Significant Age-Related Visual Declines but Preserved Binocular Summation of Contrast Sensitivity and Visual Acuity Across Adulthood.

Purpose: Binocular summation refers to the enhanced visual performance when using two eyes instead of one. Although well established in young adults, how binocular summation and its underlying mechanisms change with normal aging remains unclear. This study aimed to characterize age-related changes in binocular summation for contrast sensitivity (CS) and high-contrast visual acuity (VA) across adulthood and evaluate the mechanisms underlying this integration. Methods: We measured monocular and binocular CS and VA using the Pelli-Robson and ETDRS charts in 292 normally-sighted individuals aged 18 to 87, grouped into seven age bins. Binocular summation ratio (BSR) was computed as the ratio of binocular to better-eye sensitivity. To examine the underlying mechanisms, we fit and compared binocular summation models to the BSR data for both VA and CS. Results: Across adulthood, both binocular CS and VA declined substantially, by approximately 37% and 59%, respectively, whereas interocular differences remained modest (<15%) and stable throughout adulthood. Binocular summation was significant for both CS (∼47% gain) and VA (∼6% gain) without any significant age-related changes. Model comparisons revealed that CS summation followed a near-quadratic summation model (m = 1.85), whereas high-contrast VA summation was better described by a weighed linear model approximating signal averaging between two eyes (w1 = 1.76, w2 = 1.86), suggesting district underlying mechanisms. Conclusions: Despite significant age-related visual decline, binocular summation for both CS and VA remains robust across adulthood. These findings suggest that the cortical mechanisms supporting binocular integration are largely preserved with normal aging and provide clinically meaningful benchmarks for assessing binocular vision throughout adulthood.