Abstract: Binocular rivalry is a phenomenon wherein the two eyes receive conflicting visual images, resulting in alternating perceptual dominance rather than image fusion. It offers a unique insight into the neural basis of visual consciousness, as the visual input remains constant despite fluctuations in subjective awareness. A fundamental question in binocular rivalry research concerns identifying the visual pathway along which the competition occurs. According to early theories, rivalry primarily involves monocular neurons in the primary visual cortex (V1) or the lateral geniculate nucleus via interocular inhibition mechanisms. However, electrophysiological studies in monkeys showed stronger correlations between neural activity in higher visual areas, e.g., the inferotemporal cortex, and perceptual states, indicating competition at object-level representations. Since rivalry occurs across multiple hierarchical levels through reciprocal interactions, the hybrid model sought to reconcile these findings. Nevertheless, a critical question remains unaddressed: How might different visual conditions alter the sites at which rivalry competition is most prevalent? Although neuroimaging studies hint at this possibility, there is currently no systematic causal evidence, and this prevents us from understanding whether the brain flexibly recruits different neural resources based on stimulus characteristics.
This study employed transcranial direct current stimulation (tDCS) to systematically examine the causal roles of four candidate brain regions, namely, the occipital cortex, the right parietal cortex, the left parietal cortex, and the prefrontal cortex, across two stimulus types representing different representational levels. Eight independent experiments were conducted based on a 2 × 4 design: two stimulus types (orthogonal gratings as local-feature stimuli vs. face/house images as object-level stimuli) × four brain regions. Each experiment used a within-subjects design with three tDCS conditions, namely, anodal stimulation, cathodal stimulation, and sham stimulation. A total of 160 participants were recruited across experiments (18~22 per experiment; mean age = 20.8 years; all right-handed with normal or corrected-to-normal vision). We used high-definition tDCS with a 4 × 1 electrode montage—comprising one central electrode and four return electrodes—positioned over target regions based on the international 10-10 system. Stimulation intensity (maximum 1.0 mA) was applied for 20 min during each session. Participants completed binocular rivalry tasks using a mirror stereoscope while continuously reporting their perceptual states via key presses. Two dependent variables were measured: eye dominance strength (reflecting competitive bias and relative gain) and perceptual alternation rate (reflecting destabilization and switching frequency).
The results revealed a robust stimulus-dependent pattern of regional involvement. For grating stimuli (Experiments 1~4), occipital tDCS significantly changed eye dominance strength, F(2, 38) = 7.64, p = 0.002, η²_p = 0.28, 1 - β > 0.99. Right parietal tDCS also changed eye dominance, F(2, 38) = 5.36, p = 0.009, η²_p = 0.22, 1 - β = 0.99, and influenced the alternation rate, F(2, 38) = 3.71, p = 0.034, η²_p = 0.16, 1 - β = 0.99. Neither left parietal nor prefrontal tDCS affected grating rivalry. In marked contrast, for face/house stimuli (Experiments 5~8), left parietal tDCS significantly changed eye dominance, F(2, 38) = 3.97, p = 0.027, η²_p = 0.17, 1 - β = 0.99. Prefrontal tDCS also altered eye dominance, F(2, 38) = 4.06, p = 0.025, η²_p = 0.18, 1 - β = 0.99. Notably, neither occipital nor right parietal stimulation affected face/house rivalry. This double dissociation indicates that simple grating rivalry primarily engages the occipital cortex (early interocular competition) and the right parietal cortex (spatial attention modulation), whereas object-level rivalry recruits the prefrontal cortex (cognitive control) and the left parietal cortex (object feature integration).
This study provides causal evidence for a stimulus-dependent, flexible configuration of rivalry sites. Instead of employing a fixed neural network, the brain flexibly allocates hierarchical neural resources based on the stimulus representational level. These findings advance the hybrid model from a descriptive multi-site framework to a predictive theory that specifies how stimulus properties determine the dominant competitive loci, thereby offering new insights into the flexible neural architecture underlying visual consciousness.

Key words: binocular rivalry, neural sites, tDCS, stimulus-dependent patterns, flexible configuration