Finger movements induced by transcranial magnetic stimulation change with hand posture, but not with coil position

We attempted to map the representations of movements in 2 normal subjects by delivering five transcranial magnetic stimuli (TMS) with a focal coil to each of a grid of positions over the primary motor area (M1). Isometric forces were recorded from the contralateral index finger. Maps were made with the hand in a semiflexed "neutral" position, and with the thumb and index finger opposed in a "pincer" grip. The electromyogram (EMG) was monitored to ensure relaxation. The wrist was immobilized. In the neutral position, TMS at almost all positions produced abduction. Flexion was produced in the pincer position. Thus, while sensitive to changes in posture, TMS mapping may not be sensitive to the topographical organization of the M1 by movements as detected with direct cortical stimulation.