Removal of Gross Artifacts of Transcranial Alternating Current Stimulation in Simultaneous EEG Monitoring

Abstract

Transcranial electrical stimulation is a widely used non-invasive brain stimulation approach. To date, EEG has been used to evaluate the effect of transcranial Direct Current Stimulation (tDCS) and transcranial Alternating Current Stimulation (tACS), but most studies have been limited to exploring changes in EEG before and after stimulation due to the presence of stimulation artifacts in the EEG data. This paper presents two different algorithms for removing the gross tACS artifact from simultaneous EEG recordings. These give different trade-offs in removal performance, in the amount of data required, and in their suitability for closed loop systems. Superposition of Moving Averages and Adaptive Filtering techniques are investigated, with significant emphasis on verification. We present head phantom testing results for controlled analysis, together with on-person EEG recordings in the time domain, frequency domain, and Event Related Potential (ERP) domain. The results show that EEG during tACS can be recovered free of large scale stimulation artifacts. Previous studies have not quantified the performance of the tACS artifact removal procedures, instead focusing on the removal of second order artifacts such as respiration related oscillations. We focus on the unresolved challenge of removing the first order stimulation artifact, presented with a new multi-stage validation strategy.

Theta Phase-dependent Modulation of Perception by Concurrent Transcranial Alternating Current Stimulation and Periodic Visual Stimulation

“…However, this has technical issues primarily because tACS induces complex EEG artefacts during stimulation. Very recent work proposes methods for removing these artefacts but this is subject to verification that the underlying EEG signal is not distorted (Kohli & Casson, 2019).…”

Transient amplitude modulation of alpha-band oscillations by short-time intermittent closed-loop tACS

“…In principle, the signal to be analyzed and modulated (e.g., a signature of alpha oscillations measured in the EEG) is overwritten by the tACS signal, which is several magnitudes larger but covers the same spatial and temporal space. Thus, substantial efforts in recent studies have been directed to the development of artifact elimination methods using different techniques and experimental protocols (Witkowski et al, 2015;Kohli & Casson, 2019;Kasten et al, 2018;Noury & Siegel, 2017). However, intermittent stimulation allows one to follow another approach while still based on adaptive principles.…”

“…Thirdly, the main technical challenge for analyzing and investigating online effects during stimulation is the induction of stimulation artifacts, which exceed the measured neural signals by several magnitudes. Although there is an increasing number of propositions for overcoming this challenge and some prominent solutions (Witkowski et al, 2015;Kohli & Casson, 2019;Kasten et al, 2018;Noury & Siegel, 2017) directed to elimination of artifacts, such approaches still require sophisticated experimental designs and/or heavy computational procedures. Most human tACS studies have used stimulation protocols with a duration applied in the range of minutes.…”

Electrical properties, accuracy, and multi-day performance of gelatine phantoms for electrophysiology

“…Cuboid shaped phantoms were used in [5] to verify the performance of 3D printed EEG electrodes, and to demonstrate how the electrode performance varied with contact force. Some research groups have even applied transcranial Electrical Stimulation (tES) to these phantoms to investigate the tES artefact removal problem from EEG where a known EEG signal can now be provided [16], [17]. [18] suggested agar as an alternative to gelatine for the recording of slow EEG potentials, while recently [19] made use of an agarose gel swollen with a saline solution.…”

“…The result is that a wide number of different materials have been investigated for making electrophysiology body phantoms (gelatine [2], [5], [13], [15], [16], wax [28], silicon [28], [14]. Image taken from the public domain [20].…”