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Figure 1.6-14
Afterimage effect
Stare at the center of the flag for a
minute and then shift your eyes to
the dot in the white space next to
it. What do you see? (After tiring
your neural response to black,
green, and yellow, you should
see their opponent colors.) Stare
at a white wall and note how the
size of the flag grows with the
projection distance. But why do people blind to red and green often still see yellow? And why does yellow
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
appear to be a pure color and not a mixture of red and green, the way purple combines
red and blue? As physiologist Ewald Hering — a contemporary of von Helmholtz — noted,
trichromatic theory leaves some parts of the color vision mystery unsolved.
Hering found a clue in afterimages. If you stare at a green shape for a while and then
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®
AP Science Practice look at a white sheet of paper, you will see red, green’s opponent color. Stare at a yellow

Research square and its opponent color, blue, will appear on the white paper. (To experience this, try

the flag demonstration in Figure 1.6-14. ) Hering formed another hypothesis: Color vision
Notice how these two theories
build on each other to give us a must involve two additional color processes, one responsible for red-versus-green percep-
more complete understanding tion and one responsible for blue-versus-yellow perception.
of color vision. This is the way A century later, researchers confirmed Hering’s hypothesis, now called the opponent-

science works — theories evolve

via the scientific process. As a process theory . This concept is tricky, but here’s the gist: Color vision depends on three sets

result, some theories described in of opposing retinal processes — red-green, blue-yellow, and white-black. As impulses travel to
this textbook might look different the visual cortex, some neurons in both the retina and the thalamus are turned “on” by red
in years to come.
but turned “off” by green. Others are turned on by green but off by red ( DeValois & DeValois,
1975 ). Like red and green marbles sent down a narrow tube, “red” and “green” messages
cannot both travel at once. We see either red or green, not a reddish-green mixture. But red
and blue travel in separate channels, so we can see a reddish-blue magenta.
So how does opponent-process theory help us understand negative afterimages, as in
the flag demonstration? Here’s the answer (for the green changing to red): First, you stared
at green bars, which tired your green response. Then you stared at a white area. White con-
tains all colors, including red. Because you had tired your green response, only the red part
of the green-red pairing fired normally.
The present solution to the mystery of color vision is therefore roughly this: Color pro-
cessing occurs in two stages.

1. The retina’s red-, green-, and blue-sensitive cones respond in varying degrees to differ-
ent color stimuli, as the Young–Helmholtz trichromatic theory suggested.

2. The cones’ responses are then processed by opponent-process cells, as Hering’s theory
proposed.
opponent-process theory the
theory that opposing retinal
processes (red-green, blue-
yellow, white-black) enable Feature Detection
color vision. For example, some

e ar

e featur
e detectors located, and what do they do?

1.6-7
Wher
cells are stimulated by green 1.6-7 Where are feature detectors located, and what do they do?
and inhibited by red; others are
stimulated by red and inhibited Scientists once likened the brain to a movie screen on which the eye projected images. Then
by green. along came David Hubel and Torsten Wiesel (1979), who showed that our visual processing
feature detectors nerve cells deconstructs visual images and then reassembles them. Hubel and Wiesel received a Nobel
in the brain’s visual cortex that Prize for their work on feature detectors nerve cells in the occipital lobe’s visual cortex that
,

respond to specific features of respond to a scene’s specific visual features — to particular edges, lines, angles, and movements.
the stimulus, such as shape,
angle, or movement. Using microelectrodes, Hubel and Wiesel discovered that some neurons fired actively
when cats were shown lines at one angle, while other neurons responded to lines at a
130 Unit 1 Biological Bases of Behavior

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