Noninvasive mapping of human motor cortex

Human motor cortex was stimulated using brief, high-voltage electrical stimulation. Constant-voltage stimuli were delivered through a bipolar surface stimulator with the anode placed at multiple positions on the scalp and the cathode situated 2.5 cm anterior to the anode. Recordings were bilateral from the abductor pollicis brevis, tibialis anterior, and risorius. We averaged the amplitudes of three muscle responses obtained from stimulation of each scalp position and assigned the resultant value to that position. The findings in eight normal volunteers were similar and reproducible. The maximal responses of the right hand were obtained when stimulating over C3, of the left hand when stimulating over C4, of the right and left legs when stimulating over Cz, and of the right side of the mouth when stimulating over T3.