Magnetic stimulation of the human motor cortex: facilitation and its relationship to a visual motor task

Transcranial magnetic stimulation of the motor cortex can evoke motor responses in small hand muscles. This response can be facilitated by a background muscle contraction of the target muscle, resulting in an enhanced compound muscle action potential (CMAP) with a shorter onset latency. A number of intracortical mechanisms may account for this facilitatory effect, including, in part, direct input from visual to motor cortex. We studied the facilitation produced by a visual-motor task and compared those results with the facilitation produced by the same task without the visual cues. No differences in facilitation of amplitude or latency were observed. This suggests that there is no direct influence exerted by the visual cortex upon those elements of the motor cortex activated by a tangential magnetic stimulus, i.e., corticocortical and corticospinal neurons and their processes. Also, the large majority of facilitation (90%) was produced by a very small background voluntary contraction (less than 5% of maximum), indicating that any mild-to-moderate contraction of the target muscles will produce a consistent response for clinical measurements.