Induction of plasticity in the dominant and non-dominant motor cortices of humans.

There are clear hemispheric differences in the human motor system. Studies using magnetic resonance morphometry have shown that representation of hand muscles is larger in the dominant hemisphere than the non-dominant hemisphere. There is some limited evidence of electrophysiological differences between hemispheres. For example, it has been reported recently that there is less intracortical inhibition in the dominant hemisphere than the non-dominant hemisphere, and it has been hypothesised that this reduction in inhibition may facilitate use-dependent plasticity in the dominant motor cortex. In the present study we examined this hypothesis in human subjects by examining plasticity induction in both dominant and non-dominant hemispheres using an experimental paradigm known to induce motor cortical plasticity, namely paired associative stimulation (PAS). Additionally, we investigated changes in dominant and non-dominant hand performance on a simple ballistic training task. Short-interval intracortical inhibition (SICI) was also measured for both dominant and non-dominant hands at a range of conditioning intensities. There was significantly less SICI in the dominant motor cortical hand area than in the non-dominant hand area. PAS induced a significant, and similar, increase in motor cortical excitability in both the dominant and non-dominant hemispheres. Motor training resulted in significant performance improvement in both dominant and non-dominant hands. However, there was significantly more improvement in the non-dominant hand. The results from these studies provide some further evidence of electrophysiological differences between the motor cortices of the two hemispheres. Additionally, these findings offer no support for the hypothesis that the dominant hemisphere is positioned more favourably, due to decreased inhibitory tone, than the non-dominant hemisphere for use-dependent plasticity.