Motor cortex excitability following short trains of repetitive magnetic stimuli

Trains of repetitive transcranial magnetic stimuli (rTMS) appear to have ef fects on corticospinal excitability that outlast the duration of the train. In order to investigate the mechanism of this effect in more detail we app lied short periods of rTMS consisting of up to 20 stimuli at 5 Hz, 10 Hz or 20 Hz (rTMS) to the motor cortex at an intensity equal to resting threshol d in 11 healthy, relaxed subjects. Spinal excitability, as judged by effect s on the H-reflex or on transcranial anodal facilitation of the H-reflex, w as not affected by the rTMS. However, cortical excitability, as judged by t he effect on the size of EMG responses evoked by a suprathreshold TMS pulse , was decreased for up to 1 s after the end of rTMS. Post-train suppression was more powerful following longer trains or higher frequencies of rTMS. T he predominant suppression contrasts with previous reports of facilitation, particularly after high-frequency rTMS. A second set of experiments, howev er, showed that this could be converted into facilitation if the intensity of rTMS was increased. We conclude that the after-effects of rTMS depend on its frequency, intensity and duration. The results are consistent with a m odel in which inhibition and facilitation build up gradually during the cou rse of a conditioning train. Inhibition reaches its maximum effect after on ly a small number of stimuli, whereas facilitation takes longer. The thresh old for evoking inhibition is lower than that for facilitation. Thus if mod erate intensities of conditioning train are applied, inhibition is predomin ant after short trains, whereas facilitation dominates after long trains.