The effect of callosotomy on novel versus familiar bimanual actions: a neural dissociation between controlled and automatic processes?

The corpus collosum is the large band of fibers that connects the two cerebral hemispheres of the brain. Individuals who have had the fibers of these tracts surgically severed by callosotomy are able to draw two different spatial figures simultaneously using the left and right hands, without evidence of interactions in the spatial planning processes. Paradoxically, tasks (e.g., tying shoes) that appear to depend on spatial interactions between the left and right hands, each of which is controlled by a separate cerebral hemisphere, pose little difficulty. How can this be? In the study reported here, we observed that well-learned cooperative actions of the hands remain intact in 2 callosotomy patients, whereas actions novel to these patients are virtually impossible for them to produce without visual guidance. We infer that duplicate memory engrams of well-learned actions can be accessed by both cerebral hemispheres without callosal mediation, whereas callosal interactions are necessary for precise cross-matching of sensory information during spatial planning or perceptual-motor learning.