Binocular Summation in Healthy Presbyopic Adults: Contrast Sensitivity and Defocus Curves

Background/Objective: Binocular summation, the improvement in visual performance under binocular compared with monocular viewing, depends on the visual task and the degree of optical degradation of the retinal image. Its behavior across the full defocus curve in presbyopia remains poorly characterized. The purpose is to quantify binocular summation in healthy presbyopic adults by assessing contrast sensitivity (CS) at intermediate and near distances and by comparing monocular and binocular defocus curves to evaluate the effect of binocularity on depth of focus, the area under the curve (AUC), and the binocular summation ratio (BSR). Methods: Thirty healthy presbyopic participants (mean age 53.7 ± 5.7 years, range 45-64) were included in this observational, cross-sectional study. CS was measured with the Pelli-Robson chart at 1 m and 40 cm, and monocular and binocular defocus curves were obtained from -3.50 D to +1.50 D in 0.50 D steps. BSR, AUC and depth of focus at the 0.2 and 0.1 logMAR thresholds were calculated. Monocular and binocular values were compared with paired t-tests, applying a Bonferroni correction. Results: Binocular CS was significantly higher than monocular CS at both distances (adjusted p < 0.025), with a large, distance-independent BSR (1.68 at 1 m, 1.69 at 40 cm). BSR exceeded unity at every defocus level (10-21% gain), but after correction the binocular advantage in visual acuity remained significant only within the central range (-0.50 D to +1.00 D). At the participant level, binocularity increased the AUC at the 0.1 logMAR threshold (+50.5%, p = 0.037, d = 0.40), whereas the increase at the 0.2 logMAR threshold (+52.2%) did not reach statistical significance (p = 0.107). Conclusions: Binocularity consistently enhances visual performance in presbyopic adults, for contrast sensitivity and, with a pattern of significant improvement concentrated in the central defocus range, for visual acuity, supporting a more personalized evaluation of presbyopia.