Where's the ball? Two- and three-year-olds reason about unseen events.

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Abstract
Children 2, 21/2, and 3 years of age engaged in a search task in which they opened 1 of 4 doors in an occluder to retrieve a ball that had been rolled behind the occluder. The correct door was determined by a partially visible wall placed behind the occluder that stopped the motion of the unseen ball. Only the oldest group of children was able to reliably choose the correct door. All children were able to retrieve a toy that had been hidden in the same apparatus if the toy was hidden from the front by opening a door. Analysis of the younger children's errors indicated that they did not search randomly but instead used a variety of strategies. The results are consistent with the Piagetian view that the ability to use represen- tations to guide action develops slowly over the first years of life. Piaget (1954) characterized the sensorimotor stage as one in which the infant uses sensorimotor schemes to interact with the world. Action is not deliberative, nor does it involve true opera- tions on mental representations until the end of the sensorimotor stage in the second year of life. The child develops a true under- standing of the world slowly and by the accumulated experience of age. In the search for hidden objects, infants progress from being able to retrieve partially occluded objects, to using means-ends behavior to retrieve fully occluded objects, to being able to locate hidden objects that have undergone invisible displacements. This protracted period of the acquisition of sophisticated senso- rimotor behavior is consistent with recent neuroscientific findings. Functional imaging, lesion, and nonhuman primate single-unit studies all point to the prefrontal cortex as playing a critical role in visuospatial and nonspatial working memory. The prefrontal cor- tex receives inputs from both the dorsal and ventral visual streams, shows heightened activity during tasks involving temporal delays, and is involved in controlling visual attention (Goldman-Rakic, 1996; Petit, Courtney, Ungerleider, & Haxby, 1998; Ungerleider, Courtney, & Haxby, 1998). Moreover, the prefrontal cortex is particularly involved in tasks that require perceptuomotor action and in tasks that require integration of representation with senso- rimotor action (Luciana & Nelson, 1998; Quintana & Fuster, 1999). Significantly, the prefrontal cortex develops with an ex- tended time course over the first several years of life (Huttenlocher & Dabholkar, 1997). Diamond (1990, 1997) showed that improv- ing performance in the delayed response and the A-not-B tasks is correlated with the development of the prefrontal cortex in chil- dren and that adult nonhuman primates with lesions of the pre- frontal cortex show impaired performance on the A-not-B task and in delayed responding.

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