Changes in the cortical silent period after repetitive magnetic stimulation of cortical motor areas

The physiological mechanisms underlying the lengthening of the silent period (SP) evoked in active upper limb muscles by repetitive transcranial magnetic stimulation (rTMS) of the motor areas were studied in normal subjects, rTMS was delivered at frequencies of 1 Hz, 2 Hz, 3 Hz, 5 Hz, 10 Hz and 15 Hz and at an intensity just above the motor threshold (Mth). Trains delivered at 2 Hz, 3 Hz, 5 Hz, 10 Hz and 15 Hz significantly prolonged the cortical SP, whereas stimuli at 1 Hz did not. The first few stimuli in the train already prolonged the duration of the cortical SP: the other stimuli did not prolong it further. Motor evoked potentials remained unchanged in amplitude regardless of the frequencies and number of stimuli in the train. The effect of intensity of stimulation was studied by delivering trains at suprathreshold intensity (110% and 140% of Mth) and 3-Hz frequency and with trains at subthreshold intensity and 5-Hz and 10-Hz frequencies. SPs had a longer duration at 140% than at 110% Mth intensity. SPs elicited by 3-Hz trains at 140% and 110% Mth intensity lengthened to a similar extent over the course of the train. rTMS delivered at an intensity below Mth did not evoke cortical SPs over the course of the trains. Repetitive stimulation of the cortical forearm motor areas prolonged the duration of the cortical SP in forearm flexor muscles but failed to evoke SPs in the biceps muscles. The maximal single stimulus intensity and less intense stimuli delivered in short trains evoked SPs of similar duration. We propose that rTMS delivered in trains at frequencies higher than 1 Hz and at suprathreshold intensity prolongs the cortical SP mainly through temporal summation of inhibitory interneurones.