Investigation of steady-state pupil responses to test stimuli defined in photoreceptor stimulation

Abstract

The pupillary light reflex has been known to be useful in medical and clinical applications and the understanding its mechanisms is of great importance. Previous studies have shown that factors such as the luminance and colours of light stimuli are related to the pupillary light reflex. Light information entering the retina is first encoded by retinal photoreceptors and signals from each photoreceptor are input to the pupil control mechanism via post-receptoral processes and complex neural circuits in higher brain functions. Test stimuli of varying colour and luminance seem to be insufficient to investigate the pupil control mechanism. This is because multiple photoreceptors respond to colour and/or luminance stimuli, and the outputs of multiple photoreceptors are delivered to the pupil control mechanism or other higher brain functions. To understand the pupil control mechanism, it seems important to consider the physiological processes such as photoreceptors and post-receptoral mechanisms and possibly higher brain functions that contribute to the pupil control mechanism. The aim of this study was to investigate how each retinal photoreceptor affects the pupillary light reflex, rather than changing the colour or luminance of the test stimulus. To this end, a four-primary light stimulator was used to independently stimulate three types of cones and melanopsin photoreceptors. We measured steady pupillary responses to the test stimuli which stimulated each retinal photoreceptor independently. The test stimuli increased or decreased the stimulation to four types of photoreceptors, melanopsin, L cone, M cone and S cone. It was found how the retinal photoreceptors, melanopsin, L cone, M cone and Scone, contribute to the pupillary light reflex. The use of test stimuli defined in photoreceptor stimulation is expected to advance future pupil research.