Enhanced Pupil Contriction Elicited by 6 Hz Flickering Light in Human Participants: Evidence for Cholinergic Pathway for ipRGC Stimulation

Abstract

Purpose : Melanopsin-expressing ipRGCs play a key role in the neural circuit that drives the pupillary light reflex. Prior in vitro pharmacological data from rat ipRGCs indicate these photoreceptors are stimulated by cholinergic (muscarinic) agonists. It has been demonstrated that flickering light, especially 6 Hz, evokes acetylcholine release from starburst amacrine cells. To test the hypothesis that a cholinergic pathway for ipRGC stimulation occurs in humans, our aim was to determine whether 6 Hz flickering light evokes greater pupil constriction compared to 0 and 30 Hz stimuli and, if so, evaluate whether topical application of muscarinic antagonists diminishes the effect.

Methods : Healthy participants (n=7) received 5 min duration light stimuli of 3 flicker frequencies (0, 6 and 30 Hz), with each frequency tested at 2 irradiances (10¹², 10¹⁰ photons/s/cm²) and at 2 wavelengths (480, 620 nm). Light was delivered to the dilated left eye while the consensual pupil response was measured in the right eye. In subsequent experiments (n=3), pupil constriction to red (620 nm; 10¹² photons/s/cm²) light at 6 and 30 Hz, each delivered for 5 minutes, was assessed at 2 and 7.5 hrs after the topical administration of 0.5% tropicamide, 0.05% atropine or no drops.

Results : The mean pupil size during the 5 min exposure to either 480 or 620 nm light was significantly (P<0.01) more constricted at 6 Hz, relative to 30 Hz, at 10¹² photons/s/cm², but the effect was not observed (P>0.05) for the dimmer (10¹⁰ photons/s/cm²) red and blue lights. These results indicate the effect occurs at photopic irradiances dimmer than that required for melanopsin photoreception. In the control (no drops) condition, the pupil size difference in final min relative to first min was significantly (P<0.05) greater with 6 Hz stimuli (0.69±0.05 mm) compared to 30 Hz (0.17±0.23 mm). This effect was diminished (P>0.05) 7 hrs after atropine application (6 Hz: 0.29±0.09 mm; 30 Hz: 0.12±0.06 mm) but not (P<0.05) tropicamide (6 Hz: 0.68±0.04 mm; 30 Hz: 0.21±0.09 mm).

Conclusions : This work demonstrates that 6 Hz flickering stimuli are especially effective at eliciting pupil constriction in healthy human participants and this enhanced constriction is diminished after application of atropine. These results are consistent with the hypothesis of acetylcholine-driven stimulation of ipRGCs in humans.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.