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red and green. And people with color blindness, who cannot see either green or red, nevertheless
can still see yellow. An alternative approach to the Young-Helmholtz theory, known as the
opponent-process color theory, proposes that we analyze sensory information not in terms of
three colors but rather in three sets of “opponent colors”: red-green, yellow-blue, and white-
black. Evidence for the opponent-process theory comes from the fact that some neurons in the
retina and in the visual cortex are excited by one color (e.g., red) but inhibited by another color
(e.g., green).
One example of opponent processing occurs in the experience of an afterimage. If you stare at
the flag on the left side of Figure 4.16 "U.S. Flag" for about 30 seconds (the longer you look, the
better the effect), and then move your eyes to the blank area to the right of it, you will see the
afterimage. When we stare at the green stripes, our green receptors habituate and begin to
process less strongly, whereas the red receptors remain at full strength. When we switch our
gaze, we see primarily the red part of the opponent process. Similar processes create blue after
yellow and white after black.
Figure 4.16 U.S. Flag
The presence of an afterimage is best explained by the opponent-process theory of color perception. Stare at the flag
for a few seconds, and then move your gaze to the blank space next to it. Do you see the afterimage?
Source: Photo courtesy of Mike Swanson,http://en.wikipedia.org/wiki/File:US_flag(inverted).svg.
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