Subcortical coordination of hemisphere activity in commissurotomized patients

Despite their commissurotomy, 'split-brain' patients behave as unified individuals and seldom display signs of hesitation and dissociation in their activities. Yet our current understanding of the callosal disconnection syndrome suggests that the two isolated hemispheres cannot work in parallel and that only one hemisphere at a time is responsive to bilateral stimulation. To examine this divergence, the two hemispheres of complete forebrain commissurotomy patients were simultaneously presented with colour stimuli associated with opposite responses, and the patients had to produce a single finger response whose accuracy depended on their capacity to resolve the conflicting information. Although the two patients differed in their efficiency, they were capable of accurate performance in the bilateral conflict condition, but could not correctly cross-compare the information received by each hemisphere. This performance suggests that the two disconnected hemispheres can work in parallel and process the incoming information efficiently. The outcome of these hemispheric operations may be integrated in subcortical 'centrencephalic' areas that coordinate the activities of the two sides of the brain and maintain some unity within cerebral structures.