Eye ‘Noise’ : Body’s Heat Reduces Humans’ Vision in Dim Light, Researchers Say

Why can’t humans see well in dim light?

The answer, at least in part, lies in the heat of our bodies, Finnish researchers reported in this week’s Nature magazine. The eye is so finely tuned and sensitized by evolution, said zoologist Thomas Reuter and his colleagues at the University of Helsinki, that body heat can trigger chemical changes in the eye’s retina; this in turn sends an electrical signal to the brain--creating “noise” that swamps a dim visual signal.

Nocturnal mammals circumvent this problem to a limited degree by having bigger eyes, but cold-blooded animals have even better vision because their body temperatures are lower. Frogs, for example, can visually hunt food under conditions in which humans see neither frog nor prey.

The Finnish results are also in accord with earlier studies showing that human visual sensitivity improves at night as body temperatures drop, said neurophysiologist Maureen Powers of Vanderbilt University. But Powers cautioned that “there may be other limiting factors in visual sensitivity” that have not been identified yet.

Casts Doubt

At least one scientist casts doubt on their conclusion, however. Neuroscientist Robert Barlow of Syracuse University said the Finnish report presents “very interesting results that are in accord with the mainstream of scientific thought,” but based on his own work in the horseshoe crab, he said, “we challenge the idea that intrinsic retinal noise results from thermal (effects).”

The ultimate conclusion may be that different factors may have limiting effects in different species, he said.

The eye is a wondrously complex mechanism that has evolved to make the maximum use of extremely small amounts of light. One of its key components is a protein called rhodopsin that is found in the visual receptors at the rear of the retina.

Rhodopsin in its normal form binds with a Vitamin A derivative to form a stable compound. But when a photon--the smallest unit of light--strikes the retina, it changes the shape of one rhodopsin molecule, releasing the vitamin A. That event triggers a complex chain of reactions that ends in the transmission of an electrical signal to the brain.

But at any given temperature, a small fraction of the rhodopsin molecules will change their shape as a result of heat, also stimulating the transmission of an electrical signal to the brain and creating electrical noise. For an object to be seen, incoming light must activate nearly as many rhodopsin molecules as are being triggered by heat, Reuter said.

Thus, the higher the body temperature, the greater the amount of light required by the eye.

Reuter and his colleagues confined toads and frogs in a dark box where temperature could be controlled. They slowly increased the amount of light in the box to determine the minimum amount of light necessary for the amphibians to see and snap their tongues at white “worm-dummies.”

The researchers found that the animals’ visual sensitivity increased as the temperature dropped. An 18-degree-Fahrenheit drop in temperature produced a fourfold increase in visual sensitivity. In addition, the researchers calculated that the temperature drop theoretically should reduce the amount of thermal changes in rhodopsin by a factor of four, in close agreement with their experimental result.

“This new work . . . now establishes a firm link between thermal events at a molecular level and their behavioral consequences,” wrote physiologist H. B. Barlow of the University of Cambridge in an accompanying commentary in Nature.

Sensitivity Doubles

Powers noted that she and her co-workers had previously observed a similar effect in humans. The body temperature of humans drops as much as four degrees Fahrenheit during the night, she said, and most people’s visual sensitivity doubles at the lower temperature.

But both Powers and Barlow noted that there are exceptions to this rule. Goldfish, for example, have an increased visual sensitivity at night, even though the temperature of the water in which they are swimming--and thus their body temperature--does not change, Powers has shown.

Barlow has found that the visual sensitivity of the horseshoe crab, amazingly, increases by a factor of 1,000 during the night when the crabs are looking for mates in the dark. Somehow, he said, the crab’s brain causes the background noise in the retina to decrease to zero for two hours.

“That couldn’t happen if the noise was (simply) thermal,” he argued.

Barlow is now studying crab vision at different temperatures to try to get a clue to how the crabs can increase their visual sensitivity by such a large amount.

Despite their disagreements on the source of noise in vision, all of the researchers marvel at the complexity of the eye. Noted the editors of Nature: “That such tiny amounts of noise are detectable at all illustrates the degree to which all other aspects of eye function have been optimized by evolution.”