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Brain Topogr (2015) 28:520528 DOI 10.1007/s10548-013-0312-z
ORIGINAL PAPER
Masking the Auditory Evoked Potential in TMSEEG: A Comparison of Various Methods
Esther M. ter Braack Cecile C. de Vos
Michel J. A. M. van Putten
Received: 18 March 2013 / Accepted: 15 August 2013 / Published online: 1 September 2013 Springer Science+Business Media New York 2013
Abstract There is growing interest in combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG). Because TMS pulses are accompanied by a clicking sound, it is very likely that part of the response in the EEG consists of an auditory evoked potential (AEP). Different methods have been applied to mask the sound of TMS. However, it is unclear which masking method is most effective in reducing the AEP. In this study we explore the presumed contribution of the AEP to the response and evaluate different ways to mask the TMS clicking sound. Twelve healthy subjects and one completely deaf subject participated in this study. Eight different masking conditions were evaluated in nine hearing subjects. The amplitude of the N100P180 complex was compared between the different masking conditions. We were not able to completely suppress the N100P180 when the coil was placed on top of the head. Using an earmuff or exposing the subjects to white or adapted noise caused a small but signicant reduction in N100P180 amplitude, but the largest reduction was achieved when combining a layer of foam, placed between coil and head, with white or adapted noise. The deaf subject also showed a N100P180 complex. We conclude that both the TMS clicking sound
and cortical activation by the magnetic pulse contribute to the N100P180 amplitude.
Keywords Transcranial magnetic stimulation (TMS) Electroencephalography (EEG) Auditory
evoked potential
Introduction
Transcranial magnetic stimulation (TMS) is a widely used technique to noninvasively activate the human cortex. After the introduction in the mid-eighties (Barker et al. 1985), TMS has developed into a method that is used to study motor cortical excitation (resting motor threshold) and inhibition (short and long interval cortical inhibition) by measuring modulation of the motor evoked potential (MEP). In addition, various diagnostic and therapeutic applications have emerged, including determining the central motor conduction time for diagnosing spinal cord compression, amyotrophic lateral sclerosis or multiple sclerosis (Chen et al. 2008), and applying repetitive...