Hemispatial neglect and visual search: a large scale analysis.

Visual search tasks have standardly been divided into two categories: those in which the target is detected through a serial, attention-driven search and those in which the target is detected rapidly in parallel and, apparently, without attentional processing. Several studies have examined this distinction in patients with hemispatial neglect with the clear prediction that the former, but not the latter, should be impaired. These studies, however, have proved inconclusive. We have addressed this issue in a large sample of patients with unilateral hemispheric infarcts to the left or right hemisphere. In addition to measuring the patients' performance on both types of visual search tasks, we documented the presence and severity of neglect and of visual field defects in these same individuals. Patients with brain-damage with or without accompanying neglect were impaired at searching for the contralateral target on both forms of visual search, relative to normal control subjects, although this deficit was magnified in individuals with neglect and was also exacerbated by the presence of hemianopia. This pattern was also more pronounced in individuals with right-than with left-hemisphere lesions. The findings not only clarify the contradictory neuropsychological data but also provide clear evidence for the involvement of attentional processing in all forms of visual search.