Transcranial Magnetic Stimulation (TMS) is a non-invasive technique that gives information about brain function. It is very useful when studying areas of the nervous system related to motor activity (motor cortex, corticospinal tract, and corpus callosum). A magnetic signal given from a special instrument held close to the patient's head stimulates a small area of the brain that controls a few muscles (for example, the muscles that control one finger). Doctors put electrodes (small pieces of metal taped to areas of the body) over the muscle to measure the electrical activity the muscle produces when it makes a movement. When the magnetic signal activates those muscles the electrodes pick up and record the electrical activity of the movement that the muscles make in response to the magnetic signal.