Hemispheric asymmetry of transcallosal inhibition in man

The transcallosal connecting fibres linking corresponding projection areas of the same muscles of the right and left primary motor cortex may play an important role in control of unilateral movements. It appears 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 at the motor representation areas of the first dorsal interosseous muscle of the left and right sides. A conditioning stimulus was given to one hemisphere 10 ms prior to the test stimulus at the opposite hemisphere. The inhibition was evaluated as relative amplitude reduction. Eleven normal right-handed subjects and 11 normal left-handed subjects participated in this study. Handedness was evaluated by the Oldfield inventory. It was found that in right-handers the inhibition after stimulation of the "dominant" left hemisphere was more marked than after stimulation of the "non-dominant" right hemisphere. In contrast, the group of left-handed subjects showed inhomogeneous findings with either 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-dominant hemisphere in governing inter-hemispheric control of motor cortical connections, supporting the view that the "language-dominant" hemisphere is also "motor dominant".