Blind people may go through life in darkness, but some still set their biological clocks by the sun. The reason for this phenomenon has long been a mystery, but now Brown researchers believe they have discovered the explanation. Led by Associate Professor of Neuroscience David Berson '75, the scientists have identified a previously unknown type of light-sensitive cell deep in the retina that functions like an ocular solar panel powering the body's twenty-four-hour clock.
"It is a visual system that runs parallel to the one we have been thinking about all these years," Berson says. "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."
The Brown team's work, which was announced in the February 8 issue of Science, comes nearly 150 years after the identification of rods and cones as photo-receptors, which turn light energy into electrical impulses or brain signals, thereby making vision possible. Berson says the new, and as yet unnamed, photo-receptor is linked to the part of the brain that sets the body's circadian clock.
The cells, found in the retinas of mice, resemble a tree's twisted root system with nerve fibers that run through the optic nerve to the brain. Berson theorizes that in some blind people this connection remains intact, despite damage to their rods and cones. "This is a very simple system because it goes right from the photoreceptor cell to the target in the brain, utilizing just one synapse," Berson said. Visual perception, on the other hand, "reflects activity in the cortex that is at least four synapses away from the rod and cone."
The discovery opens up new possibilities for adjusting circadian rhythms in people. It could, for example, lead to treatments for sleep disorders and jet lag.
"We thought we knew everything about the retina," University of Virginia neuroscientist Michael Menaker told the New York Times. "Now we have two separate systems in the eye, one for vision and one for setting the clock. We have a new way of thinking about how light is interpreted by the nervous system."