Secondary Logo

Journal Logo

Institutional members access full text with Ovid®

Task-Dependent Effects on Motor-Evoked Potentials and on the Following Silent Period

Mathis, Johannes; de Quervain, Dominique; Hess, Christian W.

Journal of Clinical Neurophysiology: November 1999 - Volume 16 - Issue 6 - p 556
Original Contributions
Buy

Summary The silent period (SP) after transcranial stimulation is used as a diagnostic tool in various central nervous system disorders although no standardized experimental setup has been established. The aim of this study was to demonstrate the influence of an isotonic compared to an isometric experimental condition. The SP after transcranial magnetic brain stimulation in the biceps brachii and brachioradialis muscle was up to 130% longer when elicited during a maintain-position (isotonic) task as compared to a maintain-force task (isometric) when stimulus intensities of 5% to 25% above threshold were used. The mean SP duration in these muscles was positively correlated to the mean contraction time in both tasks. However, no such relationship was observed for the trials within the individual subjects. We speculate that the invariably longer SP of the maintain-position task was due to the different “motor set” which predictively determined the muscle behavior after the stimulus. In the maintain-position trials, the stimulus-induced long-lasting flexion movement is counteracted by a motor set aiming to relax the elbow flexors immediately after the stimulus. In the maintain-force task the contraction twitch is short and a force drop below the preset level must be prevented by a motor set aiming to contract the elbow flexors immediately after the stimulus. The latter may increase the synaptic input to the motoneuron pool and facilitate the reoccurrence of the electromyogram terminating the SP. At high-stimulus intensities the SP duration increased in both tasks, and the task-dependent differences disappeared. Therefore, when using the SP duration for diagnostic purposes, isometric conditions and high-stimulus intensities should be used.

Department of Neurology, University Hospital, CH 3010 Bern, Switzerland

Address correspondence and reprint requests to Dr. Johannes Mathis, Dept. of Neurology, University Hospital Inselspital, CH-3010 Bern, Switzerland.

Part of this paper was presented as a poster at the 14th International Congress of EEG and Clinical Neurophysiology, Florence, Italy, August 24–29, 1997.

Copyright © 1999 American Clinical Neurophysiology Society