Thursday, August 30, 2018

The YouTube Theory of Colour Vision

When writing for a general audience, vision scientists often refer to the long-, middle- and short-wavelength cone classes (L, M and S) as red, green and blue cones respectively. While this simplification may seem harmless it unfortunately has been the starting point for a cascade of misunderstandings about human colour vision. To begin with it reinforces the assumption that hues are properties residing in wavelengths of light, and then understandably leads to the assumption that the three cone types individually detect red, green and blue hues/wavelengths. Together these assumptions lead to the conclusion commonly encountered in discussions of colour vision on social media that we “only really see three colours”. In turn this conclusion has teamed up with the homunculus fallacy to spawn a model of colour vision in which the cone cells send hue signals directly to an observing brain. When the brain receives a combination of cone signals that could be produced by a “real” colour in the spectrum it “thinks it sees” that colour. This model bears little resemblance to current science but has achieved the status of orthodoxy on several online platforms.

Expositions of this model can be found in four YouTube videos recorded as having from nearly half a million to more than 18 million views: “This Is Not Yellow” (Michael Stevens, September 2012 [1]), “How We See Color” (Colm Kelleher, TED-Ed, January 2013 [2]), “Colour Mixing: The Mystery of Magenta” (Steve Mould, The Royal Institution of Great Britain, February 2013 [3]) and “Does This Look White to You?” (Dianna Cowern, October 2015 [4]). Kelleher’s version is typical: “There are three kinds of cone cells that roughly correspond to the colors red, green, and blue. When you see a color, each cone sends its own distinct signal to your brain. For example, suppose that yellow light, that is real yellow light, with a yellow frequency, is shining on your eye. You don't have a cone specifically for detecting yellow, but yellow is kind of close to green and also kind of close to red, so both the red and green cones get activated, and each sends a signal to your brain saying so.”

Based on the assumptions that hues reside in the wavelengths of the spectrum and that the function of colour vision is to detect these hues/wavelengths of monochromatic light, when we “think we see” yellow while looking at a mixture of long and middle wavelengths our brain is being “tricked” or ”lied to”; this mixture of wavelengths is only “fake yellow” [1].

In each video the verbal explanation implies that the “red and green cones” respond most strongly to “red and green wavelengths” respectively, and that there is no cone responding most strongly to “yellow wavelengths”, even when an accompanying diagram shows correctly that the “real yellow” wavelength lies near the peak of the “red cone” response [1,4]. Stevens adds the additional misconception that bright yellow objects such as lemons reflect only “real yellow” wavelengths (they in fact reflect strongly most of the long and middle wavelengths of the spectrum).

Mould [4] adds the novel idea that when the brain receives a combination of “red cone” and “blue cone” signals that could not be produced by a single wavelength it “makes up” a colour, magenta. The view that magenta alone is a “made up colour”, which is now also popular on the internet, reflects the assumption that the spectral hues are not “made up” because they “exist” in the wavelengths of the spectrum.

The YouTube explanations make no explicit reference to cone opponency, the process by which the cone responses are compared with each other beginning in the retina (rather than proceeding directly to the brain). Nor do they mention the important higher-level process of hue opponency [5], by which perceptions of hue are generated in a yet poorly understood way as combinations of red/green and yellow/blue hue components. It is in fact difficult to see how the concept of hue opponency could be grafted without causing great confusion onto an explanation that already invokes red, green and blue signals arising at the level of the cones.

In explaining colour vision science, it is important to make it clear that hues do not reside in wavelengths of light and that at the level of cone responses and cone opponency our vision detects not hues but variations in the balance of the long-, middle- and short-wavelength components of light across the visual field. Hues should not enter the narrative until the higher-level stage of hue opponency, in which detected variations in the spectral composition of light and in the spectral reflectance of objects evoke red/green and yellow/blue hue-opponent perceptions.

5 Wuerger, S., & Xiao, K. (2015). Color Vision, Opponent Theory. In R. Luo (Ed.), Encyclopedia of Color Science and Technology (pp. 413-418). Springer. doi:10.1007/978-3-642-27851-8_92-1

David J. C. Briggs (National Art School and Julian Ashton Art School, Sydney, Australia)
Dr. David Briggs has taught courses in colour for painters for twenty years and is the author of the website A recording of his recent ISCC webinar “The New Anatomy of Colour” is available through the ISCC website (

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