Previous research suggests that target templates are stored visual working memory and used to guide attention during visual search. However, observers can search efficiently even if working memory is filled to capacity by a concurrent task. The idea that target templates are stored in working memory receives support primarily from studies of nonhuman primates in which the target varies from trial to trial, and it is possible that working memory templates are not necessary when target identity remains constant, as in most studies of visual search in humans. To test this hypothesis, we asked subjects to perform a visual search task during the delay interval of a visual working memory task. The 2 tasks were found to interfere with each other when the search targets changed from trial to trial, but not when target identity remained constant. Thus, a search template is stored in visual working memory only when the target varies from trial to trial. These findings suggest that the network of brain areas involved in shifting attention during visual search tasks may be able to operate essentially independently of the anatomical areas that perform visual working memory maintenance of objects, but only if the identity of the visual search target is stable across time.

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