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


Journal of Alzheimers Disease & Parkinsonism

Author(s): Orth M, Rothwell JC

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Abstract 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 current 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. This article was published in Clin Neurophysiol and referenced in Journal of Alzheimers Disease & Parkinsonism

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