For the greater part of 150 years it was assumed that the mammalian retina contained only two classes of photoreceptors, rods and cones. However, a flurry of studies in the late 1990s and early 2000s demonstrated the existence of a third class of mammalian photoreceptors that differs greatly from rods and cones. It utilizes a different photopigment, is much less sensitive to light, responds to light far more slowly, and has far lower spatial resolution, characteristics that fit with its primary function of signaling ambient light levels (irradiance) to the brain. Most surprisingly, these photoreceptors are retinal ganglion cells (RGCs), and thus, have the unique ability to communicate directly with higher visual centers of the brain. These intrinsically photosensitive retinal ganglion cells (ipRGCs) are a rare subpopulation of ganglion cells (<5%) whose primary role is to signal light for largely subconscious, non-image-forming visual reflexes, such as pupillary constriction, neuroendocrine regulation, and synchronizing daily (“circadian”) physiological rhythms to the light/dark cycle (“circadian photoentrainment”). Recent research has revealed additional roles for ipRGCs in conscious visual perception, including brightness discrimination and contrast detection. Many visual behaviors under ipRGC control are remarkably tonic, and require prolonged integration of ambient light levels. The unique properties of ipRGCs make them well suited for regulating such behaviors.
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