DEPRESSION OF MOTOR CORTEX EXCITABILITY BY LOW-FREQUENCY TRANSCRANIALMAGNETIC STIMULATION

We studied the effects of low-frequency transcranial magnetic stimulat ion (TMS) on motor cortex excitability in humans. TMS at 0.1 Hz for 1 hour did not change cortical excitability. Stimulation at 0.9 Hz for 1 5 minutes (810 pulses), similar to the parameters used to induce long- term depression (LTD) in cortical slice preparations and in vivo anima l studies, led to a mean decrease in motor evoked potential (MEP) ampl itude of 19.5%. The decrease in cortical excitability lasted for at le ast 15 minutes after the end of the 0.9 Hz stimulation. The mechanism underlying this decrease in excitability may be similar to LTD. TMS-in duced reduction of cortical excitability has potential clinical applic ations in diseases such as epilepsy and myoclonus. Spread of excitatio n, which may be a warning sign for seizures, occurred in one subject a nd was not accompanied by increased MEP amplitude, suggesting that spr ead of excitation and amplitude changes are different phenomena and al so indicating the need for adequate monitoring even with stimulations at low frequencies.