Investigating the role of beta and gamma tACS in visual processing and conscious perception

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. To capture different aspects of visual experience, we measured objective visibility: participants’ ability to correctly categorize the masked stimulus, and subjective visibility: participants’ self-report of whether they consciously perceived the stimulus. We expected that 40hz-tACS would affect both the objective visibility and subjective visibility. Moreover, we expected that 20 Hz-tACS would selectively affect the subjective visibility. Our results showed that target visibility was selectively compromised by 20 Hz-tACS but, in contrast to our hypothesis, this effect was specific to objective visibility. Although the power of local beta oscillations increased after 20 Hz-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, our findings suggest that 20 Hz tACS may modulate target visibility, indicating a potential relationship between beta-band activity 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.