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) durin g repetitive magnetic stimulation (rTMS) trains of ten stimuli delivered at low (1 Hz) and high (7 Hz) frequencies. The effects at differ ent intensit ies of stimulation (motor threshold, MT, 115% and 130% above the MT) were a lso evaluated. rTMS was performed in eight healthy subjects with a figure-o f-eight coil placed over the hand motor area. The SP was recorded from abdu ctor pollicis brevis (APB) muscle during a voluntary contraction of 30% of maximum effort, rTMS at l-Hz frequency progressively decreased the duration of SP, whereas an alternating pattern of smaller and larger values was obs erved during trains at 7-Hz frequency and higher stimulus intensity. The fi ndings show that rTMS changes the duration of cortical SPs; the effect is p robably due to the modulation of intracortical inhibitory interneurons depe nding on the frequency and intensity of stimulation.