New graphical method to measure silent periods evoked by transcranial magnetic stimulation

Objectives: Manual methods of measuring duration of cortical silent periods (CSP) evoked by transcranial magnetic stimulation (TMS) depend upon subjec tive visual estimation of onset and offset. Because of this, the measuremen ts are susceptible to poor rater reliability. We describe a graphical metho d to measure silent periods with greater precision. The statistical process underlying this new method is simple and particularly suited to signal det ection in serially dependent data. Methods: TMS-evoked silent periods were recorded in 13 healthy subjects. Tw o investigators subjectively measured silent period duration on each subjec t to estimate rater reliability. Using the graphical method, the mean and 9 9.76% variation limits of pre-stimulus electromyogram (EMG) activity were c omputed. Each averaged trial was displayed and CSP onset and offset detecte d when post-stimulus EMG activity moved outside the 99.76% limits. Results: Maximum variation in silent period duration was 21.8 ms between th e two investigators' subjective measurements. Silent period duration measur ed with the graphical method closely approximated measurements obtained usi ng the manual method. It was possible to automate the procedure. Conclusions: This graphical method allowed precise measurement of CSP durat ion, independent of subjective estimations of onset or offset points. Furth er studies are necessary to determine if this method can provide a framewor k for other physiologic measures. (C) 2001 Elsevier Science Ireland Ltd. Al l rights reserved.