Transcranial magnetic stimulation or TMS is a neurophysiological technique that allows the induction of a current in the brain using a magnetic field to pass the scalp and the skull safely and painlessly. In TMS, a current passes through a coil of copper wire that is encased in plastic and held over the subject's head. This coil resembles a paddle or a large spoon and is held in place either by the investigator or by a mechanical fixation device similar to a microphone pole. As the current passes through the coil it generates a magnetic field that can penetrate the subject's scalp and skull, and in turn induce a current in the subject's brain. TMS is used in clinical neurophysiology to study the nerve fibers that carry the information about movements from the brain cortex to the spinal cord and the muscles. 

Technical developments in the devices used for TMS made it possible in the late 1980s to apply TMS in trains of multiple stimuli per second. This form of TMS is called repetitive TMS or rTMS. Repetitive TMS can be used to study how the brain organizes different functions such as language, memory, vision, or attention. In addition, rTMS seems capable of changing the activity in a brain area, even beyond the duration of the rTMS application itself. In other words, it seems possible to make a given brain area work more or less for a period of minutes, or even weeks when rTMS is applied repeatedly several days in a row. This has opened up the possibility of using rTMS for therapy of some illnesses in neurology, rehabilitation, and psychiatry. It is important that before agreeing to participate in a TMS study, all the questions of potential subjects or patients be answered. 

TMS, particularly rTMS, can have adverse effects. The most worrisome of which is its potential to induce a seizure or epileptic convulsion, even in subjects without any predisposing illness. The risk of adverse effects is low, but appropriate safety guidelines and precautions need to be followed, and the person performing rTMS needs to be properly trained. Subjects considering their participation in a TMS study, and patients planning to undergo rTMS for treatment of any disease should know that there are relative and absolute contraindications to TMS. It is important to discuss these with the investigators and physicians and undergo proper screening.