Masked target visibility is selectively impaired by 20 Hz transcranial alternating current stimulation

It has been proposed that both conscious and unconscious perception are associated with a feedforward sweep of oscillatory activity in the gamma band (>40 Hz), while conscious perception also requires recurrent feedback via beta band (~20 Hz) oscillations. To investigate the causal relationship between these oscillations and (un)conscious visual perception, we assessed the effect of transcranial alternating current stimulation (tACS) in the gamma (40 Hz) and beta (20 Hz) bands on the objective and subjective visibility of targets in a metacontrast backward masking task. We expected that 40hz-tACS would affect both the correct categorization of the target (i.e. objective visibility) and the reports of conscious perception (i.e. subjective visibility). Moreover, we expected that 20Hz-tACS would selectively affect the subjective visibility. Our results showed that target visibility was selectively compromised by 20Hz-tACS but, in contrast to our hypothesis, this effect was specific to objective visibility. Although the power of local beta oscillations increased after 20Hz-tACS, inter-areal beta synchrony could have nevertheless been impaired, a possibility that should be investigated in the future by means of source reconstructed high density electroencephalography recordings. In summary, we provided evidence supporting that 20Hz-tACS is capable of modulating target visibility, suggesting a possible a causal link between synchrony in this frequency band and visual perception. Future studies could build upon this result by investigating other forms of stimulation and other model organisms, further contributing to our knowledge of how conscious access causally depends on brain oscillations.