Even Monkeys Can Tell Red from Grue

Unsure of the difference between ochre and ecru? Mauve and maize? Don't feel bad, because there's at least one color distinction you can handle: warm versus cool colors. You may have thought it was made up by your art teacher to torment you, but the concept is biologically based and universal to cultures around the world. Even a monkey knows the difference.

Researchers led by Youping Xiao at Mount Sinai School of Medicine based their study, in part, on data from the World Color Survey. That project, begun in the late 1970s, asked people from 110 cultures with unwritten languages to look at hundreds of color chips and name them.

Previous analysis of this survey found that no matter how many or how few color terms a language has--even if, say, speakers group together several English-language colors under one term--there's a certain "fault line" that these groupings almost never cross. That's the boundary between warm colors (such as orange or red) and cool ones (such as green or blue--or "grue," as some cultures combine them).

Xiao and colleagues analyzed the wavelengths of light reflected by each color chip used in the World Color Survey and calculated which cone cells in the human eye those colors correspond to. We see color using three types of cone cells: S, M, and L, for short, medium, and long wavelengths of light. The researchers found that the warm-cool boundary corresponded to how much a color stimulated L versus M cone cells. So the distinction isn't one that only artsy people can see--there's a real difference in how warm and cool colors hit our eyes.

The researchers also studied brain images from macaque monkeys as they viewed various colors. Scans of the visual cortex in each macaque showed the same kind of polarity: two different clusters of neurons lit up, depending on whether a color was warm or cool.

Earlier studies had shown that colors are spatially arrayed in a "hue map" in the brain, with different patches of neurons corresponding to separate colors. No matter how many colors our brains or our languages have taught us to distinguish between, though, the anatomy of our eyes seems to make certain distinctions universal. This gives at least a partial answer to the question of whether everyone else is seeing the same colors you are. Whether you carefully sort your sweaters into chartreuse and lime or just call the whole thing "grue," you would never confuse a warm hue with a cool one.

If you want to test exactly how accurate your color perception is, try arranging the color chips on this hue test. It might make your brain feel muddled, like your hue map is missing its compass. But keep in mind that the macaques were only looking at nine different colors.

Image: from xrite.com online color challenge

Xiao, Y., Kavanau, C., Bertin, L., & Kaplan, E. (2011). The Biological Basis of a Universal Constraint on Color Naming: Cone Contrasts and the Two-Way Categorization of Colors PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0024994