HEMISPHERIC CONTROL OF UNILATERAL AND BILATERAL RESPONSES TO LATERALIZED LIGHT STIMULI AFTER CALLOSOTOMY AND IN CALLOSAL AGENESIS

Normally, simple digital or manual responses to a light stimulus in th e right or left visual hemifields are performed faster with uncrossed hand-field combinations than with crossed hand-field combinations. Bec ause of the organization of visual and motor pathways, the integration of uncrossed responses is assumed to occur within a single hemisphere , whereas a time-consuming interhemispheric transfer via the corpus ca llosum is considered to be necessary for the integration of crossed re sponses. However, callosal transfer may be dispensable for those cross ed responses which can be controlled through ipsilaterally descending motor pathways by the hemisphere receiving the visual stimulus. We inv estigated crossed-uncrossed differences (CUDs) in speed of simple visu omotor responses to lateralized flashes in one subject with total sect ion of the corpus callosum and two subjects with complete callosal age nesis. We recorded the reaction times as well as the premotor times, a s indicated by the electromyographic latencies of the prime movers, of three types of responses: a distal response involving a thumb flexion , a proximal response chiefly involving a forearm flexion and an axial response involving a shoulder elevation. Further, the three types of responses to a single lateralised flash were performed both unilateral ly and bilaterally. The three acallosal subjects showed CUDs greatly e xceeding normal values on distal responses, either unilateral or bilat eral, and on unilateral proximal responses. These abnormally long CUDs stood in sharp contrast to the insignificant CUDs exhibited by the sa me subjects on bilateral proximal responses and on unilateral and bila teral axial responses in agreement with correspondingly insignificant CUDs reported for normal subjects. These results confirm that a callos al contribution is important for the execution of fast distal and unil ateral proximal responses to a visual stimulus directed to the hemisph ere ipsilateral to the responding hand. By contrast, the other types o f crossed responses appear to be efficiently coordinated across the mi dline without the aid of the corpus callosum. This is in keeping with the hypothesis that they are governed by a bilaterally distributed mot or system which is preferentially activated for the execution of symme trical bilateral movements, employing axial and proximal limb muscles.