A paradox of control?

[From Bruce Abbott (980309.1500 EST)]

Bruce Gregory (980309.1212 EST) --

Bill Powers (980309.0906 MST)

The spectral bands of the eye overlap not slightly, but mostly, at least
for the red-green bands. Edwin Land showed that the most important
distinction in color vision is between long and short wavelengths; the same
object can appear to be different colors depending on the colors of other
objects in the same visual field -- yet a given object can be seen as the
same color even though it is illuminated by light that does not contain
that color at all, or that changes the received spectrum of light energy to
match exactly the spectrum of a different color seen under normal lighting
conditions. There is, in fact, no simple correspondence of perceived color
to the spectrum of incoming light.

There is a very nice discussion of this in the first
chapter of Oliver Sack's book _An Anthropologist on Mars_.

Yes, this has come up before. The artist described in that chapter suffered
a stroke that deprived him of the portion of the visual cortex that is
involved in color perception. Not only could the man no longer perceive
color in the visual input, he could no longer dream in color, nor could he
imagine colors. This example provides strong evidence that all these
color-perceptions arise from activity of the same brain mechanism.