Pathways of interhemispheric transfer in normals and in a split-brain subject - A positron emission tomography study

We studied with PET the intra- and interhemispheric pathways subserving a s imple, speeded-up visuomotor task. Six normal subjects and one patient with a complete section of the corpus callosum (M.E.) underwent regional cerebr al blood flow (rCBF) measurements under conditions of lateralized tachistos copic visual presentations in a simple manual reaction time paradigm. Confi rming previous behavioural findings, we found that on average crossed hand and/or hemifield conditions, i.e. those requiring an interhemispheric trans fer of information, yielded a longer RT than uncrossed conditions. This dif ference (0.7 ms) was dramatically larger (45.6 ms) in the callosum-sectione d patient M.E. In normal subjects the cortical areas selectively activated in uncrossed and crossed conditions were different. In the former condition , most activation foci were anterior to the ventral anterior commissure (VA C) plane, whereas in the latter there was a prevalent parietal and occipita l activation. This shows that a simple model in which the cortical visuo-mo tor pathways are similar in the intra- and the interhemispheric condition, with an extra callosal route for the latter, is too simplistic. Furthermore , these results suggest that the bulk of visuomotor interhemispheric transf er takes place through the widespread callosal fibres interconnecting the p arietal cortices of the two hemispheres. The pattern of activation in the t wo crossing conditions was markedly different in M.E., in whom interhemisph eric transfer might take place via his intact anterior commissure or subcor tical commissures.