CORPUS-CALLOSUM AND SIMPLE VISUOMOTOR INTEGRATION

Malcolm Jeeves was the first to demonstrate lengthened interhemispheri c transmission times in subjects with agenesis of the corpus callosum by using a simple reaction time paradigm with lateralized unstructured light stimuli and crossed and uncrossed hand responses. Uncrossed res ponses can be integrated within one hemisphere, whereas crossed respon ses require a communication between the two hemispheres. In the normal brain this communication is effected rapidly by the corpus callosum, whereas in the acallosal brain it must occur much more slowly by way o f less efficient alternative interhemispheric pathways. Using a simila r experimental paradigm we have studied normal subjects, subjects with a complete callosal agenesis and epileptic patients with surgical cal losal sections, either complete or partial. All subjects with complete callosal defects showed much lengthened interhemispheric times compar ed to normal controls. Virtually normal interhemispheric transmission times were found in subjects with partial callosal defects, whether an terior or posterior, suggesting a possible equipotentiality of differe nt portions of the corpus callosum in the mediation of crossed manual responses. In both normals and acallosals there were no crossed-uncros sed differences in reaction time when responses were made unilaterally with lower limb effecters or para-axial upper limb effecters, as well as bilaterally with upper-limb proximal and para-axial effecters. Sin ce these effecters can be controlled directly from either side of the brain via bilaterally distributed motor pathways, crossed responses us ing them, unlike crossed manual responses, do not require an interhemi spheric integration.