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Re: Quest for colours: what's basic then?

From:Javier BF <uaxuctum@...>
Date:Monday, April 26, 2004, 1:02
>Also, I don't understand how YELLOW is supposed to be a more basic visual >percept for humans than ORANGE. Our eyes pick up RED, GREEN and BLUE and >our brains figure out all the rest of the colors from that.
This is a very common misconception, prompted by the poor explantion of human colour vision that is usually taught at school and that is centered on the systems of colour reproduction and the kinds of retinal cones without any mention to the actual organization of our _perceptual_ colour space. Our eyes do not pick up RED, GREEN and BLUE _percepts_, our eyes have receptors whose peak of response occurs at the wavelengths that _only if occurring in isolation_ is perceived as the colours red, green or blue. But using the name of the colour that is perceived when only one of the receptors is stimulated as a name for that receptor is misleading, because there is much more to our colour vision than just the raw retinal signal; it is less misleading to refer to the kinds of receptors simply as L, M, H (for Low, Medium and High wavelengths). White is an epitome of purity in all cultures. In fact, white is perceived as such a pure and unmixed a colour that when scientists realized that white light could be decomposed through a prism into chromatic lights by separating the different wavelengths of the spectrum, there were people who opposed the idea saying that it was a heresy against the purity of white. Yet, the perception of such a pure colour as white results when all three cone receptors are stimulated simultaneously. That is, according to your explanation, we should _see_ white as a _mixture_ of red, green and blue. Well, I for one cannot perceive anything reddish, greenish or bluish at all in white, as I clearly see a reddish quality in orange and purple and a greenish and bluish quality in turquoise. Neither can I perceive reddishness or greenishness in yellow, which is the percept that is experienced when the 'red' and 'green' cones are stimulated simultaneously. When trying to understand the working of human vision, one has to keep in mind that the cone receptors are just the beginning of the story and that the tri-stimulus signal that is generated by them is then processed through a neuronal network that results in an _perceptual_ space organized into three axes: the luminance axis (with white and black at the extremes) and two hue axes (the red-versus-green and the yellow-versus-blue ones). It is these six basic percepts what form the actual building blocks of our colour experience, and not the three kinds of cones, which merely define the tri-stimulus signal space that is useful for prompting retinal responses by mixing lighwaves, retinal responses that only when further processed by our visual neuronal network generate in our minds the perception of the intended colours. Cheers, Javier

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Mark P. Line <mark@...>