Saccadic Reaction Times and Computational Modelling Reveal Heterogeneous Binocular Summation in Glaucomatous Visual...

Binocular summation enhances visual performance. This enhancement may be disrupted in visual conditions that introduce interocular asymmetries, e.g., glaucoma. Reaction times provide a direct index of visual processing efficiency. However, they have not been studied systematically under binocular conditions in glaucoma. This investigation quantified monocular and binocular saccadic reaction times (RTs) across the visual field using eye-movement perimetry (EMP), compared binocular summation between healthy and glaucoma participants and evaluated correspondence with computational models. Saccadic RTs were measured monocularly/binocularly at 54/56 visual-field locations in seven healthy participants and eight participants with glaucoma of varying severity. Each location was tested with six to ten repetitions under low (74%) and high (155%) contrast conditions. Data were analysed using reciprobit plots, estimation statistics and linear mixed-effects modelling. Binocular RTs were compared with statistical facilitation (Race model) and variance-weighted neural integration predictions. Healthy participants consistently demonstrated a binocular advantage, with RTs faster by ~20 ms relative to monocular viewing, closely matching statistical facilitation predictions. Glaucoma participants showed greater heterogeneity, ranging from preserved summation (~40 ms faster than monocular) to binocular RT being slower than the fastest monocular RT. Computational modelling indicated that most glaucoma cases were compatible with statistical facilitation, but some exhibited deviations suggestive of inhibitory interactions, monocular dominance or faster responses than predicted by statistical facilitation. Binocular RTs provide a sensitive index of visual processing and reveal heterogeneous binocular summation mechanisms in glaucoma. EMP under binocular conditions offers a promising approach for studying functional visual impairment beyond traditional monocular testing.