The effect of pupil size on binocular summation at suprathreshold conditions.

PURPOSE To determine the influence of retinal illuminance on monocular or binocular visual reaction time (VRT). METHODS On two normal subjects, uniform circular stimuli were presented with respect to a reference stimulus at the fovea under suprathreshold conditions, and the detection of positive and negative luminance variations was recorded. Binocular and monocular reaction times were measured (by the index finger pressing on a mouse key) for viewing with both natural and artificial pupils. RESULTS Binocular reaction times were shorter than monocular reaction times; nevertheless, this binocular-summation effect was less marked in trials with the artificial pupil. Analyses of binocular-summation ratios for contrast changes for both pupil types indicated maximum and minimum binocular-saturation values depending on contrast variations in both positive and negative luminance changes. CONCLUSIONS Binocular summation can be influenced by pupil size under suprathreshold conditions. Results are discussed in terms of retinal illuminance and cortical pupil response mechanisms.