Targeting alpha oscillations using closed-loop transcranial alternating current stimulation

Abstract Background: Transcranial alternating current stimulation (tACS) can modulate human brain oscillations noninvasively. However, tACS effects are variable and inconsistent across studies, possibly because the stimulation signal is not phase-tuned to targeted brain oscillations. Such tuning of the tACS signal is typically impeded by large stimulation artifacts, rendering closed-loop operation of tACS challenging. Objective: The objective of this study was to establish closed-loop tACS (CL-tACS) for phase-tuned targeting of occipital alpha oscillations. Methods: We used stimulation artifact source separation (SASS), a recently developed real-time compatible artifact rejection algorithm for the electroencephalogram (EEG), to recover phase information of occipital alpha oscillations during amplitude-modulated tACS (AM-tACS). AM-tACS was tuned to occipital alpha oscillations at six equidistant phase angles. To allow for offline validation of the approach, visual stimuli presented at random inter-stimulus intervals were used to generate cortical evoked responses exhibiting known inter-trial phase coherence (ITC). Results: Without SASS, the envelope of AM-tACS was phase-locked to the electrical artifact in parietooccipital EEG signals at 0° or 180° (p < 0.001), and cortical evoked responses could not be recovered. Using SASS, tuning of AM-tACS to occipital alpha oscillations could be demonstrated (p < 0.001), and ITC of cortical evoked responses could be recovered (p < 0.001). Conclusions: CL-tACS using amplitude-modulated waveforms is feasible and allows for phase-tuned targeting of ongoing brain activity such as occipital alpha oscillations. Research Category and Technology and Methods Translational Research: 8. Transcranial Alternating Current Stimulation (tACS) Keywords: closed-loop, tACS, oscillations, electroencephalography