Reliable assessment of evoked brain responses targeted by transcranial alternating current stimulation (tACS) using optically pumped magnetometers (OPMs)

Abstract Background: Millisecond-precise targeting of ongoing brain oscillations with electric stimulation is an important goal to uncover brain-behavior relationships or to treat neurological and psychiatric disorders related to abnormal brain activity. The main challenge to implement such a paradigm is to reliably assess ongoing brain activity despite stimulation artifacts that often exceed physiological signals by many orders of magnitude. Recently, we have introduced stimulation artifact source separation (SASS) to recover brain electric signals using electroencephalography (EEG) during transcranial alternating current stimulation (tACS). Due to volume conduction and susceptibility to muscle artifacts, EEG remains limited to establish millisecond- and millimeter-precise closed-loop or adaptive brain stimulation. Here we tested the feasibility of using optically pumped magnetometers (OPMs) to assess visually evoked brain responses targeted by tACS. Methods: Visually-evoked responses related to flickering visual stimuli with random intertrial intervals were assessed using 16 OPM sensors (FieldLine Inc., Boulder, USA) fitted into a custom 3D-printed array. tACS stimulation pads were placed over occipital brain regions and stimulation signals applied at random phase angles relative to the visual stimuli. Brain-evoked responses were recovered using SASS. Results: We found that (1) tACS-related stimulation artifacts remain well within the dynamic range of the OPM sensors, (2) visually evoked brain responses could be successfully recovered, allowing for real-time closed-loop stimulation paradigms. Conclusion: We found that combining OPMs and tACS is feasible and may pave the way for millisecond- and millimeter-precised closed-loop neuromodulation. Research Category and Technology and Methods Translational Research: 8. Transcranial Alternating Current Stimulation (tACS) Keywords: Source, Artifact, MEG, Optically pumped magnetometer