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

Abstract

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.

Repetitive magnetic stimulation of cortical motor areas in Parkinson's disease: Implications for the pathophysiology of cortical function

“…In an earlier study, we observed that in normal subjects rTMS delivered at frequencies of 3 and 5 Hz prolongs the duration of the SP and probably does so by altering the activity of the cortical inhibitory interneurons. 10,11 Here we now show that rTMS prolongs the cortical SP to a similar extent, also in patients with PD. The normal pattern of cortical SPs during rTMS in PD suggests that the cortical inhibitory interneurons mediating the SP as tested by rTMS function normally.…”

“…9 Besides altering the MEPs, trains of rTMS prolong the duration of the cortical SP evoked by magnetic stimuli. 10,11 Repetitive stimu-lation of the motor cortex therefore influences the activity of excitatory as well as inhibitory neural elements of cortical motor areas. We investigated the effects of rTMS on MEPs and cortical SPs in patients with PD.…”

Informing Dose-Finding Studies of Repetitive Transcranial Magnetic Stimulation to Enhance Motor Function: A Qualitative Systematic Review

“…No obvious association is apparent between either (1) intensity of single-frequency rTMS and effect or (2) number of rTMS stimuli and effect (Table 3). However, there is some suggestion from data of the effects of stimulation at 3 Hz and 5 Hz that prolongation of the silent period might be related to the number of stimuli given at the same intensity 27,46 (Table 4). The effects of combined-frequency rTMS on MEP amplitude were mostly inhibitory 33,34 or had no effect 33 using the parameters tested ( Table 6).…”

Repeated Transcranial Magnetic Stimulation (rTMS) to improve Electroconvulsive Therapy (ECT) in Treatment-Resistant Depression: A Report of Two Cases

“…However, a single session of ECT does not increase CSP 23 while several ECT increase CSP (24), a finding consistent with GABAergic activity of ECT. Transcranial magnetic stimulation also increases the CSP and depressed patients who responded to the high frequency rTMS (20Hz) of the left DLPFC have an increase of CSP and ICI [25][26][27]. Change of ICI was correlated with improved HAM-D score (Hamilton Rating Scale for Depression) [28].…”