The use of an electromagnet to induce electric potential on the brain's surface, thus forcing the area's neurons to fire. This effect is due to Faraday's law, and the induced electric field flows in the direction opposite that of current in the magnetic coil. Stimulation effects axons (as opposed to the neuronal body or its dendrites), and is most likely to occur where the axon changes shape with respect to the magnetic field, such as at bends or ends. Modern TMS is done with a figure-8 shaped coil, which focuses the magnetic field into a more localized point than a circular one, and can be "aimed" better. You should note that TMS has been used in quite a few (somewhere in the hundreds) studies since its invention in 1982, and is not an imagined or placebo effect like Mesmer's magnetic rod treatments of a hundred years ago. It's real science, folks!

Two kinds of TMS are used today. Single pulse TMS produces one pulse of magnetic energy when fired that lasts between 200 and 600 ms. Repetitive TMS (rTMS) produces a stream of pulses when fired, usually at a frequency of 10 and 60 Hz. They both use coil currents of between 5000 and 10000 amps (!), and may produce a surface magnetic field strength of up to 3 Teslas. rTMS devices cannot be turned on for very long, a few seconds at most, as heat builds up in the coils. Water-cooled rTMS devices are available, as are less expensive air-cooled ones. There appears to be no difference in effect between TMS and rTMS, but rTMS is used when the effects of lengthy stimulation are to be studied.

The first application of TMS was to map functions of areas of the brain surface. For instance, TMS applied to a point just behind the left ear caused the right hand to twitch, since that area corresponds to the portion of the motor cortex responsible for the right hand. Another example is the finding that TMS applied to the occipital cortex disturbs the brain's ability to interpret and understand sight.

Another interesting finding was that when TMS was applied to the left side of the prefrontal cortex (the brain structure just behind each eyebrow) of healthy adults, their mood changed to sadness. When applied on the right, happiness resulted. Dr. Mark S. George suggested using TMS as a treatment for major depression, by stimulating the left prefrontal cortex, which is underactive in depressed individuals. In double-blind studies in the US and elsewhere this has proven statistically to be an effective treatment, including one study in Israel that found it to be more effective than electroconvulsive therapy.

The major side effect is seizure, which may be triggered by TMS or rTMS in people who are susceptible to it. TMS never induces seizure in healthy individuals, but rTMS sometimes does. After TMS the patient often has a headache due to activation of scalp and neck muscles, but this may be treated with any mild analgesic. Older devices used for TMS tended to be very loud (> 100 dB) and become hot enough to burn the scalp, but these problems have been done away with in newer designs.