40 Hz Steady-State Visually Evoked Potentials Recovered During Oscillating Transcranial Electrical Stimulation

Abstract Objective Transcranial Electrical Stimulation and Visual Stimulation in the gamma band (30-100 Hz, especially 40 Hz) are increasingly used to study and even enhance human cognition. Combining both techniques would be of scientific and clinical value, provided that Steady-State Visually Evoked Potentials (SSVEPs) are measured concurrently to determine any neuronal effects of the electrical brain stimulation. This poses a substantial methodological challenge. We aimed to demonstrate that recording visually evoked 40 Hz activity with EEG during electrical brain stimulation is possible and to explore potential interactions. Approach We tested if electrical and visual stimulation might interact depending on which brain areas are electrically stimulated (Experiment 1; N=25) and how closely the respective frequencies match (Experiment 2; N=25). Experiment 3 (N=25) assessed how effectively the data processing pipeline can mitigate electrical artefacts and recover real evoked neuronal activity. SSVEPs were processed and analysed in the time domain using an optimised adaptive template subtraction approach. Main Results 40 Hz SSVEPs were successfully recorded during frequency-matched electrical stimulation applied between central and occipital regions. Waveform correlations revealed that SSVEPs from combined visual and electrical stimulation were more similar in shape to baseline SSVEPs from visual stimulation alone than to control data from electrical stimulation alone. Accordingly, during combined stimulation, the recovered signals were stronger in amplitude than the electrical control data. We found no evidence of interactions between electrical and visual stimulation. Significance We demonstrated that 40 Hz SSVEPs can be reliably measured with EEG during frequency-matched electrical brain stimulation, distinguishing neuronal activity from electrical or physiological confounds. This method enables fundamental and clinical researchers to combine rhythmic sensory and electrical stimulation in the gamma band and concurrently quantify neuronal electrophysiological effects. - Transcranial Electrical Stimulation - Steady-State Visually Evoked Potential (SSVEP) - Electroencephalography (EEG) - gamma - flicker - multimodal - Non-Invasive Brain Stimulation Competing Interest Statement LH, MS, PT, and JD report no conflicts of interest. JP received consulting fees from Uniqure, has a patent on Epo variants, is on the advisory board of the Sinapps2 study, and is a board member of the following societies: DGPPN, DGGPP, DZPG, DZNE. Footnotes (manuel.spitschan{at}tum.de) (josef.priller{at}tum.de) (paul.taylor{at}psy.lmu.de) (james.dowsett{at}stir.ac.uk) Upon reinspection of the data from experiments 1 and 2, optimisating the artefact removal pipeline and objectively quantifying its performance was demeed necessary. Therefore, we conducted a third experiment. While the original results from experiments 1 and 2 had to be revised, experiment 3 substantiated the methodological advance presented in our updated manuscript. Abbreviations - EEG - Electroencephalography - SSVEP - Steady-State Visually Evoked Potential - TES - Transcranial Electric Stimulation - VS - Visual Stimulation