Facilitated Event-Related Power-Modulations during Transcranial Alternating Current Stimulation (tACS) Revealed by Concurrent tACS-MEG

Abstract

Non-invasive approaches to modulate oscillatory activity in the brain are increasingly popular in the scientific community. Transcranial alternating current stimulation (tACS) has been shown to modulate neural oscillations in a frequency specific manner. However, due to a massive stimulation artifact at the targeted frequency, little is known about effects of tACS during stimulation. It remains unclear how the continuous application of tACS affects event-related oscillations during cognitive tasks. Depending on whether tACS influences pre- or post-stimulus oscillations, or both, the endogenous, event-related oscillatory dynamics could be pushed in various directions or not at all. A better understanding of these effects is crucial to plan, predict and understand outcomes of solely behavioral tACS experiments. In the present study, a recently proposed procedure to suppress tACS artifacts by projecting MEG data into source-space using spatial filtering was utilized to recover event-related power modulations in the alpha-band during a mental rotation task. MEG data of twenty-five human subjects was continuously recorded. After 10 a minute baseline measurement, participants received either 20 minutes of tACS at their individual alpha frequency, or sham stimulation. Another 40 minutes of MEG data were acquired thereafter. Data were projected into source-space and carefully examined for residual artifacts. Results revealed strong facilitation of event-related power modulations in the alpha-band during tACS application. These results provide first direct evidence, that tACS does not counteract top-down suppression of intrinsic oscillations, but rather enhances pre-existent power modulations within the range of the individual alpha (=stimulation) frequency.

Significance Statement Transcranial alternating current stimulation (tACS) is increasingly used in cognitive neuroscience to study the causal role of brain oscillations for cognition. However, online effects of tACS largely remain a ‘black box’ due to an intense electromagnetic artifact encountered during stimulation. The current study is the first to employ a spatial filtering approach to recover, and systematically study, event-related oscillatory dynamics during tACS, which could potentially be altered in various directions. TACS facilitated pre-existing patterns of oscillatory dynamics during the employed mental rotation task, but did not counteract or overwrite them. In addition, control analyses and a measure to quantify tACS artifact suppression are provided that can enrich future studies investigating tACS online effects.