Walsh & Hoyt: The Pupil Light and Near Reflexes and Their Relationship to the Autonomic Nervous System

The major function of the pupil is to aid in optimizing retinal illumination to maximize visual perception. In dim light, dilation of the pupil provides an immediate means for maximizing the number of photons reaching the retina, which supplements the dark-adaptive mechanisms of retinal-gain control. Under bright light, pupil constriction can reduce retinal illumination by up to 1.5 log units. Although this reduction in retinal illumination is only a portion of the 12-log unit range of light sensitivity of the retina, it provides an important and immediate contribution to early light adaptation. Patients with a fixed immobile pupil are usually very symptomatic under conditions of changing illumination, emphasizing the important role of the pupil in optimizing visual perception over a wide range of lighting conditions of the environment.