Abstract
BackgroundTo probe the functional role of brain oscillations, transcranial alternating current stimulation (tACS) has proven to be a useful neuroscientific tool. Because of the excessive tACS-caused artifact at the stimulation frequency in electroencephalography (EEG) signals, tACS + EEG studies have been mostly limited to compare brain activity between recordings before and after concurrent tACS. Critically, attempts to suppress the artifact in the data cannot assure that the entire artifact is removed while brain activity is preserved. The current study aims to evaluate the feasibility of specific artifact correction techniques to clean tACS-contaminated EEG data.New MethodIn the first experiment, we used a phantom head to have full control over the signal to be analyzed. Driving pre-recorded human brain-oscillation signals through a dipolar current source within the phantom, we simultaneously applied tACS and compared the performance of different artifact-correction techniques: sine subtraction, template subtraction, and signal-space projection (SSP). In the second experiment, we combined tACS and EEG on one human subject to demonstrate the best-performing data-correction approach in a proof of principle.ResultsThe tACS artifact was highly attenuated by SSP in the phantom and the human EEG; thus, we were able to recover the amplitude and phase of the oscillatory activity. In the human experiment, event-related desynchronization could be restored after correcting the artifact.Comparison With Existing MethodsThe best results were achieved with SSP, which outperformed sine subtraction and template subtraction.ConclusionOur results demonstrate the feasibility of SSP by applying it to a phantom measurement with pre-recorded signal and one human tACS + EEG dataset. For a full validation of SSP, more data are needed.
Highlights
The goal of transcranial alternating current stimulation is often the modulation of oscillatory brain activity and the concurrent demonstration of behavioral consequences of the intervention (Herrmann et al, 2013)
The amplitude spectra were in the range of the signal of interest (Figure 3); seemingly at the expense of different degrees of overcorrection, which means that non-artifact activity had been removed
signal-space projection (SSP) was the superior method in recovering the temporal information of the baseline signal both at the stimulation frequency as well as at individual alpha frequency (IAF), whereas template subtraction and sine fitting poorly recovered the baseline signal in a number of channels (Figure 5, left)
Summary
The goal of transcranial alternating current stimulation (tACS) is often the modulation of oscillatory brain activity and the concurrent demonstration of behavioral consequences of the intervention (Herrmann et al, 2013). Most studies combining tACS with electroencephalography (EEG) have demonstrated effects on oscillatory brain activity only by comparing the EEG before and after tACS (Zaehle et al, 2010; Vossen et al, 2015; Kasten and Herrmann, 2017), because EEG data recorded during stimulation are contaminated by an immense tACS-generated artifact at the stimulation frequency which exceeds the range of physiological EEG signals by several orders of magnitude. It is possible to analyze the EEG spectrum outside the tACS frequency, by applying adequate bandpass filters to the stimulated frequency band It is important, to measure the neuronal activity at the stimulation frequency, because the changes at the stimulated frequency are expected during successful entrainment (Thut et al, 2011; Kasten et al, 2018). The current study aims to evaluate the feasibility of specific artifact correction techniques to clean tACS-contaminated EEG data
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