HEMISPHERIC-ASYMMETRY OF TRANSCALLOSAL INHIBITION IN MAN

The transcallosal connecting fibres linking corresponding projection a reas of the same muscles of the right and left primary motor cortex ma y play an important role in control of unilateral movements. It appear s that they have mainly inhibitory effects. This was further evaluated by transcranial magnetic stimulation using two focal coils placed on the optimal positions, i.e. the positions with the lowest thresholds a t the motor representation areas of the first dorsal interosseous musc le of the left and light sides. A conditioning stimulus was given to o ne hemisphere 10 ms prior to the test stimulus at the opposite hemisph ere. The inhibition was evaluated as relative amplitude reduction. Ele ven normal right-handed subjects and 11 normal left-handed subjects pa rticipated in this study. Handedness was evaluated by the Oldfield inv entory. It was found that in right-handers the inhibition after stimul ation of the ''dominant'' left hemisphere was more marked than after s timulation of the ''non-dominant'' right hemisphere. In contrast, the group of left-handed subjects showed inhomogeneous findings with eithe r right- or left-side predominant inhibition. It is concluded that not handedness but hemispheric dominance contributes to the laterality of inhibition. The results point to a superior role of the language-domi nant hemisphere in governing inter-hemispheric control of motor cortic al connections, supporting the view that the ''language-dominant'' hem isphere is also ''motor dominant''.