ABSTRACT
Mammalian eyes mediate both image-forming and non-image-forming visual functions. Non-image-forming vision provides a measure of the ambient light for the purposes of synchronization of circadian clocks to light/dark cycles and regulation of pupil size,pineal melatonin production and other functions. Traditionally,people used to believe that the classical photoreceptors (rods and cones) regulate both image-forming and non-image-forming visual pathways. However,a small subset of retinal ganglion cells called intrinsically photosensitive RGCs ( ipRGCs) has been identified to be a third type of mammalian photoreceptor and determined to be photosensitive, recently. The discovery of ipRGCs has allowed for rapid progress in the past decade toward understanding the non-image-forming visual system,especially about how the circadian clock complete the synchronization with the light/dark cycle. The anatomical and developmental characteristics of ipRGCs, as well as its biological functions and regulation were reviewed in this paper.