Interhemispheric transfer in the rat

The mechanisms of interhemispheric transfer of brightness or pattern discrimination were explored in rats, using cortical spreading depression (CSD) to achieve a functional split-brain. After monocular acquisition of the task retrieval was tested during unilateral CSD. The savings were 70 and 38 per cent for the primary trace (hemisphere contralateral to the open eye during initial acquisition) and secondary trace (hemisphere ipsilateral to the open eye) respectively. The secondary trace could not be improved by initial monocular overtraining. In a second study extensive monocular training was preceded by lateralized acquisition of the discrimination task (using CSD) in the primary hemisphere. Retention testing (under CSD) revealed a transferred trace similar to the secondary trace seen when monocular training was not preceded by lateralized training of the primary hemisphere. Thus, monocular training activates interhemispheric write-in processes yielding transferred traces which cannot be improved either by initial monocular overtraining or by prior lateralized training of the primary hemisphere. These results were compared to previous findings indicating that several monocular training trials given with input mainly directed to the untrained hemisphere (after prior lateralized acquisition under CSD) initiate a more powerful transfer. It is suggested that comparison of the transcommissurally transferred information with direct sensory input to the untrained hemisphere is a prerequisite of efficient interhemispheric transfer.