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.