TMS Adaptable Auditory Control: a universal tool to mask TMS click

Abstract

Abstract Coupling transcranial magnetic stimulation with electroencephalography (TMS-EEG) allows to non-invasively probe the human thalamocortical system. However, obtaining a pure TMS evoked potential requires controlling for several confoundings, such as auditory potentials evoked (AEPs) by the TMS 'click'. An effective control for AEPs consists of playing a noise through earphones in order to mask the TMS 'click' (masking noise).Here we tested TMS Adaptable Auditory Control (TAAC), a tool that generates in real-time a masking noise adapted to the stimulator and optimized to fit the ‘click’ perception. To this aim, we systematically measured the sound level required to mask the TMS ‘click’ with the state-of-the-art masking noise (i.e. white noise and adapted noise) and with two masking noises generated through TAAC played through in-ear earphones in a group of 19 healthy subjects. TMS pulses were delivered while gradually increasing the loudness until subjects were not able to identify the TMS 'click'. We iteratively performed this procedure for all four masking noises and during both a real TMS (2 cm behind the vertex) and a sham TMS condition (on the vertex, 90° sagittal rotation). Finally, we estimated the equivalent sound level for each masking noise as measured through a Head and Torso Simulator (HATS, Bruel&Kjaer, Type4128). Results show that all masking noises were effective at the following sound levels (Medians): White noise=87.1Db; Adapted noise=88.9Db; TAAC Noise1=84.1Db; TAAC Noise2=82.9Db. Thus, we demonstrate that the sound level required to mask TMS 'click' is safe for the human auditory system for the typical duration of TMS-EEG procedures with respect to standard safety tables (e.g. 90Db for 2 hours). Furthermore, TAAC can be used to mask TMS 'click' at lower sound levels compared to currently available masking noises. Keywords: TMS-EEG, Noise masking, Auditory evoked potential, TEP