pioneered the study of infants’ perceptual and cognitive abilities with his foundational work on object knowledge and spatial cognition. Since these original studies, a wealth of information on visual-cognitive abilities has accrued in such areas as object permanence (e.g., Baillargeon, 1987
; Diamond, 1990
), spatial reasoning (e.g., Bremner, 1978
; Rieser, 1979
), and perceptual completion (e.g., Johnson, Bremner, et al., 2003
; Kellman & Spelke, 1983
). Departures from the age and motor constraints Piaget suggested originally have been claimed, sparking polemic arguments on the “true” capacities of young infants (Spelke, 1998
Modern rationalist views of infants’ understanding of the physical world have contended that infants are more sophisticated in their representations of the visual world than Piaget surmised (Spelke, Breinlinger, Macomber, & Jacobson, 1992
). Studies examining infants’ understandings of object continuity and solidity, for example, have suggested that infants as young as 2.5 months may have a rudimentary understanding of object permanence (Aguiar & Baillargeon, 1999
). In general, those arguing in favor of the precocious nature of infants’ object perception and physical understanding maintain that developmental processes are more related to infants’ sensitivity to relevant features of the objects used in the displays rather than changes in the underlying mechanisms of information-processing and object knowledge (e.g., Baillargeon, 2004
Others have argued for a more gradual emergence of object knowledge over the 1st year after birth—with infants’ understanding of the physical world built upon successively more sophisticated attentional skills, an increasing ability to integrate multiple sources of information in the visual array, and emerging manual and other exploratory skills to facilitate the acquisition of physical knowledge (e.g., Colombo, 2001
; Gibson, 1988
; Johnson, 2003
). Work on infants’ perception of causality (e.g., Cohen & Oakes, 1993
), perception of object unity (e.g., Johnson, 2004
), and object recognition (e.g., Kraebel & Gerhardstein, 2006
) has provided evidence for infants’ gradually emerging object knowledge. The present study, therefore, was motivated by this growing awareness of the importance of understanding developmental processes in providing a more accurate account of infants’ physical world than is offered by rationalist approaches.
We examined infants’ perception of the full, volumetric form of three-dimensional (3D) objects when provided only a limited view, asking if infants represent objects seen from a limited vantage as coherent volumes or instead as consisting solely of the surfaces that are visible from the current viewpoint. This question maps onto work investigating developmental processes of object and spatial knowledge and relates to work on perceptual completion in general in infancy.
The 3D nature of our world may pose a challenge to the immature visual system. Distant objects are often occluded by nearer objects, and recognition of the partially occluded object requires synthesis of the visible portions into a coherent percept. Objects come in and out of existence as they move or observers move around them, and the far sides of opaque objects are blocked from sight by their visible portions—the problem of self-occlusion. Despite the fragmented nature of the visual world, adult observers are adept at discriminating and recognizing complete forms through spatial and spatiotemporal occlusion (Kellman & Shipley, 1991
; Marr, 1982
) and through self-occlusion (Tse, 2002
). Thus, our visual experience is not composed of disjoint surfaces, fleeting percepts of moving objects, and incomplete, hollow volumes; rather, our visual system “fills in” the missing parts, resulting in a richer experience than what is visible directly.
The visual system of the adult is able to accomplish perceptual completion under many circumstances without apparent effort, but the visual experience of very young infants is substantially different. Four-month-olds perceive a moving, center-occluded rod as a unified object rather than two disjoint segments (Kellman & Spelke, 1983
), but newborn infants perceive this display as consisting of unconnected pieces (Slater, Johnson, Brown, & Badenoch, 1996
). In these investigations, infants were habituated to a display in which a rod moved laterally behind an occluder. After habituation, infants were shown two test displays, both without the occluding box: one depicting a complete, unbroken rod and in the other, two broken rod pieces of the same size as the visible portions seen previously. Consistently longer looking at either test display is interpreted as a novelty response; therefore, a posthabituation preference for the broken rod pieces would imply unity perception during habituation. Newborns preferred the unbroken rod at test, whereas older infants preferred the broken rod parts.
Other experiments examined spatiotemporal completion: the perception of occluded trajectories of moving objects. When shown a display in which a ball moves back and forth, the center portion of its trajectory occluded by a box, infants at 6 months appeared to perceive the ball continuing behind the occluder, both in an habituation experiment (Johnson, Bremner, et al., 2003
) and in an oculomotor anticipation paradigm (Johnson, Amso, & Slemmer, 2003
), but younger infants provided evidence of perceiving the visible parts of the trajectory only, failing to complete it, in parallel with experiments on perception of object unity described previously. Infants’ visual cognition thus undergoes substantial development during the first few months after birth toward an experience of a more holistic percept of the visual environment.
These and other experiments have characterized infants’ perception of spatial and spatiotemporal unity. In contrast, little is known about how infants come to understand self-occlusion and perceive volumetric unity. Work on the development of 3D object completion and infants’ understanding of self-occlusion may help elucidate how the infant’s visual system overcomes the problems imposed on it by the cluttered 3D world as well as further general learning principles.
A subsidiary goal of the present work is to understand better the nature and limits of 3D form perception in young infants. Early research showed that 4-month-olds could discriminate familiar objects in novel axes of rotation from novel rotating objects (Kellman, 1984
). Infants in these experiments, however, could not generalize 3D form across views within static displays and required either object or observer movement to discriminate novel and familiar objects (Kellman & Short, 1987
). Infants as young as 2 months are adept at picking up 3D structure from kinetic information (Arterberry & Yonas, 2000
), and by 4 months, infants are sensitive to some pictorial cues that signify 3D structure (Bhatt & Bertin, 2001
; Bhatt & Waters, 1998
; Shuwairi, Albert, & Johnson, 2007
). Infants at 2 and 4 months will complete a 3D shape when occluded in displays depicting fully rotating 3D forms (Johnson, Cohen, Marks, & Johnson, 2003
). Recent work from an operant learning paradigm provides evidence that at 3 months, infants may achieve view-invariant 3D object recognition when given the opportunity to cause the movement of various 3D forms (Kraebel & Gerhardstein, 2006
Together, this work on perceptual completion and 3D object perception shows that young infants by 4 months show some nascent understanding that objects are continuous in space and that most objects are 3D. Infants appear to continually build upon their existing knowledge and attentional skills to visually complete more complex displays and veridically perceive more complex objects. Still, it is not known when and how infants come to perceive a complete 3D volume through self-occlusion.
In the present experiment, we showed infants computer-generated displays depicting objects rotating in depth around the vertical axis (see )—either pivoting back and forth through 15°(limited view) or rotating through a full 360°(full view). Infants were shown a limited-view display during habituation, providing the opportunity to see only a few faces of the object. Once infants met the habituation criterion, they were shown two new full-view displays. One display depicted a complete solid object, and the other depicted an incomplete hollow object composed of only the surfaces seen during habituation. All the displays contained rich information for 3D form including kinetic cues (e.g., relative motion, contour deformation) and pictorial cues (e.g., shading, line junctions). We reasoned that if an infant perceived the limited-view object as complete during habituation, then during test he or she should respond to the incomplete display as novel and thus look longer at it than at the complete display. If, however, the infant showed no reliable preference for either display following habituation, then we reasoned that infant likely did not perceive the self-occluded surfaces of the limited-view object.
We tested infants at 4 and 6 months to explore developmental differences in performance. To assess whether infants in these age groups show any spontaneous preferences for incomplete or complete 3D objects, infants in a control group were shown complete and incomplete test displays without first being habituated to the limited-view object.