Modulation of intracortical inhibition induced by low- and high-frequency repetitive transcranial magnetic stimulation.

We studied the changes of duration of subsequent silent periods (SPs) during repetitive magnetic stimulation (rTMS) trains of ten stimuli delivered at low (1 Hz) and high (7 Hz) frequencies. The effects at different intensities of stimulation (motor threshold, MT, 115% and 130% above the MT) were also evaluated. rTMS was performed in eight healthy subjects with a figure-of-eight coil placed over the hand motor area. The SP was recorded from abductor pollicis brevis (APB) muscle during a voluntary contraction of 30% of maximum effort. rTMS at 1-Hz frequency progressively decreased the duration of SP, whereas an alternating pattern of smaller and larger values was observed during trains at 7-Hz frequency and higher stimulus intensity. The findings show that rTMS changes the duration of cortical SPs; the effect is probably due to the modulation of intracortical inhibitory interneurons depending on the frequency and intensity of stimulation.