Abstract
Interocular grouping occurs when two stimuli presented to two eyes form a coherent pattern during binocular rivalry. In the present study, we investigated whether gamma-band synchronization between the left and right visual occipital cortices facilitates interocular grouping during binocular rivalry. To induce interocular grouping in binocular rivalry, DĂaz-Caneja stimuli were dichotically presented (i.e., left-side concentric ring pattern and right-side horizontal square-pattern for left eye and vice versa for right eye). Because of the spatial configuration of the stimuli, interocular grouping (i.e., the percepts of the complete concentric rings and horizontal grating) can be experienced only when the visual information entering the left and right visual cortices are properly combined. Therefore, we hypothesized that inter-hemispheric synchronization between the left and right visual cortices would promote interocularly-grouped coherent percepts in binocular rivalry. Gamma frequency synchronization was specifically targeted based on the previous studies demonstrating close relationship between cortical gamma-band activity in the early visual cortex and collinear facilitation and perceptual grouping. In the gamma synchronization condition, participants’ percepts during binocular rivalry were tracked while the left and right visual areas corresponding O1 and O2 in the international 10-20 system were synchronized by 40 Hz transcranial alternating current stimulation (tACS). As a baseline condition, a sham stimulation was also applied to the same participants on a different day. As expected, the duration of interocularly-grouped coherent percepts was significantly increased while the inter-hemispheric synchronization was ongoing, compared to while such stimulation was not applied (pre-stimulation session). In the sham control condition, the durations of interocularly-grouped coherent percepts during the pre-stimulation and stimulation sessions were not significantly different. These results support the causal relationship between interhemispheric gamma-band synchronization and coherent conscious percept, implying that interocular grouping during binocular rivalry may be mediated by interhemispheric coherence of visual stimulus.