red-green reversal, in which my color space is the same as yours except for re-
flection about the blue-yellow plane, thus reversing reds and greens (see figure
1.1B). It does not suffer from problems concerning the differential lightness
of blues and yellows because my blues correspond to your blues and my
yellows to your yellows. Our particular shades of blues and yellows would be
different—my greenish yellows and greenish blues would correspond to your
reddish yellows (oranges) and reddish blues (purples), respectively, and vice
versa—but gross differences in lightness would not be a problem.
There are other candidates for behaviorally undetectable color transforma-
tions as well (see figures 1.1C and 1.1D). The crucial idea in all these versions of
the inverted spectrum argument is that if the color solid were symmetric with
respect to some transformation—and this is at least roughly true for the three
cases illustrated in figures 1.1B–1.1D—there would be no way to tell the dif-
ference between my color experiences and yours simply from our behavior. In
each case, I would name colors in just the same way as you would, because
these names are onlymediatedby our own private experiences of color. It is the
sameness of the physical spectra that ultimately causes them to be named con-
sistently across people, not the sameness of the private experiences. I would
also describe relations between colors in the same way as you would: that focal
blue is darker than focal yellow, that lime green is yellower than emerald
green, and so forth. In fact, if I were in a psychological experiment in which my
tas kwas to rate pairs of color for similarity or dissimilarity, I would ma ke the
same ratings you would. I would even pic kout the same unique hues as you
would—the ‘‘pure’’ shades of red, green, blue, and yellow—even though my
internal experiences of them would be different from yours. It would be ex-
tremely difficult, if not impossible, to tell from my behavior with respect to
color that I experience it differently than you do.^2
I suggested that red-green reversal is the most plausible form of color trans-
formation because a good biological argument can be made that there should
be some very small number of seemingly normal trichromats who should be
red-green reversed. The argument for suchpseudo-normal color perceptiongoes
as follows (Nida-Ru ̈melin, 1996). Normal trichromats have three different pig-
ments in their three cone types (figure 1.2A). Some people are red-green color
blind because they have a gene that causes their long-wavelength (L) cones to
have the same pigment as their medium-wavelength (M) cones (figure 1.2B).
Other people have a different form of red-green color blindness because they
have a different gene that causes their M cones to have the same pigment as
their L cones (figure 1.2C). In both cases, people with these genetic defects lose
the ability to experience both red and green because the visual system codes
both colors by taking the difference between the outputs of these two cone
types. But suppose that someone had the genes forbothof these forms of red-
green color blindness. Their L cones would have the M pigment, and their M
cones would have the L pigment (figure 1.2D). Such doubly color blind indi-
viduals would therefore not be red-green color blind at all, but red-green-
reversed trichromats.^3 Statistically, they should be very rare (about 14 per
10,000 males), but they should exist. If they do, they are living proof that this
color transformation is either undetectable or very difficult to detect by purely
behavioral means, because nobody has ever detected one!
14 Stephen E. Palmer