In the previous two sections, I described the capacities of newborns to detect and remember key information about the visual world, information that is important for specifying objects: their segregation from one another and their relative distances, and the retention of this information for brief intervals sufficient to support recognition upon repeated encounters. I also described models of development object perception. These models demonstrated that a naïve system, given appropriate perceptual, cognitive, and motor skills and a suitable environment in which to learn, can perceive objects as complete and persistent despite occlusion, and can act on objects by detecting relevant information about their properties. Does the developmental process in human infants accord with these findings?
6.1. Infants learn about objects via association
Consider first the possibility that infants learn about object occlusion via association. How might this work? The Mareschal and Johnson (2002) model learned to perceive partly hidden objects as complete in two ways: by associating objects with different visual cues in the input (i.e., texture, motion, junctions, and orientation), and by associating different views of objects with each other—a fully visible, complete rod that moved behind the occluder and then became partly occluded. The Mareschal et al. (1999) model was exposed to an object moving on a repetitive trajectory and quickly learned to predict its reappearance from behind an occluder. The model was set up to predict the location of the moving object based on its preceding position and trajectory, and the model maintained a memory trace of it when it was rendered invisible by the occluder. Is there evidence for similar processes in human infants?
To my knowledge, no one has tested the possibility that infants learn about objects by associating individual visual attributes with their coherence and persistence across occlusion, though the contributions of such visual attributes to perceptual completion have been investigated. Spatial completion has been observed in young infants (younger than 6 months) only when the rod parts are aligned, and moving in tandem behind the occluder (Johnson, 1997, 2004), in displays with the four visual cues examined in the Mareschal and Johnson (2002), but in the absence of one or more cues, spatial completion is disrupted (Kellman & Spelke, 1983). And spatiotemporal completion has been observed in young infants only when the trajectory is horizontal, not angled, and when the spatiotemporal gap imposed by the occluder is relatively short (Bremner et al., 2005, 2007; Johnson, Bremner, et al. 2003). But it is not clear that these studies provide evidence that association per se is an important mechanism of development in perceiving object occlusion.
Such evidence comes from experiments by Johnson, Amso, and Slemmer (2003), who examined 4- and 6-month-old infants’ responses to object trajectory displays by recording predictive eye movements. We reasoned that a representation of a moving object would be revealed by a consistent pattern of fixations toward the far side of the occluder upon its occlusion. Infants were tested in one of four conditions. In the baseline condition, infants were shown the ball-box display depicted in Fig. 4 as eye movements were recorded with a corneal-reflection eye tracker. The display was presented for eight 30-s trials. In the random condition, infants viewed eight presentations of displays that were identical to the ball-box stimulus except the ball’s point of reemergence after occlusion was randomized (left or right). In this case, anticipation offers no gain to the observer, who is just as likely to make perceptual contact with the ball if the point of gaze remains where the object moved out of view. (We hypothesized that anticipations in the random condition might be random eye movements themselves.) In the training condition, infants were first presented with four trials of the ball only, fully visible on its lateral trajectory (no occluder), followed by four trials with the ball-box display, as in the predictable condition. Finally, in the generalization condition, infants first viewed four trials with a vertical unoccluded trajectory, followed by four trials with a partly occluded horizontal trajectory.
In the baseline condition, 6-month-olds produced a significantly higher proportion of anticipatory eye movements than 4-month-olds, and a comparison of 4-month-olds’ performance in the baseline versus random conditions revealed no reliable differences. This latter finding implies that any predictive eye movements we observed by 4-month-olds were actually not based on a mental representation of the occluded object and its motion, but instead were simply random eye movements scattered about the display that, by chance, happened to fit the criteria for categorization as predictive. Moreover, 4-month-olds’ performance in the baseline condition did not improve across trials (as would be expected if the infants learned the repetitive sequence). In fact, there was a significant decline in anticipations across trials. These results indicate that eye movement patterns may have been driven more in the older age group by a veridical representation of the object on its path behind the occluder.
However, 4-month-olds in the training condition showed reliably more predictive eye movements relative to 4-month-olds in the baseline condition. Comparisons of the two 6-month-old groups, in contrast, revealed no significant differences. The boost in anticipation performance seen in the 4-month-old training group generalized from exposure to the vertical trajectory orientation, implying that infants in the training condition were not simply trained for facilitation of horizontal eye movements, but instead true representation-based anticipations.
How long does this effect of training last? Johnson and Shuwairi (2009) addressed this question with a replication of the Johnson, Amso, et al. (2003) experiment: Baseline and training conditions with 4-month-olds yielded similar results as the previous study. We extended these findings with three additional conditions: a delay condition, in which a 30-min wait was imposed between training (with an unoccluded trajectory) and test (with a partly occluded trajectory), and a reminder condition, identical to the delay condition except for the addition of a single additional training trial immediately before test. Performance in the delay condition was not significantly different from that of baseline, implying that the gains produced by brief training did not survive the 30-min interruption prior to test. However, eye movement anticipations were facilitated by the reminder condition to the same extent as in the (immediate) training condition. (A fifth condition, brief training, consisted of a single training trial prior to immediate test, and this did not have any discernible effect on performance.)
Taken together, these findings suggest that there are consequential changes around 4 months after birth in representations of moving, occluded objects (Johnson, Bremner, et al. 2003). Such representations are sufficiently strong by 6 months to guide anticipatory looking behaviors consistently when viewing predictable moving object event sequences. Four-month-olds’ anticipations under these conditions provided little evidence of veridical object representations. However, a short exposure to an unoccluded object trajectory induces markedly superior performance in our tracking task in this age group, and with a reminder, this training effect can last for a period of time outside the scope of short-term memory. These findings also help clarify the role of associative learning in object perception development. Infants did not seem to learn by viewing repetitive events that are perfectly predictable to adults (otherwise infants in the baseline conditions would have begun to show increased levels of anticipation after several trails viewing the occluded trajectory). Instead, infants learned by associating views of the fully visible object trajectory and the partly occluded object trajectory.
6.2. Infants learn about objects via “active assembly”
The Schlesinger et al. (2007) model discussed in the previous section highlighted two aspects of visual development that might have a key role in development of spatial completion: growth of horizontal connections among neurons and circuits in cortical visual area V1, and recurrent processing, which, we reasoned, served to compare aspects of the visual scene. How might these influence developing object perception skills?
Burkhalter and colleagues (Burkhalter, 1991; Burkhalter et al., 1993) have reported evidence for developments in horizontal connections in V1 from deceased fetuses and infants, across the period from 26 weeks postconception to 7 months after birth, but the precise role of these developments in object perception has not been documented. However, there are findings from experiments on spatial completion that bear on this question. Two-month-old infants have been found to perceive spatial completion in displays with a relatively narrow occluder, such that the rod parts are close together across the gap imposed by occlusion, but not when the occluder is wide (Johnson, 2004). (Similar findings were obtained in studies of spatiotemporal completion—reducing gap size facilitates perception of completion here as well.) Older infants are less susceptible to effects of widening this gap (Johnson, 1997). In addition, 4-month-old infants are more likely to look back and forth across a wide gap at the rod parts than are 2-month-olds (Johnson & Johnson, 2000). These results are to be expected if the visual system becomes better able to link aligned edges across a gap as connections between receptive fields become strengthened.
Other experiments examined the possibility that spatial completion develops from a constructive process—which I have termed active assembly—serving to integrate parts of the visual scene into a coherent whole, in like fashion to recurrent processing discussed previously. Amso and Johnson (2006) and Johnson, Slemmer, and Amso (2004) observed 3-month-old infants in a spatial completion task and recorded infants’ eye movements with a corneal reflection eye tracker during the habituation phase of the experiment. We found systematic differences in oculomotor scanning patterns between infants whose posthabituation test display preferences indicated unity perception and infants who provided evidence of perception of disjoint surfaces: “Perceivers” tended to scan more in the vicinity of the two visible rod segments, and to scan back and forth between them (Fig. 11). In a younger sample, Johnson et al. (2008) found a correlation between posthabituation preference—our index of spatial completion—and targeted visual exploration, operationalized as the proportion of eye movements directed toward the moving rod parts, which we reasoned was the most relevant aspect of the stimulus for perception of completion. (Precise localization of the point of gaze can be a challenge for these very young infants, attested by the fact that targeted scans almost always followed the rod as it moved, rarely anticipating its position.)
A relation between targeted visual exploration and spatial completion does not by itself pinpoint a causal role. Such evidence would come from experiments in which individual differences in oculomotor patterns were observed in both spatial completion and some other visual task, and this was recently reported by Amso and Johnson (2006). We found that both spatial completion and scanning patterns were strongly related to performance in an independent visual search task in which targets, defined by a unique feature (either motion or orientation) were placed among a large set of distracters. There were substantial individual differences in successful search, both in terms of detecting the target and the latency to do so, and these differences mapped clearly onto the likelihood of spatial completion. This finding is inconsistent with the possibility that scanning patterns were tailored specifically to perceptual completion, and instead suggests that a general facility with targeted visual behavior leads to improvements across multiple tasks.
Targeted visual exploration may make a vital contribution to the emergence of veridical object perception. As scanning patterns develop, they support binding of disparate visual features into unified percepts—active assembly of coherent objects from surface fragments, confirming the outcome of the Schlesinger et al. (2007) model of visual development. With the emergence of selective attention and other perception-action systems, infants become increasingly active participants in their own perceptual development rather than passive recipients of information. Active engagement of the infant’s visual attention is consistent with a key tenet of Piagetian theory—the central role of the child’s own behavior in cognitive development—and with a constructivist view—the building of structure from constituent pieces. The following section describes another of these perception-action systems, visual-manual exploration, and its role in constructing volumetric objects.
6.3. Infants learn about objects via visual-manual exploration
The Kuperstein (1988) and Bullock et al. (1993) models demonstrated that when perception and action develop in tandem their coordination can be an emergent property, with each influencing the other to the benefit of the exploratory capacity of the organism. Developments in perception and action have long been of interest to developmental psychologists, and there has been recent evidence to show that 3D object completion emerges as a consequence of improvements in infants’ motor skills. Two types of motor skill, both of which develop rapidly at the same time that 3D object completion seems to emerge—4 to 6 months—may play a particularly important role: self-sitting and coordinated visual-manual exploration. Independent sitting frees the hands for play and promotes gaze stabilization during manual actions (Rochat & Goubet, 1995), and, therefore, self-sitting might encourage coordination of object manipulation with visual inspection as infants begin to play with objects, providing the infants with multiple views. In addition, manipulation of objects—touching, squeezing, mouthing—may promote learning about object form from tactile information.
To examine these possibilities, Soska, Adolph, and Johnson (2010) observed infants between 4.5 months and 7.5 months in a replication of the Soska and Johnson (2008) habituation experiment with the rotating wedge stimuli (Fig. 6). In the same testing session we assessed infants’ manual exploration skills by observing spontaneous object manipulation in a controlled setting and obtained parental reports of the duration of infants’ sitting experience. We reasoned that infants who had more self-sitting experience would in turn show a greater tendency to explore objects from multiple viewpoints and therefore have more opportunities to learn about objects’ 3D forms outside the lab. Thus, within this age range, individual differences in self-sitting experience and coordinated visual-manual exploration were predicted to be related to individual differences in infants’ looking preferences to the complete and incomplete object displays, our index of 3D object completion.
Our predictions were supported. We found strong and significant relations between both self-sitting and visual-manual coordination, from parents’ reports and the motor skills assessment, and 3D object completion performance, assessed with the habituation paradigm. We recorded a number of other motor skills to explore how widespread the relations were within the perception-action systems under investigation, such as grasping, holding, and manipulation without visual inspection, and none were related to 3D object completion.
Self-sitting experience and coordinated visual-manual exploration were the strongest predictors of performance on the visual habituation task. The results of a regression analysis yielded evidence that the role of self-sitting was indirect, influencing 3D completion chiefly in its support of infants’ visual-manual exploration. Self-sitting infants performed more manual exploration while looking at objects than did nonsitters, and visual-manual object exploration is precisely the skill that provides active experience viewing objects from multiple viewpoints, thereby facilitating perceptual completion of 3D form. These results provide evidence for a cascade of developmental events following from the advent of visual-motor coordination, including learning from self-produced experiences.