Development of Reaching and the A Not B Error

Studies of reaching behavior in human infants and monkeys suggest that development of such behavior involves both the ability to plan and execute sequences of action and the ability to inhibit certain reflexive actions or dominant response tendencies.

Piaget suggested that infants have difficulty in understanding objects and their properties, including spatial relations. However, it seems that infants, even as young as 5 months old, can understand the object concept but they have difficulty demonstrating this understanding by their reaching behavior. Reaching behavior of infants has been studied by Adele Diamond and colleagues using several paradigms. In one paradigm infants were presented with a Plexiglas box with a building block inside or outside the box. Infants were able to retrieve the object when a direct line of reach was possible (the object was in front of the box touching its front wall or in the middle of the box, away from its front wall). However, when the object was placed inside the box touching its front wall infants were unsuccessful in retrieving it. To retrieve the object in the latter situation there is a need to execute a sequence of two movements in order to avoid touching the front wall of the box—one away from the object and a second in the direction of the object. Moreover, when the infants touched the front of the box they reflexively grasped it or withdrew their hands. Seven-month-old infants rarely continued their reaching toward the object. In contrast, 10-month-old infants were much less likely to show these reflexive behaviors upon touching the edge of the box. These infants were able to retrieve the object in these circumstances. It seems that the older infants developed the ability to execute a reach that requires a change of direction and to inhibit reflexive reactions of the hand.

Infants also have difficulty detouring around a barrier to retrieve an object. Here, the infant's task is to retrieve the object from a transparent box that has an open side. Infants 6.5-7 months old reach straight through the side at which they are looking at the object. If they see the object through the open side, they can retrieve it. Otherwise, they cannot retrieve the object and they do not try alternative reaching behaviors. Toward the end of the first year of life they can look through a closed side but retrieve the object from any open side of the box. In order to develop this ability infants need to inhibit the tendency to reach according to the line of sight.

In another paradigm the infant is presented with an object in one of two places, A or B. The locations are covered to hide the object from sight and the infant is then allowed to search for it by lifting the covers. After the infant retrieves the object, the object is again placed in one of the two locations and the search task continues. Suppose the object is first hidden at location A and the infant retrieves it successfully. If the object is then hidden at the second location B, the infant will often try to retrieve the object from A, even though the infant watched the experimenter hiding the object at B. This is the A not B error. Infants continue to make the A not B error from about 7.5 to 12 months of age, as long as the delay between hiding and retrieval is incremented as the infant gets older. Perhaps the most striking aspect of the phenomenon is that infants appear to know that the object is at B despite the fact that they reach toward A. Visual habituation and other visual memory tests indicate that infants remember the correct location. Sometimes, in the search task, the infant fixates B while he or she is reaching toward A. Moreover, infants show the A not B error even when the covers are transparent and the object can be seen. In order to prevent this type of error there is a need to hold details of the task and the situation briefly in short-term memory, and there is a need to inhibit the tendency to reach to A. The tendency to reach to A develops because reaching to A was reinforced earlier by successful finding of the object, and it increases with the number of times the infant has retrieved the object from location A before it is hidden at location B. The ability to meet both of these needs imposed by the task improves during infancy. Short-term memory improves as evidenced by the longer retention interval between hiding and allowing the infant to reach for the object needed to elicit the A not B error, and inhibition of the incorrect response tendency improves as evidenced by the eventual disappearance of the A not B error.

It seems from this evidence that during the second half of the first year of life infants begin to gain control over their reaching actions. They can inhibit interfering automatic tendencies and demonstrate planned and goal-directed control over manual behavior. The gradual ascendance of planned and goal-directed behavior over reflexive or practiced stimulus-action routines has been modeled both by Diamond and colleagues and Stuart Marcovitch and Philip Zelazo as changes with age in the relative strength or dominance of competing systems, one like working memory and the other like conditioned or procedural learning. Evidence from lesions and single-cell recordings suggests that the increase in dominance of the working memory-like system is achieved, at least in part, through the maturation of several components of frontal cortex: the supplementary motor area (SMA) and the dorsolateral prefrontal cortex. Reflexive grasping is released in adult humans following lesions of the SMA, and the same is true in the case of lesioned monkeys. Lesions of the dorsolateral prefrontal cortex in monkeys produce the A not B error and difficulties inhibiting the urge to reach straight ahead to retrieve an object.

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