Transcranial magnetic stimulation (TMS) is a noninvasive method to excite neurons in the brain. The excitation is caused by weak electric currents induced in the tissue by rapidly changing magnetic fields (electromagnetic induction). This way, brain activity can be triggered or modulated without the need for surgery or external electrodes.

The TMS-induced neuronal excitation is based on the Faraday’s principles of electromagnetic induction. A brief pulse of current flowing through a coil of wire generates a magnetic field. If the magnitude of this magnetic field changes in time, then it will induce a secondary current in any nearby conductor.

For brain stimulation a pulse is produced in a coil held over a subject’s head. As a brief pulse of current is passed through it, a magnetic field is generated that passes through the subject’s scalp and skull with negligible attenuation. This time-varying magnetic field induces an electric current in the subject’s brain, causing depolarization of cellular membranes and thereby neuronal activation.

In many experiments, single pulses of TMS are applied over the motor cortex. The stimulation of the motor cortex is able to trans-sinaptically activate the corticospinal system and to produce a response in controlateral extremity muscles, the motor evoked potential (MEP) that can be recorded by means of electrodes. Amplitude of MEPs is used as a measure of corticospinal excitability. The amplitude of these potentials is modulated by the behavioural context. Thus, the modulation of MEPs amplitude can be used to assess the central effects of various experimental manipulations.