S. Romeo et al., Changes in the cortical silent period after repetitive magnetic stimulation of cortical motor areas, EXP BRAIN R, 135(4), 2000, pp. 504-510
The physiological mechanisms underlying the lengthening of the silent perio
d (SP) evoked in active upper limb muscles by repetitive transcranial magne
tic 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 H
z and at an intensity just above the motor threshold (Mth). Trains delivere
d 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 al
ready 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 effe
ct of intensity of stimulation was studied by delivering trains at suprathr
eshold 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 corti
cal SPs over the course of the trains. Repetitive stimulation of the cortic
al 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 train
s evoked SPs of similar duration. We propose that rTMS delivered in trains
at frequencies higher than 1 Hz and at suprathreshold intensity prolongs th
e cortical SP mainly through temporal summation of inhibitory interneurones
.