Abstract
Electroencephalographic (EEG) signals evoked by transcranial magnetic stimulation (TMS) are usually recorded with passive electrodes (PE). Active electrode (AE) systems have recently become widely available; compared to PE, they allow for easier electrode preparation and a higher-quality signal, due to the preamplification at the electrode stage, which reduces electrical line noise. The performance between the AE and PE can differ, especially with fast EEG voltage changes, which can easily occur with TMS-EEG; however, a systematic comparison in the TMS-EEG setting has not been made. Therefore, we recorded TMS-evoked EEG potentials (TEPs) in a group of healthy subjects in two sessions, one using PE and the other using AE. We stimulated the left primary motor cortex and right medial prefrontal cortex and used two different approaches to remove early TMS artefacts, Independent Component Analysis and Signal Space Projection—Source Informed Recovery. We assessed statistical differences in amplitude and topography of TEPs, and their similarity, by means of the concordance correlation coefficient (CCC). We also tested the capability of each system to approximate the final TEP waveform with a reduced number of trials. The results showed that TEPs recorded with AE and PE do not differ in amplitude and topography, and only few electrodes showed a lower-than-expected CCC between the two methods of amplification. We conclude that AE are a viable solution for TMS-EEG recording.
Highlights
The concurrent use of transcranial magnetic stimulation and electroencephalography (TMS-EEG) has recently emerged as a powerful and widely used non-invasive stimulation method to investigate human brain function [1,2,3]
This work suggested that Active electrode (AE) offer worse performance when fast voltage changes are involved; we reasoned that this might have represented an issue in the TMS-EEG setting, due to the high frequency content of the cortical signal, especially in the first 70 ms [7]
The present results allow to conclude that AE can be used in the TMS-EEG setting with outcomes very similar to passive electrodes (PE)
Summary
The concurrent use of transcranial magnetic stimulation and electroencephalography (TMS-EEG) has recently emerged as a powerful and widely used non-invasive stimulation method to investigate human brain function [1,2,3]. The mentioned capability of AE to provide clean signals with suboptimal impedance levels seems to be limited when the EEG exhibit sudden voltage changes, such as with ERPs; this may be due to a slower slew rate (i.e., the capability of an amplifier to quickly and accurately follow changes in voltage) at the preamplification stage. This issue might be relevant when recording TMS-EEG signals, due to the rapidly changing temporal profile of TMS-induced EEG responses and artefacts [9,10]. These findings warrant a formal comparison between PE and AE in the TMS-EEG setting
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