Contextual Binocular Imbalance Impairs Local Stereopsis.

Purpose: Binocular imbalance is known to inhibit stereopsis. This study investigates whether an imbalanced context around stereo stimuli also affects local stereopsis and explores the underlying mechanisms. Methods: Three experiments were conducted with normally sighted participants. Experiment 1 measured local stereo detection thresholds under three context conditions: binocular balance (0.5 vs. 0.5 contrast), left-eye dominance (0.8 vs. 0.2 contrast), and right-eye dominance (0.2 vs. 0.8 contrast). Experiment 2 assessed the modulation of the imbalance effect by context-target collinearity. Experiment 3 examined the imbalance effect with binocular fusion and rivalry context stimuli. Results: In experiment 1, the average stereo threshold was 62.4 arcsec in the binocular balance condition, elevated to 111.4 arcsec in the left-eye dominance (P = 0.003), and 114.7 arcsec in the right-eye dominance (P < 0.001), with no significant difference between the two imbalance conditions (P = 0.650). Experiment 2 showed that context-target collinearity modulated the imbalance effect, resulting in a smaller threshold elevation in the non-collinear condition (P = 0.011). Experiment 3 revealed significant main effects of imbalance (P = 0.031) and rivalry (P = 0.004), with no significant interaction (P = 0.966). Conclusions: Contextual binocular imbalance inhibits local stereopsis, an effect modulated by collinearity and similarly observed in both binocular integrative and suppressive contexts. These findings suggest that lateral connectivity in the primary visual cortex (V1) plays a fundamental role in stereopsis generation, offering novel approaches for clinical interventions aimed at restoring binocular balance and stereopsis.