Absolute luminance detection

Visual perception rarely relies on estimates of absolute luminance. Instead, we recognize objects by how they reflect light, and these percepts are largely independent of overall illumination. However, a host of behaviors rely on absolute luminance estimates, including reflexes such as pupil constriction and the entrainment of our circadian clocks. Only in the last two decades have researchers revealed that the retina has a specialized, evolutionarily ancient set of circuits to encode information about absolute luminance. These circuits rely on the photopigment melanopsin, expressed in a subset of retinal ganglion cells (RGCs), enabling them to sense light intrinsically. Since this landmark discovery, the field has grown rapidly, and much is known about intrinsically photosensitive (ip)RGCs, where they project in the brain, and the many aspects of physiology they influence. It has become clear that ipRGCs are a diverse family with heterogeneous phototransduction pathways, upstream synaptic circuits, and downstream projections in the brain. This chapter covers the current state of our understanding of how absolute luminance is represented in the retina by ipRGCs.