Feature Selectivity for Circadian Regulation within Melanopsin Photoreceptors

The day-night cycle drives profound environmental changes, which animals anticipate using the circadian clock. The clock synchronizes with this cycle by extracting features of light that indicate solar elevation. It uses a specialized dynamic range to sense relevant photon rates and precise, extended temporal integration to encode total photon counts. This feature selectivity is distinct from that of visual perception and is thought to arise in the brain. We have found it within the retina's M1 melanopsin photoreceptors, which signal light to the clock. M1s have heterogeneous intrinsic and synaptic drives, and collectively account for the dynamic range of circadian photoregulation. Both individually and as a population, M1s approach the level of temporal integration observed behaviorally. M1s and behavior match closely when the pupillary light reflex, another M1-driven function, dynamically shapes retinal illumination. Thus, the earliest steps of photoreception are tuned to signifiers of Earth's rotation.