This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Biologists and linguists alike have long puzzled over the universal human tendency to see and name colors as a combination of four "pure" hues—red, yellow, green and blue. Most people and most cultures do not perceive red as a combination of orange or purple, for example, whereas they naturally perceive orange as a combination of yellow and red.
We also perceive some colors as opposites of one another: red is the opposite of green; blue, the opposite of yellow. We would never describe a color as reddish-green or bluish-yellow. Biologists have tried and failed to reconcile these four primary opposing colors with the three kinds of cone cells in our eyes that detect different wavelengths of light.
Similarly, our subjective experience of color can't be explained by the network of neurons that process vision in the brain. Yet, as painstakingly documented by linguist Paul Kay and the late anthropologist Brent Berlin at UC Berkeley, dozens of human languages, including many unwritten ones, categorize color based on these four hues, hinting at a unique aspect of human perception. "We tend to think of red as just red, not as a mixture," said Berkeley postdoctoral fellow Alexander Belsten.
"This same unique property holds for green, blue and yellow: Each appears perceptually pure rather than as a mixture of neighboring hues." Belsten and Berkeley neuroscientist Bruno Olshausen have now come up with a theory that explains this conundrum. They demonstrate that the natural world—as distinct from the more colorful human-created world—displays a restricted palette of colors that the human brain, for the sake of simplicity, represents as combinations of only four pure colors. Basically, a combination of just four opposing hues—red versus green and blue versus yellow—provides the simplest way to encode the range of colors found in nature.
"Think about this like north, south, east and west. If you say you're going north, that means you're not going south," said Olshausen, director of Berkeley's Redwood Center for Theoretical Neuroscience and professor of optometry and vision science. "That's what's kind of striking about this study.
It captures exactly this idea of opponency that physiologists described back in the 19th century, which is that blue and yellow appear to be opposites and that red and green appear to be opposites. This has puzzled color vision researchers for a long time." The new theory offers "a possible resolution of two historically competing accounts of color vision that emerged around the same time and were vigorously debated in the late 19th century: Hermann von Helmholtz's theory that color vision begins with three receptor types versus Ewald Hering's theory of the four unique hues and their opponent nature," said Belsten, who is first author of a paper about the theory published July 14 in the Journal of the Optical Society of America A. "We now know that both are valid accounts for how we see color.
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