An artifact-robust framework for measuring tES effects during stimulation.

Objective.Transcranial electric stimulation (tES) is a promising technique to non-invasively modulate human brain activity. However, stimulation artifacts in EEG and MEG recordings severely hinder the study of its online effects.Approach.Here, we introduce a new approach to account for these artifacts. The approach rests on two key ideas. First, we focus on interactions of tES with intrinsic brain activity, which are absent for tES artifacts. Second, rather than removing artifacts, we compare composite signals that share similar artifacts but potentially differ in the interaction of tES with intrinsic brain activity. We follow a simple logic: if tES does not interact with intrinsic brain activity, then neural activity during simultaneous sensory stimulation and tES should equal the linear superposition of the neural effects of each applied alone. Any deviation from this prediction provides evidence for a neural interaction.Main results.We tested this approach in a proof-of-principle MEG study, applying 10 Hz transcranial alternating current stimulation (tACS) during rest and during a 10 Hz visual flicker. We compared neural activity during simultaneous stimulation with that predicted by the linear superposition of flicker and tACS alone and found a phase-dependent interaction between tACS and flicker-evoked brain activity.Significance.Our work establishes a novel approach to investigate online effects of tES and suggests a state-dependent interaction of tACS with human brain activity.