Understanding Psychology (10th Ed)

trichromatic theory of color vision
The theory that there are three kinds 
of cones in the retina, each of which 
responds primarily to a specifi c range 
of wavelengths.
FIGURE 7 
To someone with normal vision, the hot-air balloon in the foreground appears 
with regions of very pure red, orange, yellow, green, blue, and violet, as well as off -white; and 
the balloon in the rear is a bright shade of red-orange. (b) A person with red-green color 
blindness would see the scene in part (a) like this, in hues of blue and yellow. (c) A person 
who is blue-yellow blind, conversely, would see it in hues of red and green.
(a)
(b)
(c)
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112 Chapter 
4 
Sensation and Perception
Because trichromatic processes do not provide a full explanation of color vision, 
alternative explanations have been proposed. According to the
 opponent-process 
theory of color vision ,
fi rst proposed by German physiologist Ewald Hering in the 
19th century, receptor cells are linked in pairs, working in opposition to each other. 
Specifi cally, there are a blue-yellow pairing, a red-green pairing, and a black-white 
pairing. If an object refl ects light that contains more blue than yellow, it will stimulate 
the fi ring of the cells sensitive to blue, simultaneously discouraging or inhibiting the 
fi ring of receptor cells sensitive to yellow—and the object will appear blue. If, in 
contrast, a light contains more yellow than blue, the cells that respond to yellow will 
be stimulated to fi re while the blue ones are inhibited, and the object will appear 
yellow (D. N. Robinson, 2007).
The opponent-process theory provides a good explanation for afterimages. When 
we stare at the yellow in the fi gure, for instance, our receptor cells for the yellow 
component of the yellow-blue pairing become fatigued and are less able to respond 
to yellow stimuli. In contrast, the receptor cells for the blue part of the pair are not 
tired, because they are not being stimulated. When we look at a white surface, the 
light refl ected off it would normally stimulate both the yellow and the blue receptors 
equally. But the fatigue of the yellow receptors prevents this from happening. They 
temporarily do not respond to the yellow, which makes the white light appear to be 
blue. Because the other colors in the fi gure do the same thing relative to their specifi c 
opponents, the afterimage produces the opponent colors—for a while. The afterimage 
lasts only a short time, because the fatigue of the yellow receptors is soon overcome, 
and the white light begins to be perceived more accurately. 
We now know that both opponent processes and trichromatic mechanisms are 
at work in producing the perception of color vision, but in different parts of the visual 
sensing system. Trichromatic processes work within the retina itself, whereas oppo-
nent mechanisms operate both in the retina and at later stages of neuronal processing 
(Baraas, Foster, & Amano, 2006; Chen, Zhou, & Gong, 2004; Gegenfurtner, 2003).