This study will use transcranial magnetic stimulation, or TMS (described below), to examine how the brain controls muscle movement to prevent unwanted movements in surrounding muscles. For example, when a person moves a finger, a part of the brain called the cortex prevents unwanted movements in other fingers by a process called cortical inhibition. In people with the muscle disorder dystonia, cortical inhibition does not work properly and patients suffer from uncontrolled and sometimes painful movements. A better understanding of how this process works in normal people may shed light on what goes wrong in dystonia and how the condition can be treated.