People's perception of color changes depending on the season, new research suggests. 

In particular, people see yellow differently on a grey day in the middle of winter, compared with how they see it on a summer day with green foliage all around.

The odd effect might exist to help keep eyesight keen even when conditions change drastically, said study co-author Lauren Welbourne, a doctoral candidate in psychology at the University of York in England.

"This process is very useful, because you can adapt to these huge seasonal changes in environmental color, and continue to see and discriminate between colors accurately," Welbourne said in a statement. [7 Mysteries of the Human Body]

Visual system

Color perception is a complicated process that involves many elements, from the light receptors in the eye (called rods and cones) to the visual regions of the brain. It can even be affected by psychiatric conditions — scientists have found that people with depression see the world as grayer than do their happier peers.

Research also shows that color perception is highly variable, both among individuals and among cultures. This may extend to the words that cultures use to explain the colors they see. For instance, people in one culture may not even have words for certain colors, whereas others may lump colors together differently (for example, Russians categorize the colors that are called light blue and dark blue in the United States as being different colors from one another entirely).

However, scientists have long noticed that across cultures, for four colors — red, yellow, green and blue — people can identify a specific range of color, within a narrow wavelength of light, that they perceive as a pure color, with no hint of any other color in it, Welbourne said.  Other colors, such as orange, can never be perceived without some hints of other colors like red or yellow, she added.

But while people of every culture can identify these four unique colors, they don't see "unique red" or "unique green" at the same wavelengths. Interestingly, what people perceive as pure yellow is similar across many cultures, unlike other hues. Welbourne and her colleagues wondered whether there was something special about how the human eye receives yellow light, or whether environmental factors played a role in people's perceptions of the color yellow.

Unique color or variable?

To answer that question, researchers asked 67 men and women to enter a darkened room, allow time for their eyes to adjust, and then turn a knob on a machine that displayed different colors until they felt they had hit pure yellow. The team repeated the same procedure in January and in June.

It turned out that the average setting changed from winter to summer, the researchers reported Aug. 4 in the journal Current Biology.

The team said it suspects this type of color shifting — essentially like tuning the color balance on a television — may be a way for the human visual system to compensate for differences in the environment. Winters in York are dull and grey, but during the summer, leaves are on the trees and the grass is green. Although it isn't clear exactly how the change works, it could be that the shift in people's perception of which shade of yellow is pure yellow is a visual adaptation to compensate for that seasonal change, Welbourne said.

The bigger question is how exactly people's perception can change so markedly.

"There are several possibilities as to where this process occurs — it could be within the eye, in the wiring that comes after the cones, or it could be at various other locations in 'visual areas' of the brain," Welbourne said.

The shift probably does not occur over the course of just one day, the researchers said.

"Some studies have suggested this type of process would occur over several weeks," Welbourne told Live Science. 

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