INTERHEMISPHERIC INTEGRATION OF SIMPLE VISUOMOTOR RESPONSES IN PATIENTS WITH PARTIAL CALLOSAL DEFECTS

Because of the organization of visual and motor pathways, simple manua l responses to a light stimulus in the right or left visual hemifields are performed faster with uncrossed hand-field combinations than with crossed hand-field combination. Uncrossed responses can be integrated within a single hemisphere, whereas crossed responses require a time- consuming interhemispheric transfer via the corpus callosum which is r eflected in the difference between crossed and uncrossed reaction time s. We investigated crossed-uncrossed differences (CUDs) in speed of si mple visuomotor responses to lateralized flashes in seven subjects wit h an anterior section of the corpus callosum sparing the splenium and in one subject with an agenetic absence of the splenium due to a cereb rovascular malformation. There was no evidence of an abnormal prolonga tion of the CUDs in any of these subjects, in sharp contrast with the very long CUDs exhibited by an epileptic subject with a complete callo sal section and two subjects with total callosal agenesis tested in th e same experimental situation [1]. The normality of the CUDs in the su bjects with partial callosal defects was not due to a postoperatory re organization of interhemispheric communication, since there was no ind ication of an increased CUD in a patient tested as early as 5 days aft er the anterior callosotomy. These results are compatible with the ass umption that both anterior and posterior callosal routes may subserve the integration of speeded manual responses to a visual stimulus direc ted to the hemisphere ipsilateral to the responding hand.