Previous studies of visuospatial attention indicated that the isolated cerebral hemispheres of split-brain patients maintain an integrated, unitary focus of attention, presumably due to subcortical attentional mechanisms. The present study examined whether a unitary attentional focus would also be observed during a visual search task in which subjects scanned stimulus arrays for a target item. In a group of four commis-surotomy patients, the search rate for bilateral stimulus arrays was found to be approximately twice as fast as the search rate for unilateral arrays, indicating that the separated hemispheres were able to scan their respective hemifields independently. In contrast, the search rates for unilateral and bilateral arrays were approximately equal in a group of six normal control subjects, suggesting that the intact corpus callosum in these subjects is responsible for maintaining a unitary attentional focus during visual search.

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