It’s been known for ages that information on light levels is passed from the two retinas via a special small nerve from each eye to the SCN, but the mechanism of this is now more clear. As well as rods and cones, cells which are sensitive to light and give us black-and-white and colour vision there is a third type of light-sensitive retinal cell. These are far less in number than rods and cones, and react to light by expressing the pigment melanopsin, so they are known as mRGCs (melanopsin-expressing retinal ganglion cells). Not only do these cells send information to the SCN but they also control pupil size. And now it seems they also contribute to our visual image formation as axons from the mRGCs have been traced onwards from the SCN to visual processing centres. What does this imply? It gives some idea of how seriously sight impaired people can still detect levels of brightness, plus the possibility in the future of engineering melanopsin-expressing cells to improve or restore sight.
Fred Rieke, Timothy M. Brown, Carlos Gias, Megumi Hatori, Sheena R. Keding, Ma’ayan Semo, Peter J. Coffey, John Gigg, Hugh D. Piggins, Satchidananda Panda, Robert J. Lucas. Melanopsin Contributions to Irradiance Coding in the Thalamo-Cortical Visual System. PLoS Biology, 2010; 8 (12)