A new study led by Lauren Welbourne from the University of York, UK, has revealed that humans see things differently in summer compared with winter.

Humans identify four unique hues – blue, green, yellow and red – that do not appear to contain mixtures of other colors. Unique yellow (UY) is particularly interesting to scientists as it is stable across large populations – everyone agrees what UY looks like despite the fact that people’s eyes are often very different.

Ms Welbourne and his colleagues from the University of York wanted to discover why UY is so stable and what factors might make it change.

They thought that this color might depend not on the biology of the eye but on the color of the natural world.

“What we are finding is that between seasons our vision adapts to changes in environment. 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,” said Ms Welbourne, PhD student at the University of York’s Department of Psychology and first author of the paper published in the journal Current Biology.

The team measured 67 participants in York, UK, in both the winter and summer, and found a significant seasonal change in UY settings.

The participants were placed in a darkened room, allowed to adjust to the light and then on a colorimeter asked to adjust a dial backwards and forwards until they felt they had reached the point where it had reached UY – with no hint of a green or red.

“I take lots of measurements of the setting in both seasons, and find a shift in the average setting between seasons,” Ms Welbourne said.

“This is the first time natural changes in the environment have been shown to affect our perception of color. For me as a vision scientist it is fascinating as it is telling us more about how visual processing works.”

“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. This can have knock on effects on the way we diagnose and treat visual disorders.”

It provides an example of how humans constantly adapt to their surroundings.

“Many places in the world have very different environments throughout the year. Think about the changes that the rainy season brings to India, or winter and summer in the arctic. So 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,” Ms Welbourne said.

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Lauren E. Welbourne et al. 2015. Human colour perception changes between seasons. Current Biology, vol. 25, no. 15, pR646–R647; doi: 10.1016/j.cub.2015.06.030