For this procedure, participants sit in front of a computer monitor with the index, middle, ring, and little fingers placed flat on four corresponding buttons of a response pad. They are asked to press one of the four buttons as fast as they can in response to an asterisk displayed on the monitor. The position of the asterisk indicates which button to push. Subjects perform nine sets of 120-button pushes with a 15-minute rest between each set. During the rest period, subjects undergo transcranial magnetic stimulation (described below), which causes twitches in the hand muscles. The electrical activity corresponding to the twitches is recorded by attaching electrodes (small metal disks) to the skin over the hand muscles.