New research has disclosed a completely new mechanism for vision, showing it's still possible to see even when the retina has been completely destroyed.
Melanopsin is known as the light sensor that sets the body's biological clock, and has been known to play a role in the perception of brightness, through so-called melanopsin-expressing retinal ganglion cells (mRGCs).
But a team of biologists has discovered a new role for mRGCs during image-forming vision, and suggests that these cells could support vision, even in people with advanced retinal degeneration.
"Millions of people worldwide suffer varying degrees of blindness because of rod and cone degeneration or dysfunction, but many of them can still perceive differences in brightness," says senior author Satchidananda Panda of the Salk Institute for Biological Studies.
"Melanopsin-expressing RGCs typically survive even complete rod and cone loss and could explain the light responses under these conditions."
It seems that the ability to convert light into electrical signals in the mammalian retina is not, as thought, restricted to only two types of photoreceptor, rods and cones.
The third type, discovered by Panda, is only present in a few thousand cells embedded in the deeper layers of the retina. Melanopsin, a photopigment that measures the intensity of incoming light, is fundamentally different from the classical rod and cone opsins which help us see. It's much less sensitive to light, for a start, and has far less spatial resolution.
Panda and his collaborators at the University of Manchester traced individual mRGCs' axons from the retina through the circadian clock and onward. They discovered that the axons reached all the way to the lateral geniculate nucleus (LGN), the primary processing center for visual information received from the retina.
"We found widespread light responses in the LGN and visual cortex, even in mice lacking functional rods and cones, which are often used as a model of advanced retinal degeneration," says Panda.
Panda believes it might one day be possible to restore vision to blind people through gene therapy with a re-engineered melanopsin.
"The density of mRGCs in the retina is too low for any meaningful resolution," he says. "But if we could express melanopsin in a greater number of cells, we might be able to increase resolution to a point that allows blind people to safely navigate their environment."