Interhemispheric facilitation of the hand area of the human motor cortex.

We investigated interaction of the human bilateral hand motor area using two magnetic stimulators. A conditioning shock over the motor cortex facilitated the response evoked in the ipsilateral first dorsal interosseous muscle by a test shock over the contralateral motor cortex given under the limiting conditions of a small conditioning shock over a very small area and a small test shock. This effect was so weak that it was not elicited under slightly different conditions. We conclude that an excitatory, but weak, connection between strictly homotopic areas of the bilateral hand motor cortices exists in humans.