Binocular summation of pupil light reflex with different light stimulation in each eye

Binocular summation of the pupil light reflex refers to the phenomenon where the pupillary constriction elicited by binocular light stimulation is greater than the expected response based on monocular stimulation. An increase in the total flux reaching the retina influences pupil constriction. Under binocular conditions, the combined light entering both eyes is greater than that under monocular conditions, which can affect the pupil's response. However, previous research indicates that equal luminance presented either to one eye or distributed equally between the two eyes cannot adequately explain the effect of binocular summation. This study aimed to investigate how pupil constriction differs when the total illuminance stimulating the eyes remains constant, but the distribution of illuminance between the left and right eyes varies. Nine participants viewed a flickering target stimulus at 1 Hz. We compared the average pupillary constriction across various interocular illuminance ratios, including a monocular condition (only one eye exposed to the total illuminance) and binocular conditions where the total illuminance was divided unequally or equally between the two eyes. The results showed that when the total illuminance stimulating the eyes was constant, pupil constriction was significantly stronger as the illuminance distribution between the eyes approached equality (i.e., a ratio closer to one), with binocular conditions generally causing stronger average constriction than monocular conditions. Furthermore, analysis of the power spectrum density at 1 Hz revealed no significant difference between the tested illuminance conditions, suggesting that the phasic response was similar despite differences in tonic constriction.