Non-Image-Forming Functions of Intrinsically Photosensitive Retinal Ganglion Cells

Life on this planet is heavily influenced by light, the most critical external environmental factor. Mammals perceive environmental light mainly through three types of photoreceptors in the retina—rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs). The latest discovered ipRGCs are particularly sensitive to short-wavelength light and have a unique phototransduction mechanism, compared with rods and cones. Piles of evidence suggest that ipRGCs mediate a series of light-regulated physiological functions such as circadian rhythms, sleep, metabolic homeostasis, mood, development, and higher cognitions, collectively known as non-image-forming vision. Recent advances in systems neuroscience, driven by modern neural circuit tools, have illuminated the structure and function of the neural pathways connecting the retina to subcortical regions, highlighting their involvement in an array of non-image-forming functions. Here we review key discoveries and recent progress regarding the neural circuit mechanisms employed by ipRGCs to regulate diverse biological functions and provide insights into unresolved scientific questions in this area.