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During the past century, vision scientists focused mainly on rods and cones as the light sensors of the eye. It took Foster, an outsider coming from the circadian-biology community, to uncover some of the first evidence for a third type of photoreceptor. ... ... Ignacio Provencio, a former graduate student of Foster's now at the University of Virginia, Charlottesville, identified the light-sensitive molecule melanopsin in the mouse and primate ganglion layer5 ? a network of retinal cells that was only thought to relay signals from rods and cones to the brain . The presence of this 'photopigment' suggested that some of these cells might also sense light and serve as a new class of photoreceptor. ... under low light conditions, rods can set the body's clock, but some groups have suggested that under different conditions cones can as well. Perhaps more surprisingly, researchers have found that ipRGCs may contribute to visual perception. Hattar and others fluorescently labelled ipRGCs in mice to trace the projections of these cells to the brain. They found that ipRGCs reach into more brain regions than expected, including centres involved in visual processing: the dorsal lateral geniculate nucleus (LGN) and the superior colliculus. Mice without functioning rods and cones, but with intact ipRGCs, could even discriminate patterns in a visual test Researchers now think that ipRGCs and rods compensate for each other and may collectively be allowing the eyes and brain to respond to light across a wide range of brightness levels And for humans, who have a smaller percentage of ipRGCs than mice, experiments are beginning to show how the cells might contribute to physiology and behaviour. Steven Lockley, a neuroscientist at Brigham and Women's Hospital in Boston, Massachusetts, and his colleagues tested the reaction times of 16 healthy volunteers while they were exposed to either blue or green light for 6.5 hours. Those exposed to blue light had faster reaction times and fewer attention lapses when they were asked to report when they heard a sound etc |
