Masking the Auditory Evoked Potential in TMS–EEG: A Comparison of Various Methods

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 N100-P180 complex was compared between the different masking conditions. We were not able to completely suppress the N100-P180 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 significant reduction in N100-P180 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 N100-P180 complex. We conclude that both the TMS clicking sound and cortical activation by the magnetic pulse contribute to the N100-P180 amplitude.

Mapping dynamical properties of cortical microcircuits using robotized TMS and EEG: Towards functional cytoarchitectonics

“…First, from results not shown in this report, we did not find any correlation between the intensity of stimulation, which is directly linked to the power of both the click sound and the induced muscle twitch, and the amplitude of this component, which should be large in case of purely auditory or sensorimotor artifacts (Hegerl and Juckel, 1993). Second, this component has already been found and discussed in numerous studies in TMS-EEG (Braack et al, 2015;Chung et al, 2015). It has been shown to reflect a consistent aftereffect of the stimulation rather than a pure auditory or somatosensory artifact.…”

Investigating TMS–EEG Indices of Long-Interval Intracortical Inhibition at Different Interstimulus Intervals

“…However, this confounding influence is significantly reduced by white noise masking during stimulation [52], in addition to the use of independent component analysis [27], both of which were utilised within the current study. Furthermore, as the P180 can still be elicited in deaf subjects [52], and is significantly reduced in individuals with progressive myoclonus epilepsy [53], it seems likely that it has contributions from TMS-induced cortical activity.…”

“…Furthermore, as the P180 can still be elicited in deaf subjects [52], and is significantly reduced in individuals with progressive myoclonus epilepsy [53], it seems likely that it has contributions from TMS-induced cortical activity.…”

A Multimodal Imaging- and Stimulation-based Method of Evaluating Connectivity-related Brain Excitability in Patients with Epilepsy

“…One particularly important such biological artifact is the auditory evoked-potential produced by the TMS coil "click", which is known to contribute to the magnitude of the TMS-evoked potential, particularly at 100 and 180 milliseconds 55,67,68 when the TMS-evoked potential is also typically maximal. One method that has been shown to minimize the TMS auditory evoked potential is noise masking via the use of white or colored noise, with the addition of a thin layer of foam between the coil and scalp 10,55 . In the absence of such noise masking, differences in the TMS auditory-evoked potential could potentially contribute to differences in the evoked activity between subject groups (although not the observed differences in the evoked potentials between different sites within subjects.…”

“…NOTE: Another option would be to utilize earphones playing white noise or colored noise (with spectral features matching those of the TMS click) throughout the recording process, at a volume sufficient to mask the auditory click produced by TMS; this would have the added benefit of minimizing the potential confound of TMS-induced auditory evoked potentials 10,55 . Of note, a thin layer of foam between the coil and scalp is also necessary to minimizing the auditory evoked potential.…”