Different processes govern sight, light...( A Johns Hopkins University biologist...),Health

A Johns Hopkins University biologist, in research with implications for people suffering from seasonal affective disorder and insomnia, has determined that the eye uses light to reset the biological clock through a mechanism separate from the ability to see.

The findings suggest that patients with trouble sleeping or seasonal depression -- disorders that can be linked to lack of exposure to daylight -- could benefit from development of easier, more available tests to determine if they are able to detect light properly for functions distinct from normal sight, said Samer Hattar, assistant professor of biology in the university's Zanvyl Krieger School of Arts and Sciences.

It seems that even if individuals have normal sight, they might be having a malfunction that is contributing to their inability to detect light, which can adversely affect their biological clocks, Hattar said.

Writing in the Advance Online issue of Nature (available at http://www.nature.com ) and in the May 1 print issue, Hattar and colleagues reported that they genetically modified mice so that a particular set of retinal ganglion cells cells that receive input from the rods and cones of the animals eyes and send information to the brain no longer functioned.

The mice were still able to use light to see normally, but had great difficulty synchronizing their circadian rhythms to light/dark cycles, the constant lengthening or shortening of daylight hours that occurs depending on the time of year.

Prior research in the field leads the researchers to believe that because the rodents internal, biological clocks are out of sync with the solar day, the rodents would have difficulty learning and sleeping on a regular, 24-hour cycle. The team has not yet tested that hypothesis.

This research illustrates that there are two distinct pathways for the two different aspects of light detection: image-forming and
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