The tACS Challenge: Test of Rhythmic Modulation of Visual Perception by Occipital Stimulation

Transcranial alternating current stimulation (tACS) is a non-invasive technique to modulate brain rhythms and test causal links between neural oscillations and cognition. However, conflicting findings and methodological limitations—including small sample and confounding peripheral stimulation—have raised concerns regarding its reliability. A multi-centre, pre-registered study in which we participate (https://tacschallenge.github.io) aim at testing whether 10 Hz occipital tACS induces phasic modulation of visual perception by directly affecting cortical excitability. A total of 880 participants from 45 laboratories will perform a visual detection task, identifying brief brightness changes in the lower peri-foveal visual fields, while receiving stimulation under three active conditions—occipital tACS, retinal control, and cutaneous control—and one passive sham condition. The two active control conditions are designed to mimic peripheral stimulation of the retina and skin without direct brain stimulation, thus enabling a clear differentiation between cortical and confounding effects. A confirmatory result would show that occipital tACS rhythmically modulates perception, with either stronger effects or different phase preferences than those observed in the control conditions. Phasic effects will be assessed via regression models on single-trial data using EEG-recorded stimulation phase. Hypotheses are evaluated through frequentist and Bayesian frameworks. Additionally, analyses examine individual alpha frequency (IAF) as a moderator of entrainment, testing whether the absolute difference between IAF and 10 Hz predicts stronger effects. With power to detect effects as small as d = 0.11, this large-scale study will evaluate whether conventional-intensity 10 Hz tACS entrains cortical alpha activity and modulates perception, with implications for current theoretical models.