Language condition infants looked longer to the box with incorrect number of objects according to linguistic labels. The results show that infants can individuate objects in a complex object individuation paradigm considerably earlier than previously thought and that linguistic cues enforce their own preference in object individuation. The results are consistent with the idea that when language and visual information are in conflict, language can exert an influence on how young infants reason about the visual world.     

Will any doll do? 12-month-olds' reasoning about goal objects

Infants as young as 5 months of age view familiar actions such as reaching as goal-directed (Woodward, 1998), but how do they construe the goal of an actor's reach? Six experiments investigated whether 12-month-old infants represent reaching actions as directed to a particular individual object, to a narrowly defined object category (e.g., an orange dump truck), or to a more broadly defined object category (e.g., any truck, vehicle, artifact, or inanimate object). The experiments provide evidence that infants are predisposed to represent reaching actions as directed to categories of objects at least as broad as the basic level, both when the objects represent artifacts (trucks) and when they represent people (dolls). Infants do not use either narrower category information or spatiotemporal information to specify goal objects. Because spatiotemporal information is central to infants' representations of inanimate object motions and interactions, the findings are discussed in relation to the development of object knowledge and action representations.     

Object individuation and event mapping: developmental changes in infants’ use of featural information

The present research examined the development of 4.5‐ to 7.5‐month‐old infants’ ability to map different‐features occlusion events using a simplified event‐mapping task. In this task, infants saw a different‐features (i.e. egg‐column) event followed by a display containing either one object or two objects. Experiments 1 and 2 assessed infants’ ability to judge whether the egg‐column event was consistent with a subsequent one‐column display. Experiments 3 and 4 examined infants’ ability to judge whether the objects seen in the egg‐column event and those seen in a subsequent display were consistent in their featural composition. At 7.5 and 5.5 months, but not at 4.5 months, the infants successfully mapped the egg‐column event onto the one‐column display. However, the 7.5‐ and 5.5‐month‐olds differed in whether they mapped the featural properties of those objects. Whereas the 7.5‐month‐olds responded as if they expected to see two specific objects, an egg and a column, in the final display the 5.5‐month‐olds responded as if they simply expected to see ‘two objects’. Additional results revealed, however, that when spatiotemporal information specified the presence of two objects, 5.5‐month‐olds succeeded at tagging the objects as being featurally distinct, although they still failed to attach more specific information about what those differences were. Reasons for why the younger infants had difficulty integrating featural information into their object representations were discussed.     

Developmental changes in the discrimination of dynamic human actions in infancy

Recent evidence suggests that adults selectively attend to features of action, such as how a hand contacts an object, and less to configural properties of action, such as spatial trajectory, when observing human actions. The current research investigated whether this bias develops in infancy. We utilized a habituation paradigm to assess 4-month-old and 10-month-old infants' discrimination of action based on featural, configural, and temporal sources of action information. Younger infants were able to discriminate changes to all three sources of information, but older infants were only able to reliably discriminate changes to featural information. These results highlight a previously unknown aspect of early action processing, and suggest that action perception may undergo a developmental process akin to perceptual narrowing.     

What’s the object of object working memory in infancy? Unraveling ‘what’ and ‘how many’

Infants have a bandwidth-limited object working memory (WM) that can both individuate and identify objects in a scene, (answering ‘how many?’ or ‘what?’, respectively). Studies of infants’ WM for objects have typically looked for limits on either ‘how many’ or ‘what’, yielding different estimates of infant capacity. Infants can keep track of about three individuals (regardless of identity), but appear to be much more limited in the number of specific identities they can recall. Why are the limits on ‘how many’ and ‘what’ different? Are the limits entirely separate, do they interact, or are they simply two different aspects of the same underlying limit?We sought to unravel these limits in a series of experiments which tested 9- and 12-month-olds’ WM for object identities under varying degrees of difficulty. In a violation-of-expectation looking-time task, we hid objects one at a time behind separate screens, and then probed infants’ WM for the shape identity of the penultimate object in the sequence. We manipulated the difficulty of the task by varying both the number of objects in hiding locations and the number of means by which infants could detect a shape change to the probed object. We found that 9-month-olds’ WM for identities was limited by the number of hiding locations: when the probed object was one of two objects hidden (one in each of two locations), 9-month-olds succeeded, and they did so even though they were given only one means to detect the change. However, when the probed object was one of three objects hidden (one in each of three locations), they failed, even when they were given two means to detect the shape change. Twelve-month-olds, by contrast, succeeded at the most difficult task level.Results show that WM for ‘how many’ and for ‘what’ are not entirely separate. Individuated objects are tracked relatively cheaply. Maintaining bindings between indexed objects and identifying featural information incurs a greater attentional/memory cost. This cost reduces with development. We conclude that infant WM supports a small number of featureless object representations that index the current locations of objects. These can have featural information bound to them, but only at substantial cost.     

Individuation of Actions from Continuous Motion

Three studies explored how infants parse a stream of motion into distinct actions. Results show that infants (a) can perceptually discriminate different actions performed by a puppet and (b) can individuate and enumerate heterogeneous sequences of such actions (e.g., jump-fall-jump) when the actions are separated by brief motionless pauses, but (c) are not able to individuate such actions when embedded within a continuous stream of motion. Combined with previous research showing that infants can individuate homogeneous actions from an ongoing stream of motion, these findings suggest that infants can use repeating patterns of motion in the perceptual input to define action boundaries. Results have implications as well for infants' conceptual structure for actions.     

Color-function categories that prime infants to use color information in an object individuation task

There is evidence for developmental hierarchies in the type of information to which infants attend when reasoning about objects. Investigators have questioned the origin of these hierarchies and how infants come to identify new sources of information when reasoning about objects. The goal of the present experiments was to shed light on this debate by identifying conditions under which infants’ sensitivity to color information, which is slow to emerge, could be enhanced in an object individuation task. The outcome of Experiment 1 confirmed and extended previous reports that 9.5-month-olds can be primed, through exposure to events in which the color of an object predicts its function, to attend to color differences in a subsequent individuation task. The outcomes of Experiments 2-4 revealed age-related changes in the nature of the representations that support color priming. This is exemplified by three main findings. First, the representations that are formed during the color-function events are relatively specific. That is, infants are primed to use the color difference seen in the color-function events to individuate objects in the test events, but not other color differences. Second, 9.5-month-olds can be led to form more abstract event representations, and then generalize to other colors in the test events if they are shown multiple pairs of colors in the color-function events. Third, slightly younger 9-month-olds also can be led to form more inclusive categories with multiple color pairs, but only when they are allowed to directly compare the exemplars in each color pair during the present events. These results shed light on the development of categorization abilities, cognitive mechanisms that support color-function priming, and the kinds of experiences that can increase infants’ sensitivity to color information.     

Infants' ability to use luminance information to individuate objects

Recent research indicates that infants first use form and then surface features as the basis for individuating objects. However, very little is known about the underlying basis for infants' differential sensitivity to form than surface features. The present research assessed infants' sensitivity to luminance differences. Like other surface properties, luminance information typically reveals little about an object. Unlike other surface properties (e.g. pattern, color), the visual system can detect luminance differences at birth. The outcome of two experiments indicated that 11.5-month-olds, but not 7.5-month-olds, used luminance differences to individuate objects. These results suggest that it is not the age at which infants can detect a feature, but the nature of the information carried by the feature, that determines infants' capacity to individuate objects.     

The human first hypothesis: identification of conspecifics and individuation of objects in the young infant

How do infants individuate and track objects, and among them objects belonging to their species, when they can only rely on information about the properties of those objects? We propose the Human First Hypothesis (HFH), which posits that infants possess information about their conspecifics and use it to identify and count objects. F. Xu and S. Carey [Cognitive Psychology, 30(2), 111-153, 1996] argued that before the age of 1 year, infants fail to use property information. To explain their results, Xu and Carey proposed the Object First Hypothesis (OFH), according to which infants under 1 year of age have only the general concept of physical object to identify and count objects. We show that infants have a more extensive knowledge of sortals than that claimed by the OFH. When 10-month-olds see one humanlike and one non-humanlike object, they successfully identify and count them by using the contrast in their properties, as predicted by the HFH. We also show that infants succeed even when they make a decision based on differences between two close basic-level categories such as humanlike objects and doglike objects, but fail when they have to use differences within the human category. Thus, infants treat "human" as a basic sortal, as predicted by the HFH. We argue that our results cannot be accounted for by general purpose mechanisms. Neither the strong version of the OFH and its explanation in terms of object indexing mechanisms [A. M. Leslie, F. Xu, P. Tremoulet, & B. J. Scholl, Trends in Cognitive Sciences, 2(1), 10-18, 1998] nor explanations in terms of task demands [T. Wilcox & R. Baillargeon, Cognitive Psychology, 37(2), 97-155, 1998] are sufficient to explain our results.     

Object manipulation facilitates kind-based object individuation of shape-similar objects

Five experiments investigated the importance of shape and object manipulation when 12-month-olds were given the task of individuating objects representing exemplars of kinds in an event-mapping design. In Experiments 1 and 2, results of the study from Xu, Carey, and Quint (2004, Experiment 4) were partially replicated, showing that infants were able to individuate two natural-looking exemplars from different categories, but not two exemplars from the same category. In Experiment 3, infants failed to individuate two shape-similar exemplars (from Pauen, 2002a) from different categories. However, Experiment 4 revealed that allowing infants to manipulate objects shortly before the individuation task enabled them to individuate shape-similar objects from different categories. In Experiment 5, allowing object manipulation did not induce infants to individuate natural-looking objects from the same category. These findings suggest that object manipulation facilitates kind-based individuation of shape-similar objects by 12-month-olds.     

How infants grasp two adjacent objects: effects of perceived display composition on infants’ actions

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

Featural and configural face processing in adults and infants: a behavioral and electrophysiological investigation

We sought to elucidate the behavioral and electrophysiological correlates of face processing, in adults and infants, by manipulating either the featural or configural information within the face. Two different experiments are reported. In these experiments, event-related potentials (ERPs) were recorded from the scalp while adult, 8-month-old, and 4-month-old participants completed configural-change and featural-change face tasks. The infants also completed a behavioral visual paired-comparison task with featural and configural face changes. ERP results reveal hemispheric differences in processing featural but not configural changes for the N170 in adults. Furthermore, featural and configural changes are processed differently within the right and left hemispheres. The right hemisphere N170 is significantly greater for configural compared to featural changes. The left hemisphere N170, however, exhibits the opposite effect. Infant data suggest that similar to adults, 8-month-old, but not 4-month-old participants, exhibit similar hemispheric differences between featural and configural changes for the P400 component. Behavioral results suggest increased sensitivity to both featural and configural face changes in 8-month-olds compared to 4-month-olds.