The last 20 yrs have witnessed important progress in the study of infant object perception and cognition. Fantz, R. L., 1964. Visual experience in infants: Decreased attention to familiar patterns relative to novel ones. Science 146, pp. 668–670.Fantz (1964) first pioneered the methodology of preferential looking with infants and showed that we can study perceptual discrimination in very young infants using this method. Subsequently, Spelke (1985) and others extended the use of this method to ask questions beyond simple perceptual discriminations. For example, do infants perceive objects as three-dimensional? Do infants understand that objects are cohesive and do not leave parts of themselves behind while moving through space? Do infants comprehend that two solid objects cannot occupy the same space at the same time? Under what conditions do infants arrive at representations of two as opposed to one object in an event?

The general method of these studies exploits the fact that infants (as well as adults) tend to look longer at new and unexpected events. Infants are shown the same event or objects repeatedly, and their looking times are recorded. With each repetition, the infant’s looking time declines; that is, infants “habituate.” When the infant’s looking time has reached some pre-set criterion (usually 50% of the initial looking times summed over three trials), test trials begin. Infants are alternately shown an expected outcome (an outcome that is consistent with adults’ understanding of the physical or social world) and an unexpected outcome (an outcome that is inconsistent with adults’ understanding of the physical or social world). If infants have the same understanding of the events shown during habituation as adults, they should look longer at the unexpected outcome relative to the expected outcome. This particular version of the methodology is often called the “visual preference for violation of expectancy” paradigm. It is in one sense akin to the measure of reaction time: It takes the infant longer to process an anomalous event/outcome than one that is consistent with their general model of the world.

Many researchers have contributed extensively to the literature of how infants understand the physical world around them. Spelke and her colleagues have discovered several principles that guide young infants’ perception and reasoning of physical objects. First, physical objects are cohesive; they move as wholes, and they do not leave parts of themselves behind. Second, objects obey principles of continuity and solidity; they move on spatiotemporally continuous paths, and two objects cannot occupy the same space at the same time. Third, objects act on each other upon contact; that is, there is no action at a distance. These principles stay at the core of our mature understanding of physical entities as adults. Against the backdrop of a strong Piagetian tradition in developmental psychology, which claims that young infants experience a “blooming, buzzing confusion” as opposed to coherent three-dimensional objects in their surroundings, Spelke and her collaborators have shown that some of our deepest beliefs about how physical objects should behave may have their roots in early infancy, perhaps given innately. Other researchers have focused on infants’ reasoning about specific types of physical events such as occlusion and support, and how infants perceive the causal relations among objects.

These research enterprises have been followed up by many laboratories. Much controversy has been generated over (a) the nature of the infants’ representations when they show such early competence; (b) how these new findings should be reconciled with the highly robust and replicable findings by Piaget and his associates; and (c) how these early representations are related to the mature cognitive system in adults.

My research focuses on a particular aspect of object representations, namely the issue of object individuation and object identity. Specifically, how do infants arrive at representations of multiple objects and trace their identity through time and space? What sources of information do infants employ in this process? We know that adults use at least three sources of information in object individuation: Spatiotemporal information, object property information, and object kind information. Spatiotemporal information refers to generalizations such as one object cannot be at two places at the same time and objects travel on spatiotemporally continuous paths. Object property information refers to how we can use general Gestalt principles of good form, good continuation, and relevant featural differences (e.g., color, shape, size, or texture) in object individuation. Lastly, object kind information refers to our knowledge about specific categories of objects. For example, size change may or may not indicate a change of identity depending on whether the entity under consideration is biological or not. In other words, our criteria for object individuation and object identity are kind-relative.

Bower (1974) was the first to suggest that young infants may use spatiotemporal information to individuate objects before they use object property information to do so. In a series of experiments, Bower found that before five months of age, infants’ tracking behavior was interrupted if a moving object stopped abruptly, but not if an object (e.g., a toy bunny) had apparently turned into a different object (e.g., a toy truck). He concluded that infants at five months represent moving and stationery objects as distinct objects: When an object stops moving, it is no longer the same object. However, when a toy bunny apparently has turned into a toy truck, infants up to five months are unable to use the property differences to arrive at a representation of two distinct objects. As we will see below, although Bower’s particular spatiotemporal rule may be incorrect and his results have been difficult to replicate, his insight about the relative importance of spatiotemporal and object property information in early object individuation may well be correct.

In recent years, much more research has been conducted on how infants resolve the ambiguity concerning the number of objects in an event or a scene. Ambiguity in object individuation can arise in at least three ways. First, two fully visible adjacent segments may or may not belong to the same object. Second, two segments may be occluded at their boundary such that it is ambiguous whether they belong to a single object or not. Third, infants are constantly in situations where they have to determine whether two encounters with an object are one object seen on two different occasions or are two numerically distinct objects.