Effects of stimulus size and contrast on binocular rivalry in adults with anisometropic amblyopia

The amblyopic eye, with reduced visual acuity, has limited ability to compete for perceptual dominance with the non-amblyopic fellow eye during binocular rivalry, likely due to diminished excitatory input or active central foveal suppression. This study investigated whether enhancing the visibility of the amblyopic eye, by increasing stimulus contrast or enlarging stimulus size, could help restore binocular rivalry in adults with anisometropic amblyopia. Using a standard binocular rivalry paradigm, we manipulated the contrast presented to the amblyopic eye and the size of rival stimuli. Results showed that the amblyopic eye exhibited little or no perceptual dominance at smaller stimulus sizes under an equal contrast condition, but dominance increased with larger stimulus sizes and increased contrast, while reducing fellow-eye dominance. However, overall perceptual dominance of the amblyopic eye remained substantially lower than that of the fellow eye and of normal-vision observers, even with enlarged stimuli or high contrast. We found no significant correlation between the fellow-eye dominance bias and the depth of amblyopia, stereoacuity, or interocular suppression in this cohort. Interestingly, individuals with amblyopia showed a much higher proportion of mixed perception compared to normal-vision observers, suggesting qualitatively altered interocular interactions in adults with anisometropic amblyopia. Nevertheless, these findings indicate that increasing the visibility of the amblyopic eye can partially restore its perceptual dominance, offering new insights into the mechanisms of suppression in amblyopia and suggesting potential directions for developing treatment strategies that target binocular vision.