Pathways of interhemispheric transfer in normal, split-brain subject. A positron emission tomography study.

Abstract We studied with PET the intra- and interhemispheric
pathways subserving a simple, speeded-up visuomotor
task. Six normal subjects and one patient with a
complete section of the corpus callosum (M.E.) underwent
regional cerebral blood flow (rCBF) measurements
under conditions of lateralized tachistoscopic visual presentations
in a simple manual reaction time paradigm.
Confirming previous behavioural findings, we found that
on average crossed hand and/or hemifield conditions, i.e.
those requiring an interhemispheric transfer of information,
yielded a longer RT than uncrossed conditions. This
difference (0.7 ms) was dramatically larger (45.6 ms) in
the callosum-sectioned 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 (VAC) plane, whereas in the latter
there was a prevalent parietal and occipital activation.
This shows that a simple model in which the cortical visuo-
motor 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
transfer takes place through the widespread callosal fibres
interconnecting the parietal cortices of the two
hemispheres. The pattern of activation in the two crossing
conditions was markedly different in M.E., in whom
interhemispheric transfer might take place via his intact
anterior commissure or subcortical commissures.