Interhemispheric comparisons in a man with complete forebrain commissurotomy

Revisiting the attentional bias in the split brain

Previous research has revealed a strong right bias in allocation of attention in split brain subjects, suggesting that a pathological attention bias occurs not only after unilateral (usually right-hemispheric) damage but also after functional disconnection of intact right-hemispheric areas involved in allocation of attention from those in the left hemisphere. Here, we investigated the laterality bias in spatial attention, as measured with the greyscales task, in two split-brain subjects (D.D.C. and D.D.V.) who had undergone complete callosotomy. The greyscales task requires participants to judge the darker (or brighter) of two left-right mirror-reversed luminance gradients under conditions of free viewing, and offers an efficient means of quantifying pathological attentional biases in patients with unilateral lesions. As predicted, the results of the two split-brain subjects revealed a pathological rightward bias in allocation of attention, suggesting strong dependence on a single hemisphere (the left) in spatial attention, which is opposite to what one expects from people with intact commissures, and is remarkable in that it occurs in free viewing. In that sense both split-brain patients are behaving as though the brain is indeed split, especially in D.D.C. who had undergone partial resection of the anterior commissure in addition to complete callosotomy, whereas the anterior commissure is still intact in D.D.V. The findings support the view that the commissural pathways play a significant role in integration of attentional processes across cerebral hemispheres.

Interhemispheric visual matching in the split brain

Three split-brained subjects, one (N.G.) with full forebrain commissurotomy and two (V.P. and J.W.) with callosotomy, made same-different judgments about pairs of visual stimuli that were flashed either unilaterally or bilaterally. In separate blocks of trials, the stimuli could differ in luminance, size, or color. In the bilateral condition, only J.W. scored above chance, and only minimally, on the luminance and size tasks, and none of the subjects scored above chance on the color task. Accuracy was generally much higher, especially for V.P. and J.W., when the stimuli were unilateral. These results confirm that there is little or no interhemispheric transfer of the visual attributes of luminance, size or color in the split brain.