WHAT IS THE ROLE OF THE CORPUS-CALLOSUM IN INTERMANUAL TRANSFER OF MOTOR-SKILLS - A STUDY OF 3 CASES WITH CALLOSAL PATHOLOGY

Intermanual transfer for a skilled motor task was studied in two patie nts with total callosal agenesis, and one with an acquired partial cal losal lesion and clinical evidence for disturbed transfer of motor sig nals. Patients had to draw meaningless figures with one upper extremit y (original learning, OL) and to reproduce their mirror-reversals ther eafter with the other side (transfer learning, TL). Both directions of intermanual transfer were tested in two conditions, that is, between either proximal or distal muscle groups. Transfer was evaluated by com paring OL and TL performance at the same effector. The main variable o f interest was movement time during the first eight trials of OL and T L. All three patients displayed a significant benefit for transfer fro m the dominant to the non-dominant hand but not vice versa during prox imal motor activity. When compared with the performance of healthy sub jects tested in almost identical conditions in a;previously reported s tudy, the proximal transfer behavior was found to be similar for all p atients and the normal group. Although patients exhibited no significa nt benefit for distal transfer, their non-dominant-to-dominant distal transfer was above the normal range. The similar transfer pattern of t he patients and healthy subjects when using proximal musculature sugge sts that proximal transfer may be subserved by identical extracallosal pathways, most probably by the ipsilaterally descending motor systems . Since non-dominant-to-dominant distal transfer was found to be disad vantageous in healthy subjects, the patients' relative superiority in this condition may reflect missing callosal influences of an inhibitor y nature.