Divided visuo-spatial attention systems with total and anterior callosotomy

The role of the corpus callosum in the inter-hemispheric integration of the visuo-spatial attention system, was investigated in patients with a total callosotomy or with an anterior callosal section. Subjects produced simple reaction times (RTs) to visual targets shown to the left or right visual he mifield. Preceding the target by an interval of 500 ms, arrow cues predicti ng the target location were shown left and right of the point of ocular fix ation. For a majority of total and anterior callosotomy patients, results w ith valid focused cues (both arrows pointing to the target location) and wi th divided-attention cues (arrows pointing away from fixation) did not diff er and both conditions produced shorter RTs than with neutral cues (equal s igns). In contrast, neurologically intact subjects showed equal RTs with di vided-attention and neutral cues, whereas valid focused cues produced reduc ed RTs relative to neutral cues. These results indicate that most split-bra ins, in contrast to normal observers, are capable of directing their attent ion to left and right visual field locations simultaneously, and therefore that each cerebral hemisphere controls its own visuo-spatial attention mech anism. (C) 1999 Elsevier Science Ltd. All rights reserved.