What we perceive as the color yellow changes with the seasons, as our world goes from winter grays to summer greens and back again, scientists say.
Researchers at the University of York in Britain working to understand how people process color looked at how humans identify the four primary colors — red, yellow, blue and green — that are considered "unique," because they do not contain a mixture of other colors.
They focused on unique yellow for its interesting quality of being stable across populations — in other words, all people agree on what that yellow looks like in spite any differences in their visual sense.
The same is not true of unique red, blue or green, making yellow a fascinating research subject, the researchers explain.
They theorized the perception of unique yellow — and any change in that perception — might not depend so much on the biological structure of the human eye but rather on the color of the surrounding natural world.
"What we are finding is that between seasons our vision adapts to changes in environment," says study leader and psychology doctoral student Lauren Welbourne. "So in summer when there is a much larger amount of foliage, our visual system has to account for the fact that on average we are exposed to far more green."
In contrast, winter, with its much reduced foliage, appears much more gray, and our vision will compensate so that, surprisingly, it changes what we perceive yellow as looking like.
"It's a bit like changing the color balance on your TV," Welbourne says.
In experiments conducted in June and again in January, 67 men and women were seated in a darkened chamber until their eyes adjusted to the lower light level.
They were then told to make adjustments to a dial on an instrument known as a colorimeter one way or the other until they believed the machine was displaying a perfect unique yellow, without a hint of red or green.
Recording the final machine settings in both winter and summer showed a difference in the average setting, Welbourne reported in the journal Current Biology.
The findings have yielded a new understanding of the complex nature of our visual system, she says, and is the first time instance in which naturally occurring cycles in our environment have been seen to affect the perception of color.
"Although there's no disorder that this can fix, the more we learn about how vision and color in particular is processed, the better we can understand exactly how we see the world," she says. "This can have knock on effects on the way we diagnose and treat visual disorders."
