described a series of infants’ behaviors that provided evidence for an emerging ability to keep track of objects that became occluded. According to Piaget’s theory, active search, initiated by the children, is a critical feature of object concepts. Active visual search behavior emerged after 4 months, marking the beginnings of “true” search and, ultimately, object permanence. Among these behaviors was visual accommodation to rapid movements (e.g., when infants would respond to a dropped object by looking down toward the floor), behaviors observed by 6.5 months. Visual accommodation was proposed to become more consistent as infants gained manual experience with objects, providing direct experience with dropping and retrieval and developing in tandem with reconstruction of partly occluded objects from visible fragments and removal of covers to gain access to occluded objects. Piaget suggested that the accumulation of these behaviors and experiences was a critical developmental mechanism in imparting object concepts. In the current article, we examine this possibility directly in 4-month-olds by providing different levels of training and delays between training and test.
Recently, researchers have recorded eye movements in infants as they view repetitive events in which objects move behind an occluder and subsequently reemerge. The question is the extent to which infants produce anticipatory eye movements toward the place of reemergence, implying a functional representation of the object that guides oculomotor behavior. By definition, anticipatory behavior meets the requirements for inchoate object permanence described previously.
This research has led to three conclusions. First, older infants produce a greater proportion of oculomotor anticipations (vs. reactive eye movements) relative to younger infants. Johnson, Amso, and Slemmer (2003)
presented infants with events in which a small target moved on a horizontal center-occluded trajectory and found that 6-month-olds produced a higher proportion of anticipatory eye movements directed toward the moving target (M
= 43.6%) relative to 4-month-olds (M
= 29.5%). Gredebäck and von Hofsten (2004)
reported continued improvements in predictive tracking up until 12 months. Rosander and von Hofsten (2004)
found no evidence of predictive tracking in infants younger than 12 weeks; however, anticipations became more consistent by 21 weeks. These results accord with the possibility that representations of occluded objects are initially weak and gradually strengthen across the first year after birth (Johnson & Munakata, 2005
; Piaget, 1937/1954
Second, 4 months is a time of transition toward veridical perception of occlusion (Bremner et al., 2005
; Johnson, Bremner, et al., 2003
), and oculomotor anticipation performance at this age can be facilitated. For example, Johnson et al. (2003)
found that performance at 4 months is enhanced by “training,” that is, viewing an unoccluded object trajectory immediately preceding the occlusion stimulus. Baseline performance was higher in von Hofsten, Kochukhova, and Rosander’s (2007)
study (47–50%), perhaps because von Hofsten and colleagues included only trials on which infants were determined to look at the object both before and after occlusion, whereas Johnson and colleagues included trials on which the objectwas not fixated prior to occlusion. Differences in performance may also stem from differences in stimulus complexity, for example, three-dimensional displays used by von Hofsten and colleagues versus high-contrast two-dimensional video displays used by Johnson and colleagues.
Third, by 6 months, infants begin to deal effectively with nonlinear trajectories, showing spatially accurate predictive eye movements when a target moves on a partly occluded circular (Gredebäck & von Hofsten, 2004
) or angled path (Kochukhova & Gredebäck, 2007
). In contrast, 4-month-olds construe nonlinear occlusion trajectories as discontinuous, and perception of continuity is disrupted when a linear object path is oriented diagonally relative to the occluder and background (Bremner et al., 2007
This research is broadly consistent with Piaget’s original descriptions of infant performance. There is little evidence of systematic predictive behavior prior to 4 months, after which time anticipations become robust and flexible, and performance continues to improve with age. It seems unlikely, however, that direct manual experience with objects is a principal developmental mechanism underlying predictive behavior given that oculomotor anticipations begin to become established prior to the onset of functional goal-directed reaching and manual object manipulation in developmental time (Konczak, Borutta, Topka, & Dichgans, 1995
The goal of the current article was to examine another key tenet of Piagetian theory. Piaget (1937/1954)
suggested that active search for hidden objects stems from the accumulation of repetitive behaviors as infants interact with objects. As noted previously, it is doubtful that manual object manipulation skills precipitate oculomotor anticipations. Nevertheless, the suggestion that the accrual of experience may strengthen predictive behavior has not been investigated systematically to our knowledge.
Recent experiments provide mixed evidence for short-term gains in predictive performance, gains that hypothetically might arise from repeated exposure to a target object that moves in a perfectly predictable manner (to adults). Rosander and von Hofsten (2004)
found that predictive performance improved across four cycles of motion in 21-week-olds, who showed decreasing eye movement latencies as a function of trial. In contrast, Gredebäck and von Hofsten (2004)
and Johnson et al. (2003)
found that the proportion of oculomotor anticipations declined across trials in infants ranging from 4 to 9 months. Therefore, infants do not seem to capitalize on the predictable nature of trajectory occlusion stimuli.
However, in Johnson et al.’s (2003)
study, anticipations in 4-month-olds were more frequent following exposure to a fully visible moving target. This facilitative effect was a product of training with a target moving either horizontally (M
= 46.3% anticipations after training) or vertically (M
= 49.7% anticipations after training); in the occlusion stimulus that followed, target trajectory always was horizontal. Results from the vertical condition imply that infants were not trained simply to produce horizontal eye movements; rather, anticipations were geared toward the moving object itself. Ensuing predictive performance in both of these conditions was comparable to performance of untrained 6-month-olds (M
A lingering question is whether the enhancement in performance is fleeting or more permanent, and it is unknown whether effects of training would survive delays. Here we address this question and the larger question of how infants begin to learn and remember trajectory information when presented with occlusion stimuli at an age when object representations may be difficult to form or access.
We examined performance under five conditions using methods first described by Johnson et al. (2003)
to assess 4-month-olds’ oculomotor behavior as they viewed trajectory occlusion stimuli (). In the baseline
condition, infants viewed a series of occlusion displays. In the full training
condition, infants viewed the same occlusion displays following a period of training with unoccluded object trajectories. In the delay
condition, we examined effects of a 30-min delay between training and test. In the reminder
condition, we considered the possibility that postdelay memory for occlusion might be prompted by a brief “reminder” trial. The final condition, brief training
, was designed to control for the possibility that the brief reminder trial itself induced the facilitative effect independent of earlier training.
Top: Trajectory occlusion display used to test oculomotor anticipations in all five experiments. Bottom: Unoccluded trajectory display used as a training stimulus in the full training, delay, reminder, and brief training conditions.