Modulatory effect of melanopsin activation on contrast sensitivity and pupil response

Abstract

Melanopsin is a photopigment expressed in a small group of retinal ganglion cells called ipRGCs. This photopigment mediates non-image forming functions as circadian rhythms and pupillary responses. Involvement of melanopsin activation in human vision was reported, however, its function is not clear. In this study, we assessed melanopsin activation as a modulator of visual and pupillary responses.A Five-primary photostimulator, which allowed us to selectively stimulate each photoreceptor (Cao et al, J. Vis, 2015), produced temporal sinusoidal cone stimulation (L-, M- and S-cone excitations in phase, while keeping rods and melanopsin activation constant) at 1000 photopic td. We obtained cone contrast sensitivity (CS) from 1 to 16Hz using a staircase procedure and flicker pupillary responses (PR) at 1Hz and 3Hz (contrast = 9%) for two melanopsin activation background levels: Mel-Low (356.4 melanopic td) and Mel-High (447.5 melanopic td). Significant differences were found for CS at 3Hz with Mel-High producing higher CS than Mel-Low (p < 0.05). Pupillary responses for condition Mel-Low were delayed by 21ms from condition Mel-High this phase difference was only found at 3Hz (p < 0.05).Our results showed the modulatory effect of melanopsin activation on contrast sensitivity and pupil responses. An increment in melanopsin activation produced better visual sensitivity and faster pupil response at one specific temporal frequency. These findings suggest that melanopsin activation could modulate contrast detection and adaptation mechanisms, depending on the dynamics of the stimulation.