Object Concept, Development of
OBJECT CONCEPT, DEVELOPMENT OF
The object concept is the knowledge that objects are permanent, independent entities that exist in space and time even when one cannot perceive or act on them. Humans would be almost unable to function without this knowledge. Although children clearly acquire the object concept early in development, researchers disagree about exactly when and how they acquire it.
The Swiss researcher Jean Piaget proposed the earliest comprehensive account of object concept development in the 1930s. Piaget believed that children gradually construct the concept over the first two years of life in a predictable and universal series of six stages. From birth to three or four months (Stages 1 and 2), infants do not truly perceive objects; they merely recognize stimulation associated with their own subjective experience, such as the reaction of pleasure connected with the sight of a caregiver or an attractive toy. By two months, infants turn to look at an object that makes a sound, demonstrating an integration of vision and hearing that gives objects greater solidity. Between four and eight months (Stage 3), infants noticeably progress toward acquiring the object concept. For example, infants visually or manually follow the path of an object that they drop and return to an object after dropping it out of sight. They also retrieve partially hidden objects and uncover their own faces in order to see, as in games of peek-aboo. From eight to twelve months (Stage 4), infants search for a completely hidden object and generalize their search to different objects and different covers or barriers. They do not, however, generalize to different locations. Infants who find a hidden toy in one location continue to search for it there even after seeing the toy hidden in a new location. Between twelve and eighteen months (Stage 5), infants incorporate location information into their object knowledge. They track the hidden object to its most recent hiding location, provided they see the toy hidden there (visible displacement). However, from eighteen to twenty-four months (Stage 6), infants find the hidden object in a new location even without seeing it hidden there (invisible displacement). According to Piaget, this behavior demonstrates that infants fully acquire the object concept between eighteen and twenty-four months.
Although the behaviors in Piaget's manual search tasks are highly replicable, many researchers disagree with his interpretations. When tasks are simplified, infants appear sensitive to hidden objects much earlier than Piaget proposed. Most research focuses on Stages 3 and 4, from four to twelve months. For example, in the violation-of-expectation method, infants simply watch events involving hidden objects while their looking times are measured. In one series of studies described by Renee Baillargeon in 1993, infants between four and seven months watched a barrier hide an object by rotating in front of it like a draw-bridge. In one event, the barrier bumped into the hidden object and then rotated back to its starting position (a possible event). In another event, the barrier rotated through the space occupied by the hidden object (an impossible event) before returning to its starting position. Infants looked significantly longer at the impossible event, suggesting they remembered the hidden object, expected the barrier to stop when it reached the object, and were surprised by the violation of this expectation. In many studies, described by Baillargeon in 1993 and 1998 and by others, infants between four and eight months looked longer at impossible than possible events across a range of objects, barriers, and displays. Such evidence suggests that infants in Piaget's Stage 3 not only mentally represent the existence of hidden objects but also other properties such as the location, height, and number of hidden objects, and even whether objects' behavior follows principles of gravity and inertia. These results suggest that infants understand the object concept earlier than Piaget proposed.
Why Do Looking and Reaching Differ?
Why do infants between four and eight months perform better on looking than reaching tasks? Researchers have proposed quite different explanations, resulting in considerable controversy. According to one account endorsed in 1993 by Baillargeon and by others, manual search tasks like those Piaget used may be inadequate for measuring infants' object knowledge; such tasks require infants to move a cover or barrier in order to retrieve an object. Infants may know the hidden object exists but are unable to demonstrate this knowledge by retrieving the object because of a secondary deficit in means-end ability. That is, infants may have difficulty planning a sequence of reaching for the cover and moving it out of the way (the means) in order to retrieve the object (the end). In contrast, infants can demonstrate their knowledge in violation-of-expectation studies because they do not require means-end skill. Other work has challenged this account. For instance, Yuko Munakata and colleagues in 1997 demonstrated that when means-end demands were equated for retrieving visible and hidden objects, infants succeeded more with visible objects. Thus, a means-end deficit cannot by itself account for infants' search problems. Whether the object is hidden or visible has a significant effect on infants' success.
According to other researchers, infants' longer looking at impossible than possible events may not reflect genuine object knowledge. Instead, infants' looking times may reflect simpler perceptual preferences for a familiar event or a novel event. According to researchers such as Richard Bogartz and others, interpretations that transcend perceptual explanations by appealing to mental representation may be too elaborate. Although this work has clearly demonstrated the importance of perceptual factors, such challenges cannot account for all violation-of-expectation results. Thus, cognitive factors also appear to affect infants' looking times, suggesting that infants may indeed have object knowledge early in development.
To explain the difference between looking and reaching performance, other accounts propose that the two tasks tap either different paths of knowledge or representations of different strengths. For example, according to Gavin Bremner, violation-of-expectation studies tap perceptual capacities that become gradually incorporated with development into infants' capacities for action in search tasks. Likewise, Munakata and colleagues in 1997 proposed that infants' object representations gradually become stronger with development. Infants may succeed in looking tasks with a weak representation of the hidden object but fail on reaching tasks because active search requires a stronger object representation.
Although researchers have fervently debated interpretation of violation-of-expectation studies, another line of research supports the position that infants have early sensitivity to hidden objects. In this approach, researchers present infants with a manual search task but hide the object in the dark instead of with a cover or barrier. The object becomes hidden when the lights go off, but infants can retrieve it with a simple direct reach instead of a means-end action. Using the reaching-in-the-dark method, researchers such as Rachel Clifton and colleagues found that infants between six and seven months reached for objects in the dark. In combination with violation-of-expectation results, these findings support the view that infants have object knowledge before they demonstrate it in means-end tasks.
In addition to research on infants' object knowledge between four and eight months (Stage 3), infants' behavior with hidden objects from eight to twelve months (Stage 4) has also generated many studies. As described earlier, infants younger than twelve months have difficulty tracking a hidden object that changes location. For instance, infants who find a hidden toy in location A continue to search for it there, even after seeing the toy hidden in location B. Although this "A-not-B error" is easy to replicate in reaching versions of the task, researchers such as Ayesha Ahmed and others reported that infants erred less often in looking versions of the task. Like violation-of-expectation results with four- to eight-month-olds, this finding suggests that infants know more than they can demonstrate in manual search tasks.
Why then do infants make the A-not-B error in their manual search? Researchers disagree. Piaget originally proposed that infants cannot separate the object's existence from its location. In contrast, Esther Thelen and colleagues suggested the error results not from inadequate object knowledge but from the normal development of the dynamic processes of looking, remembering, planning for action, and reaching. This dynamic systems account proposes that these processes are continuously meshed or embodied such that knowledge cannot be separated from perception, memory, and reaching. Memory and reaching experience also play crucial roles in a different explanation of the A-not-B error that Adele Diamond reiterated in 1998. According to Diamond, the error results from a combination of limitations in both memory and inhibition. Infants' memory for the toy in location B fades quickly, and they have trouble inhibiting the previously reinforced reach to location A. This account shares some features with an alternative account of the A-not-B error that Munakata described in 1998. According to this account, competition between latent memory traces for location A and active memory traces for location B explains the error. Infants' repeated experience in finding the toy in location A results in a latent bias toward A. Infants also form active memory traces for location B, maintaining an accessible representation of the hidden toy at B. However, the strong latent memory traces for A override the weaker active memory for B, resulting in the error.
Alternatives to Piaget's Theory
Beyond these accounts, which address specific phenomena of object concept development (e.g., the A-not-B error or the difference between looking and reaching performance), researchers have proposed more comprehensive theories of the development of the object concept as a whole. Some accounts, like that posed by Elizabeth Spelke, propose that some degree of core object knowledge is present from birth. Other theories, like that described by Baillargeon in 1998, suggest that from birth there is a specialized learning mechanism that guides infants' acquisition of object knowledge. According to this account, infants initially form a broad category for events involving hidden objects and gradually expand their understanding of such events by identifying the important factors that affect it. In similar accounts, Fei Xu and others suggest that infants construct object representations from knowledge pathways that are initially separate. In particular, infants encode object motion and location information earlier in development than they encode object properties or features, only later incorporating both kinds of information in their object representations. Likewise, the 1997 account of Munakata and colleagues proposes that knowledge is graded, with object representations becoming stronger with development.
Piaget originally proposed that infants gradually develop the object concept from birth to age two. However, evidence collected from simplified tasks that measure looking suggests that infants have some object knowledge several months earlier than Piaget believed. Despite challenges to interpretations of looking studies and divergent explanations for why infants perform better in looking than reaching tasks, most researchers agree that looking studies demonstrate early sensitivity to hidden objects. Results from reaching-in-the-dark studies support this conclusion. Findings from looking versions of the A-not-B task likewise suggest that infants know more about objects than they demonstrate in their manual search, though researchers disagree about why infants err in the reaching version of the task. The goal for future research is to thoroughly test the predictions of these various accounts, with the hope of establishing a comprehensive framework for understanding infants' acquisition of the object concept.
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