Callosal pathways for simple visuomotor control in man.

Subjects with complete defects of the corpus callosum, whether congenital or acquired, are very slow in reacting with each hand to a stimulus in the ipsilateral half of the visual field. This is due to the fact that, because of the organization of visual and motor pathways, the hemisphere receiving the stimulus is different from that controlling the response. The interhemispheric transfer necessary for performing the response, which normally is effected by the corpus callosum, must rely in the acallosal subjects on inefficient interhemispheric extracallosal pathways. However patients with complete callosal defects do not show abnormally long crossed responses when they react to the lateralized light stimulus with a whole-arm movement, e.g. a shoulder elevation, rather than with the sole hand. These types of crossed responses appear to be efficiently coordinated across the midline without the aid of the corpus callosum, i.e. by a bilaterally distributed motor system which is preferentially activated for the execution of movements employing axial and proximal limb muscles. Subjects with partial callosal defects do not show any increase in the reaction time of crossed manual responses, suggesting that the intact callosal routes can subserve the integration of speeded crossed manual responses. Bilateral synchronization of hand and arm movements may utilize the corpus callosum as well as other substrates for cross-midline motor coordination.