Topographic mapping of human motor cortex with transcranial magnetic stimulation: Homunculus revisited.

The purpose of this study was to non-invasively evaluate the homuncular organization of the motor cortex in man. We used transcranial magnetic stimulation to induce motor evoked potentials (MEP's) in Abductor Pollicis Brevis (APB) and Biceps Brachii (BB) muscles of 10 healthy volunteers. The practicality and accuracy of magnetic stimulation to scalp sites one cm apart was increased by the application to the scalp of a flexible nylon grid with grid size of 1x1 cm. Responsive scalp sites collectively contributed to distinct but overlapping muscle representational areas for the two muscles. The topography of these motor maps along and slightly anterior to the central fissure corresponds to the homuncular configuration as described by Penfield and Boldrey in 1937.