Identifying the ipRGC scotoma through the pupillary light reflex

Abstract

The pupil light reflex (PLR) is mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs), therefore topographical characteristics of ipRGCs might impact the PLR. The purpose of this study was to identify the ipRGC scotoma located in the central fovea through pupillary measurements. We measured the PLR to red and blue stimuli in young participants without ocular diseases. The stimulus conditions consisted of rings with an outer diameter of 3° and an inner diameter that was larger than (thin ring), similar to (middle ring) or smaller than (wide ring) the average physiological human ipRGC scotoma (~1.47°); an entire 3° disk (full), and a central small (0.8°) disk within the thin ring (thin+disk). We recorded the pupil size one second before (baseline), during (one second of stimulation), and five seconds after (post-illumination pupil response, PIPR) the stimulus presentation. Each stimulus condition was repeated 30 times grouped in six runs. Trials were removed from the analysis if blinks occurred or if the gaze deviated (>0.5°) from the fixation cross.When pupil recordings were normalized to the baseline, the pupillary constriction during the stimulation phase followed the area value of the stimulus conditions for both the blue and the red stimuli. To analyze the dynamics of the time course in the PIPR phase, we normalized the pupil data to the constriction peak. The PIPR time courses clustered in two groups (full, wide ring, and middle ring) and (thin ring and thin+disk), which matches the effect of the ipRGC scotoma. Interestingly, these clusters were revealed after an additive normalization for the blue stimulus, and after a multiplicative normalization for the red stimulus.Our results suggest that the ipRGC scotoma can be unveiled by the PLR measurements, and the PLR is mediated by normalization mechanisms potentially affecting signals from ipRGCs and cones in a different fashion.