The cortical silent period: intrinsic variability and relation to the waveform of the transcranial magnetic stimulation pulse

Objective: To assess the variability of the duration of the contralateral cortical silent period (CSP) between individuals and to assess the effect of different transcranial magnetic stimulation (TMS) pulse waveforms. Methods: In Expt. 1, CSP duration, and the motor-evoked potential (MEP) amplitude and area were measured in the first dorsal interosseous muscle (FDI) of 11 subjects on 3 separate occasions using a TMS intensity of 150% active motor threshold (AMT). In Expt. 2, the stimulation intensity was varied between 100% AMT and 150% AMT. In both sets of experiments, 3 types of TMS pulse were used: monophasic posterior-anterior (PA) induced cur-rent in the brain, monophasic anterior-posterior induced current (AP), and biphasic PA/AP stimulation. Results: Experiment 1: Between-subject variation in CSP duration was high. In addition, the duration after PA stimulation was significantly shorter than after AP or PA/AP stimulation. However, there was a good correlation between CSP duration and the area, or amplitude, of the MEP. This meant that calculating the ratio of duration/amplitude or duration/area reduced intersubject variability and eliminated differences between TMS pulses. Experiment 2: increasing stimulation intensity increased the mean value of all parameters, but with significantly lower values for PA than other forms of stimulation. The ratios of duration/amplitude or duration/area did not differ between current flow directions and Were relatively constant for intensities 130-150% AMT. Conclusions: Between-subject variation in the duration of the CSP is high. A given intensity of stimulation (expressed in %AMT) produces a shorter CSP for PA stimulation than for AP or PA/AP stimulation. Significance: If the ratio (CSP duration)/(MEP size) is calculated, then intersubject variability is reduced, and TMS pulse type differences are eliminated. (C) 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.