For nearly 150 years, scientists considered rods and cones to be the eyes only photoreceptors cells that turn light energy into electrical impulses. Many cells in the eye and brain respond to light but only because they are linked to the rods and cones by complex pathways. These cells are responsible for the nervous systems sensitivity to patterns, objects and movement in the visual world.
Now there is a third photoreceptor, say scientists at Brown. The new cell resides deeper in the retina than rods and cones and looks remarkably different, more like the underside of a canopy of twisted tree branches.
The scientists dub the new cell an intrinsically photosensitive ganglion cell. It also turns light energy directly into brain signals. These signals govern the bodys 24-hour clock, they say, adding that this retinal input is what helps people get over jet lag.
In the February 8 issue of Science, the researchers describe the new cells, discovered in the retinas of rats, and their direct pipeline to the brain. The cells send out nerve fibers which travel within the optic nerve and connect with the clock region in the brain.
We think this population of cells plays a role in setting the circadian clock and probably in a variety of other functions where all the brain needs to know is how bright it is, said lead author David Berson, associate professor of neuroscience. It is a visual system that runs parallel to the one we have been thinking about all these years. Now we have to rethink how the retina works and how the brain understands what is going on in the visual world. This is a new kind of representation of light by the nervous system, a new way for the brain to react to the visual environment.