Abstract
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.