A pupillary contrast response in mice and humans: Neural mechanisms and visual functions.

In the pupillary light response (PLR), increases in ambient light constrict the pupil to dampen increases in retinal illuminance. Here, we report that the pupillary reflex arc implements a second input-output transformation; it senses temporal contrast to enhance spatial contrast in the retinal image and increase visual acuity. The pupillary contrast response (PCoR) is driven by rod photoreceptors via type 6 bipolar cells and M1 ganglion cells. Temporal contrast is transformed into sustained pupil constriction by the M1's conversion of excitatory input into spike output. Computational modeling explains how the PCoR shapes retinal images. Pupil constriction improves acuity in gaze stabilization and predation in mice. Humans exhibit a PCoR with similar tuning properties to mice, which interacts with eye movements to optimize the statistics of the visual input for retinal encoding. Thus, we uncover a conserved component of active vision, its cell-type-specific pathway, computational mechanisms, and optical and behavioral significance. [Display omitted] • Contrast elicits robust pupil constriction in mice and humans • A cell-type-specific retinal pathway mediates the pupillary contrast response • Pupil size shapes spatial frequency content in the retinal image • Pupil constriction enhances contrast sensitivity and visual acuity in mice The pupil is well known to constrict in response to increases in luminance. Here, Fitzpatrick et al. show that temporal contrast also drives pupil construction via a cell-type-specific retinal circuit in mice and humans. The pupillary contrast response enhances high spatial frequency contrast in the retinal image and improves visual acuity. [ABSTRACT FROM AUTHOR]