Frog eyes take a view of dim world.

Frog eyes take a view of dim world

What stops us from seeing better in dim light? Scandinavian researchers have succeeded in showing something that scientists have suspected for decades: Limits to visual sensitivity lie largely in the temperture of the eye of the beholder.

Photons of light are registered in the retina when they hit molecules of the pigment rhodopsin and change those molecules' shape. Soon after this was discovered, scientists theorized that normal molecular motion occasionally would lead some rhodopsin molecules to change shape without being struck by light, thus introducing a source of "noise" into the system. If too few photons entered the eye, the light signal would be swamped by this background noise.

Molecular motion -- and therefore noise -- increases as the temperature rises, so, all other things being equal, cooler animals should be able to see in the dark better than warmer animals. That is the prediction tested by A.C. Aho and his colleagues from the University of Helsinki in Finland and the University of Copenhagen's August Krogh Institute in Denmark. Under very dim light, the scientists showed a white worm "dummy" to frogs and toads at different temperatures, and made a record of when the cold-blooded amphibians snapped at it.

The object of amphibian desire was waved under a glass plate in front of a frog or toad in a dim chamber at 10[deg.] to 20[deg.]C. The results show a linear relationship between the theoretical amount of background noise in the retina (calculated from the animal's body temperature) and the minimum amount of light the animals needed to see their prey, report the scientists in the July 28 NATURE.

Humans were tested with the same apparatus, although they weren't expected to snap at the worms. The warm-blooded humans (with body temperatures of 37[deg.]C) needed eight times as much light to see the worms, which is a direct extension of the linear relationship between temperature and sensitivity found in the data from frogs and toads.

in a commentary accompanying the article, Horace Barlow of the University of Cambridge in England cautions that although the data go a long way toward showing that body temperature is the dominant factor in determining visual sensitivity in dim light, one wouldn't necessarily expect to see such a linear relationship between temperature and vision because of other factors that play a part in the visual system.