Transcranial magnetic stimulation (TMS) is a method to stimulate the human brain and other neural tissues noninvasively, making use of electromagnetic induction by rapidly time-varying electric currents. It was first introduced in 1985 [1] and is unique among different methods collectively termed noninvasive brain stimulation (NIBS) in that it can evoke action potentials in the underlying neurons without pain. This is quite in contrast with weak transcranial electric stimulation including transcranial direct current stimulation and transcranial alternating current stimulation that are not strong enough to produce action potentials, and with high-voltage electric stimulation introduced earlier [2] that causes substantial pain under the stimulating electrodes or associated neck muscle contraction.

The use of TMS for research as well as for clinical applications can be categorized into two ways: single-, paired- (or sometimes triple-) pulse TMS investigates the instantaneous effect of the stimulation, whereas repetitive TMS (rTMS) aims at inducing long-lasting aftereffects, probably through synaptic plasticity. Single- or paired-pulse TMS can be used to investigate the functions of various intra- and intercortical facilitatory and inhibitory circuits of the brain. They can also be used to temporarily disrupt the function of a certain part of the brain, a phenomenon known as the virtual lesion effect. The effect of rTMS lasts even after the stimulus pulses have ceased, and its effect can be measured by behavioral tasks conducted later at time points temporally separated from the stimulation. Such applications can lead to novel, nonpharmacological treatments of neurological and psychiatric disorders.

Physiology underlying TMS has been extensively studied using motor evoked potential (MEP) obtained by stimulation of the primary motor cortex (M1) as a readout. In this article, we first review basic mechanisms and functionality of (r)TMS mainly based on evidence from M1 stimulation. We also cover variability and other related issues later. Past, present, and future issues are also summarized elsewhere, with some emphasis on human behavior [3].