198 Chapter 6 Sensation and Perception
“color-blind” people are actually color deficient.
Usually they cannot distinguish red and green; the
world is painted in shades of blue, yellow, brown,
and gray. In rarer instances, they may be blind to
blue and yellow and may see only reds, greens,
and grays. Color deficiency is found in about 8
percent of white men, 5 percent of Asian men, and
3 percent of black men and Native American men
(Sekuler & Blake, 1994). Because of the way the
condition is inherited, it is uncommon in women.The opponent-Process Theory. The opponent-
process theory applies to the second stage of color
processing, which occurs in ganglion cells in the
retina and in neurons in the thalamus and visual
cortex of the brain. These cells, known as oppo-
nent-process cells, either respond to short wave-
lengths but are inhibited from firing by long
wavelengths, or vice versa (DeValois & DeValois,
1975). Some opponent-process cells respond in
opposite fashion to red and green, or to blue and
yellow; that is, they fire in response to one and
turn off in response to the other. (A third system
responds in opposite fashion to white and black
and thus yields information about brightness.)
The net result is a color code that is passed along
to the higher visual centers. Because this code
treats red and green, and also blue and yellow,
as antagonistic, we can describe a color as bluish
green or yellowish green but not as reddish green
or yellowish blue.
Opponent-process cells that are inhibited by
a particular color produce a burst of firing when
the color is removed, just as they would if the op-
posing color were present. Similarly, cells that fire
in response to a color stop firing when the color
is removed, just as they would if the opposing
color were present. These facts explain why we areopponent-process
theory A theory of color
perception that assumes
that the visual system
treats pairs of colors as
opposing or antagonistic.
information about edges, angles, shapes, motion,
brightness, texture, and patterns and figure out
that a chair is a chair and the thing next to it is a
table. The perception of any given object probably
depends on the activation of many cells in far-
flung parts of the brain and on the overall pattern
and rhythm of their activity. Moreover, experi-
ences can modify and shape the brain’s circuits.
Thus, some of the brain cells that seem especially
responsive to faces respond to other things as well,
depending on a person’s experiences and interests.
In one study, “face” cells fired when car buffs ex-
amined pictures of classic cars but not when they
looked at pictures of exotic birds, whereas the ex-
act opposite was true for birdwatchers (Gauthier
et al., 2000). Cars, of course, do not have faces.How We See Colors Lo 6.11
For more than 300 years, scientists have been
trying to figure out why we see the world in liv-
ing color. We now know that different processes
explain different stages of color vision.The Trichromatic Theory. The trichromatic the-
ory (also known as the Young-Helmholtz theory) ap-
plies to the first level of processing, which occurs
in the retina of the eye. The retina contains three
types of cones. One type responds maximally to
blue, another to green, and a third to red. The
thousands of colors we see result from the com-
bined activity of these three types of cones.
Total color blindness is usually the result of a
genetic variation that causes cones of the retina to
be absent or malfunctional. The visual world then
consists of black, white, and shades of gray. Many
species of animals are totally color-blind, but the
condition is extremely rare in human beings. Mosttrichromatic theory A
theory of color percep-
tion that proposes three
mechanisms in the visual
system, each sensitive to
a certain range of wave-
lengths; their interaction
is assumed to produce all
the different experiences
of hue.
Get Involved! a Change of Heart
Opponent-process cells that switch on or off in
response to green send an opposite message—
“red”—when the green is removed, producing a
negative afterimage. Stare at the black dot in the
middle of this heart for at least 20 seconds. Then
shift your gaze to a white piece of paper or a white
wall. You may get a “change of heart”: an image of
a red or pinkish heart with a blue border.