Line bisection in the split brain.

The authors examined line bisection in 4 patients with resection of the corpus callosum and in 22 control participants. The control participants showed a leftward bias, especially with the left hand, implying right-hemispheric dominance in spatial attention. Two patients with anterior callosotomy showed similar biases, suggesting that the anterior callosum plays only a small role. A patient with complete callosotomy showed a strong right bias, regardless of hand use. A patient with posterior callosotomy showed the opposite pattern: a strong left bias, regardless of hand use. These data suggest that the posterior corpus callosum normally plays a role in line bisection and that the resection of the posterior corpus callosum produces consistent bias. The direction of the bias depends on which hemisphere assumes control.