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Eye's light-detection system revealed

A research team led by Johns Hopkins scientists has discovered that a special, tiny group of cells at the back of the eye help tell the brain how much light there is, causing the pupil to get bigger or smaller. The findings, which appeared in the Jan. 10 issue of Science, largely complete the picture of how light levels are detected in the eye.

"This tiny group of cells, together with rods and cones, are the bulk of the eye's mechanisms for detecting levels of light and passing that information to the brain," says King-Wai Yau, Ph.D., professor of neuroscience and a Howard Hughes Medical Institute investigator at the Johns Hopkins School of Medicine.

The team previously had shown that this set of retinal cells, all of which contain a protein called melanopsin, are naturally sensitive to light. They also showed that the cells connect to the brain in such a way that they are poised to control how the pupil reacts to light and how animals adapt to day and night.

The new work proves that these melanopsin-containing cells, a subset of so-called retinal ganglion cells, are in fact a working part of the body's light-detection system and complement the light-detecting role of rods and cones, which also convey information about the color, shape and movement of objects.

"Rods and cones provide high sensitivity to light, allowing the pupil to constrict, but melanopsin-containing cells seem to be crucial for completing the pupil's response in bright light," says Samer Hattar, Ph.D., a postdoctoral fellow in neuroscience at Johns Hopkins. "Without melanopsin, the pupil fails to constrict fully, even in very bright light."

First authors Hattar and Robert Lucas, Ph.D., of the Imperial College, London, measured how small the pupil of each of two kinds of "knockout" mice became when exposed to known amounts of light. One set of mice were missing the gene for the melanopsin protein, the others lacked rods and cones. In mice without mela
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Contact: Joanna Downer
jdowner1@jhmi.edu
410-614-5105
Johns Hopkins Medical Institutions
14-Jan-2003


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