Surround inhibition in human motor system.

In sensory systems, a neural mechanism called surround inhibition (SI) sharpens sensation by creating an inhibitory zone around the central core of activation. In the motor system, the functional operation of SI remains to be demonstrated, although it has been hypothesized to contribute to the selection of voluntary movements. Here we test this hypothesis by using transcranial magnetic stimulation of the human motor cortex. The motor evoked potential of the little finger muscle is suppressed or unchanged during self-paced, voluntary movements of the index finger, mouth or leg, despite an increase in spinal excitability. This result indicates that motor excitability related to little finger movement is suppressed at the supraspinal level during these movements, and supports the idea that SI is an organizational principle of the motor system.