No evidence for a substantial involvement of primary motor hand area in handedness judgements: a transcranial magnetic stimulation study.
Twelve right-handed volunteers were asked to judge the laterality of a hand stimulus by pressing a button with one of their toes. Judgements were based on two-dimensional drawings of the back or palm of a right or left hand at various orientations. Suprathreshold single-pulse transcranial magnetic stimulation (TMS) was given to the left primary motor hand area (M1-HAND) at 0, 200, 400, 600, 800 or 1000 ms after stimulus onset to probe the functional involvement of the dominant left M1 at various stages of handedness recognition. We found that mean reaction times and error rates increased with angle of rotation depending on the actual biomechanical constraints of the hand but suprathreshold TMS had no influence on task performance regardless of the timing of TMS. However, the excitability of the corticomotor output from the left M1-HAND was modulated during the reaction. Judging left hand drawings was associated with an attenuation of motor-evoked potentials 300-100 ms before the response, whereas judging right hand drawings facilitated the motor-evoked potentials only immediately before the response. These effects were the same for pictures of backs and palms and were independent of the angle of rotation. The failure of TMS to affect task performance suggests that there is no time window during which the M1-HAND makes a critical contribution to mental rotation of the hand. The modulation of motor-evoked potentials according to the laterality of the stimulus indicates a secondary effect of the task on corticomotor excitability that is not directly related to mental rotation itself.