Transcranial magnetic stimulation (TMS) is a non-invasive method for exciting neurons in the brain. The excitation is caused by electric currents induced in the tissue by rapidly alternating magnetic fields (electromagnetic induction). In this way, brain activity may be triggered or modulated without surgery or externally applied electrodes. This technique is used to study the circuitry and connectivity of the brain. Repetitive transcranial magnetic stimulation, known as rTMS, may produce longer-lasting changes in electro-cortical function.
EEG (or ERP) recording during TMS stimulation is an important technique providing support in the determination of cognitive processes. TMS stimulation induces a strong electrical field that can saturate the recording amplifiers for longer periods. Furthermore, even if the amplifier does not reach a saturation point, the TMS pulse induces an artifact in the EEG data that can last for hundreds of milliseconds.