The role of the corpus callosum in visual orienting: importance of interhemispheric visual transfer.

In a tachistoscopic visual search task, the effects of ipsi- and contralateral distractors on target search were investigated in two complete commissurotomy patients. Pop-out distractors slowed the search for contralateral targets in both patients, i.e. search was not independent in both hemifields. In normals, we previously observed an extinction-like asymmetry in that distractors in the right visual hemifield interfered with target search in the left visual hemifield, but not vice versa. This pattern was also found in one of our patients, N.G., whereas the other, L.B., showed a reversed laterality effect. While N.G. is able to transfer visual shape information between hemispheres, L.B. is not. The data suggest that the reversal of the contralateral distractor asymmetry in L.B. is due to the disruption of ipsilateral visual input to the right hemisphere.