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Arthur Cukiert, M.D., PhD and Cesar Timo-Iaria, M.D., PhD.
Clinical Neurophysiology Laboratory, University of Sao Paulo School of Medicine, Sao Paulo SP, Brazil
Rationale: To study the patterns of inter- and intrahemispheric transfer in the cat by means of transcallosal evoked potential mapping.
Methods: Thirty-five adult cats were anesthetized with quetamine and submitted to extensive bilateral craniotomy thus exposing most of the neocortex. After stimulation of multiple points covering the entire convexity and mesial regions with bipolar electrodes, transcallosal evoked potentials were recorded over the contralateral hemisphere, starting at the homologous site and then procceeding in both the antero-posterior and medio-lateral directions.
Serial percortical and callosal sections were performed to further investigate the possible routes of evoked-potentials spreading.
Results: The sites associated with dense callosal connectivity showed a maximal amplitude of evoked potentials over the homologous region; the sites associated with few or no callosal connectivity (especially posterior suprasylvian and marginal regions) were associated with a maximal amplitude of evoked potentials at regions located much more anteriorly. The percortical and callosal sections showed that an L-shaped connection, comprising an intrahemispheric route followed by a more anterior interhemispheric pathway, was responsible for this asymetric findings in acallosal regions.
Conclusion: Posterior quadrant acallosal regions or regions poorly connected by the corpus callosal may use L-shape connections to make interhemispheric integration possible. Posteriorly located epileptic foci may generate secondary bilateral synchrony in the anterior (frontal) regions without the presence of actual seizure generators in these anterior regions.