Gamma-band synchronization in visual cortex induced by tACS promotes contour integration

The Rhythm Theory of cognition posits that various human perceptions and cognitions are mediated by the oscillatory activity of the human brain. Particularly, it has been demonstrated that gamma-band oscillations are involved in the integration of visual information, playing a significant role in object perception. Recently, we demonstrated that interocular grouping during binocular rivalry was enhanced by inter-hemispheric gamma-band synchronization induced by transcranial alternating current stimulation (tACS) (Yoon & Hong, 2023). This previous result suggests that contour integration across visual hemifields may be mediated by synchronized neural oscillation in the gamma-band frequency. To directly test this hypothesis, we investigated whether gamma-band synchronization between the left and right visual occipital cortices facilitates the detection of a colinear line composed of three short bars. The target line was presented within a square window (6 by 6 visual angle) where 397 distractor bars with random orientations were simultaneously presented. The location of the target was chosen from one of eight predefined positions along an imaginary circle (2.5-degree radius). Participants were instructed to report the orientation of the target line (left-tilted or right-tilted from the vertical orientation) as accurately and quickly as possible. Gamma-band synchronization between hemispheres was induced by applying 40 Hz tACS to the left and right visual areas corresponding to O1 and O2 in the international 10-20 system. The data revealed that the response time to detect the target line presented across two hemifields indeed decreased during stimulation and post-stimulation compared to the response times measured before stimulation (baseline). In the sham control condition, the response times for detecting the target line were not significantly different between pre-stimulation and post-stimulation. We also observed decreased response times for the within-hemifield target locations after gamma-band tACS. Together, our results indicate the causal role of gamma-band oscillation in contour integration within and between hemispheres.