An automated method to determine the transcranial magnetic stimulation-induced contralateral silent period Zafiris J. Daskalakis a , Gregory F. Molnar b , Bruce K. Christensen a , Alexandra Sailer b , Paul B. Fitzgerald c , Robert Chen b, * a Centre for Addiction and Mental Health, Schizophrenia and Continuing Care Program, University of Toronto, Toronto, Ontario, Canada b Division of Neurology, Krembil Neuroscience Centre and Toronto Western Research Institute, University of Toronto, Toronto, Ontario, Canada c Dandenong Psychiatry Research Centre, Dandenong, Victoria, Australia Accepted 5 February 2003 Abstract Background: The transcranial magnetic stimulation (TMS)-induced contralateral silent period (CSP) refers to a period of interruption of voluntary muscle activity measured in tonically active muscles. The length of the CSP is generally interpreted to reflect cortical inhibition. The determination of the return of voluntary motor activity is typically accomplished via visual inspection of the electromyography (EMG) waveform and may be subject to inaccuracy on the part of the rater. Objective: To present and evaluate an automated method (AM) to determine the CSP. Methods: The CSP of 11 healthy controls was recorded using stimulus intensities 20 and 50% above the resting motor threshold (RMT). The mean CSP duration obtained by the two raters using visual inspection and our automated approach were compared. Results: The interclass correlation coefficient (ICC) between the two raters and the AM was 0.99 at 150% of RMT and was 0.97 at 120% of RMT. The level of pre-stimulus EMG amplitude and sampling rate did not affect agreement between the AM and more conventional visually guided methods. Conclusions: Our study demonstrates that this AM is a simple, objective and reliable approach for CSP determination. Significance: The CSP is an important neurophysiological measure of cortical inhibition and its determination by our AM provides a more objective and automated approach compared to visually guided methods. q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Science Ireland Ltd. All rights reserved. Keywords: Contralateral silent period; Transcranial magnetic stimulation; Motor cortex; Inhibition; Automated method 1. Introduction Transcranial magnetic stimulation (TMS) of the con- tralateral motor cortex during voluntary muscle activity produces a motor evoked potential (MEP) followed by a period of cessation of electromyographic (EMG) activity that is termed the contralateral silent period (CSP) (Cantello et al., 1992). The early part of this CSP is at least partly related to decreased spinal motor neuron excitability, while the late part (after about 50 ms) is due to intracortical inhibitory mechanisms (Chen et al., 1999; Fuhr et al., 1991; Triggs et al., 1993). The CSP has been shown to be abnormal in many neurological (Cantello et al., 1992) and psychiatric disorders (Daskalakis et al., 2002; Ziemann et al., 1997) reflecting altered cortical inhibition and may be a key to understanding the pathophysiology of these disorders. How to demarcate both the start and end of the CSP is often the subject of debate. Some investigators regarded the start of the CSP as the MEP onset, while others used MEP offset or the time of delivery of TMS. Measuring the end of the CSP is even more difficult. The return of voluntary EMG activity, marking the end of the CSP, is frequently imprecise. Terms such as the absolute CSP and relative CSP have been introduced to provide some uniformity when deriving this measure. The end of the absolute CSP is defined as the point in which any EMG activity returns. In contrast, the end of the relative CSP is defined as time when Clinical Neurophysiology 114 (2003) 938–944 www.elsevier.com/locate/clinph 1388-2457/03/$30.00 q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S1388-2457(03)00038-5 * Corresponding author. Toronto Western Hospital, 5W445, 399 Bathurst Street, Toronto, Ontario, Canada M5T 2S8. Tel.: þ1-416-603-5927; fax: þ 1-416-603-5768. E-mail address: robert.chen@uhn.on.ca (R. Chen). CLINPH 2002111