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

>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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