Hemispheric differences for visual search: serial vs parallel processing revisited.

Subjects were tachistoscopically presented with arrays of two, three or four stimuli to the right or left hemisphere and judged whether all of the items were the same or whether one was physically different from the rest. Separate groups of right-handed subjects viewed letters of featurally similar symbols as stimuli items. Faster and more accurate responding was obtained for left hemisphere presentations for bot same and different response judgments. Response time was independent of array size, with same judgments made faster than different judgments for both visual field conditions. Extensive practice shortened reaction time and decreased error rate, but did not change the pattern of hemisphere or judgment effects. Virtually identical results were observed for both stimulus conditions. These findings suggest that the left hemisphere can process information in parallel when the task situation requires featural analysis of stimulus materials.