We have known for 10 years about melanopsin-containing retinal ganglion cells (mRGCs). Research over the past decade has shown that they play an important role in reflexive responses to light, such as pupil constriction and regulation of the body's sleep-wake cycle. But they did not appear to be involved in vision.
In July, researchers tagged the mRGCs with a blue protein to see where the cells occur in the mouse eye. When they tracked the cells' axons from the eye into the brain, they saw that many of them terminated in the lateral geniculate nucleus (LGN) and into parts of the mouse brain involved in conscious vision, not just the parts of the brain that control unconscious responses to light.
Timothy Brown, a neuroscientist at the University of Manchester in the United Kingdom tested a range of light intensities, from starlight to bright daylight, in mice without rods nor cones and found that light as intense as daylight fired up the LGN.
Brown and colleagues also looked at whether mRGCs might also send information to the LGN in mice with normal vision. "We found that approximately 40% of the brain cells that process visual signals appear to receive information from mRGCs," says Brown, whose team reports its work today in PLoS Biology. What the researchers don't yet know is whether mRGCs can sense variations in brightness across the visual field.
The next step will be to do the complementary experiment, i.e., show that mice lacking melanopsin have inferior vision based on their behavior.