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Module 1.6b

your finger moves. Why do you see light? Why at the left? This happens because your retinal
cells are so responsive that even pressure triggers them. But your brain interprets their firing
as light. Moreover, it interprets the light as coming from the left — the normal direction of
light that activates the right side of the retina.

Color Processing

ound us?
ceive color in the world ar
How do we per
1.6-6 How do we perceive color in the world around us?
1.6-6
We talk as though objects possess color: “A tomato is red.” Recall the old question, “If a tree
falls in the forest and no one hears it, does it make a sound?” We can ask the same of color:
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If no one sees the tomato, is it red?
The answer is No. First, the tomato is everything but red, because it rejects (reflects) the Young–Helmholtz
long wavelengths of red. Second, the tomato’s color is our mental construction. As Sir Isaac trichromatic (three-color)

Newton (1704) noted, “The [light] rays are not colored.” Like all aspects of vision, our percep- theory the theory that the retina
Copyright © Bedford, Freeman & Worth Publishers.
tion of color resides not in the object itself but in the theater of our brain; even while dreaming, contains three different types
of color receptors — one most
we usually perceive things in color. Likewise, air molecules striking the eardrum are silent and sensitive to red, one to green,
scent molecules have no smell. Our brain creates experiences of sight, sound, and smell. one to blue — which, when
One of vision’s most basic and intriguing mysteries is how we see the world in color. stimulated in combination, can
How, from the light energy striking the retina, does our brain construct our experience of produce the perception of any
such a multitude of colors? color.
Modern detective work on the mystery of color vision began in the nineteenth century,
when German scientist Hermann von Helmholtz built on the insights of an English physicist,
Thomas Young. Both knew that any color can be created by combining the light waves of
three primary colors — red, green, and blue. So Young and von Helmholtz’s research led to a
hypothesis: The eye must have three corresponding types of color receptors.
®
Researchers later confirmed the Young–Helmholtz trichromatic (three-color) AP Science Practice
theory by measuring the responses of various cones to different color stimuli. The retina does Research
indeed have three types of color receptors, each especially sensitive to the wavelengths of Recall from Unit 0 that a hypoth-
red, green, and blue. When light stimulates combinations of these cones, we see other colors. esis is a falsifiable prediction that
For example, the retina has no separate receptors especially sensitive to yellow. But when red can be used to check the theory or
and green wavelengths stimulate both red-sensitive and green-sensitive cones, we see yellow. produce practical applications of it.
Said differently, when your eyes see red and green without blue, your brain says yellow. By testing Young and Helmholtz’s
hypothesis, researchers supported
Worldwide, about 1 in 12 males and 1 in 200 females have the genetically sex- their theory of color vision.
linked condition of color-deficient vision. Most with color vision deficiency are not entirely
“colorblind”: They simply lack functioning red- or green-sensitive cones, or sometimes both.

Their vision — perhaps unknown to them, because their lifelong vision seems normal — is
monochromatic (one-color) or dichromatic (two-color) instead of trichromatic, making

it impossible to distinguish the red and green in Figure 1.6-13 ( Boynton, 1979 ). Dogs, too,

lack receptors for the wavelengths of red, giving them only limited, dichromatic color vision Figure 1.6-13
( Neitz et al., 1989 ).
Color-deficient vision
The photo in image (a) shows
how people with red-green
deficiency perceived a 2015
Ben Solomon/The New York Times/Redux Pictures of Americans like me that are red-
Buffalo Bills versus New York Jets
football game. “For the 8 percent
green colorblind, this game is a
nightmare to watch,” tweeted one
fan. “Everyone looks like they’re
on the same team,” said another.
The photo in image (b) shows

viewers with normal color vision.
(a) (b) how the game looked for those

Sensation: Vision Module 1.6b 129

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