Senescent effects on binocular summation for contrast sensitivity and spatial interval acuity

Purpose. Age effects on binocular summation, a cortically mediated visual function, were compared for resolution acuity, contrast sensitivity (CS), and spatial interval (SI) hyperacuity. Method. The binocular and monocular thresholds of healthy, optimally-corrected healthy young (mean age 21.3 years) and old (mean age 69.9 years) observers were determined for acuity, Sl discrimination, and CS at 1.0, 2.0, 4.0, 8.0 and 18.0 c/deg. Results. No age effects were observed on monocular or binocular Sl discrimination. The binocular summation ratios (BSRs) did not exceed the expected probability gain for resolution acuity or SI discrimination in either age group. Older observers showed a binocular inhibition effect on the SI task. On the CS task, the BSRs of the young significantly exceeded those of the old only at 18.0 c/deg. Conclusions. 1.) Binocular summation of high spatial frequency contrast information, which may be less robust in the senescent visual system, did not appear to be related to interocular CS differences, 2.) Sl discrimination of high contrast, well-separated targets appears to be unaffected by aging, and 3.) Binocular neural summation on SI discrimination may be more likely to be seen with targets that are narrowly separated or low in contrast.