Signal-space projection suppresses the tACS artifact in EEG recordings

Vosskuhl, Johannes; Mutanen, Tuomas P.; Neuling, Toralf; Ilmoniemi, Risto J.; Herrmann, Christoph S. · 2019 · Crossref

DOI: 10.1101/823153

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Summary

This study addresses the critical challenge of removing transcranial alternating current stimulation (tACS) artifacts from concurrent electroencephalography (EEG) recordings. While tACS is a valuable tool for modulating brain oscillations, the resulting electrical artifact typically overwhelms neural signals, forcing researchers to rely on pre- and post-stimulation comparisons rather than analyzing online effects. The authors aimed to evaluate and compare the feasibility of three artifact correction techniques—sine subtraction, template subtraction, and signal-space projection (SSP)—to determine which method best preserves underlying brain activity while suppressing the tACS artifact. The research employed a two-part experimental design. First, a phantom head study provided full control over signal sources. A spherical, three-compartment phantom mimicking human head conductivity properties contained a dipolar current source driven by pre-recorded human EEG signals. Simultaneous tACS (10 Hz, 50 and 150 µA) was applied to the phantom’s surface. This setup allowed for precise quantification of artifact removal and signal preservation by comparing cleaned data against a known baseline. Second, a human study validated the best-performing method. Participants performed a mental rotation task, known to induce event-related desynchronization (ERD) in alpha oscillations, while receiving concurrent tACS (500 and 1000 µA). The success of artifact correction was assessed by the ability to recover this expected ERD. In the phantom experiment, SSP significantly outperformed sine subtraction and template subtraction. Sine fitting resulted in substantial overcorrection and poor recovery of temporal and spatial information, particularly in the 5–15 Hz range. Template subtraction performed moderately but showed higher variability across channels. SSP, combined with source-informed reconstruction to minimize spatial distortions, achieved the highest correlation with baseline data and the most accurate recovery of amplitude and phase spectra. Statistical analyses confirmed that SSP preserved the individual alpha frequency amplitude without significant distortion, whereas other methods introduced significant errors. In the human experiment, SSP successfully suppressed the tACS artifact, allowing for the clear recovery of the alpha ERD associated with the mental rotation task. The findings demonstrate that signal-space projection is a feasible and superior method for correcting tACS artifacts in EEG data. By enabling the reliable recovery of neural oscillations during stimulation, this approach allows for the direct analysis of online tACS effects, overcoming a major limitation in the field. This advancement supports more rigorous investigations into the causal role of brain oscillations by permitting simultaneous stimulation and recording without the confounding influence of massive electrical artifacts.

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