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Colorblindness

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Colorblindness
96
FIGURE
Chapter 3 Sensation and Perception
3.10
Individual Differences in
Cone Types
These photographs show that people can
differ widely from one another in the distribution of blue, green, and red cones in
their retinas (Roorda & Williams, 1999).
J.W., whose retina is shown in Part A, has
an especially high population of red cones,
whereas green cones predominate in A.N.,
whose retina is shown in Part B. Both have
normal color vision, but J.W. will be somewhat more sensitive to long wavelengths
of light, whereas A.N. will be somewhat
more sensitive to light of medium wavelengths.
(A)
(B)
The Opponent-Process Theory of Color Vision Although essentially correct, the
trichromatic theory cannot explain some aspects of color vision, such as afterimages. To
see an afterimage, stare at the black dot in the flag in Figure 3.11 for thirty seconds, and
then look at the black dot in the white space below it. What was yellow in the original
image will be blue in the afterimage. What was green before will appear red, and what
was black will now appear white.
This type of observation led Ewald Hering to offer another theory of color vision,
called the opponent-process theory. Hering suggested that color-sensitive visual elements in the eye are arranged into three kinds of pairs and that the members of each
pair oppose, or inhibit, each other. Each element signals one color or the other (red or
green, blue or yellow, black or white), but never both. This theory explains color afterimages. When one member of an opponent pair is no longer stimulated, the other is
activated. So, in Figure 3.11, if the original image you look at is green, the afterimage
will be red.
Summing Up Together, the trichromatic and opponent-process theories encompass
FIGURE
3.11
Afterimages Produced by the
Opponent-Process Nature of
Color Vision
Stare at the black dot in the
flag for at least thirty seconds,
by
and then focus on the dot in
the white space below it. The afterimage
you will see can be explained by the opponent process theory of color vision. What
colors appeared in the afterimage you saw?
doing
2
learn
most of what we now know about the complex process of color vision. We see color
because our three types of cones have different sensitivities to different wavelengths.
We sense different colors when the three cone types are stimulated in different ratios.
Because there are three types of cones, any color can be produced by mixing three pure
wavelengths of light. But there is more to it than that. The cones connect to ganglion
cells containing pairs of opposing elements that respond to different colors and inhibit
each other. This arrangement provides the basis for afterimages. Therefore, the trichromatic theory explains color vision as it relates to rods and cones, whereas the opponentprocess theory explains color vision as it relates to the ganglion cells. Both theories are
needed to account for the complexity of our visual sensations of color. (“In Review:
Seeing” summarizes our discussion of vision.)
F e a t u r e Ty p e : I n R e v i e w
Colorblindness
opponent-process theory A theory of
color vision stating that the visual elements sensitive to color are grouped
into red-green, blue-yellow, and blackwhite pairs.
Cones normally contain three kinds of chemicals, each of which responds best to a
particular wavelength of light. People who have cones containing only two of these
three color-sensitive chemicals are described as colorblind (Carroll et al., 2004). They
are not really blind to all color, but they discriminate fewer colors than do other people, as Figure 3.12 shows. Red-green colorblindness, for example, means that reds and
greens look the same brownish gray color. Colorblindness is more common in men
than in women.
97
in review
Seeing
Online Study Center
Improve Your Grade
Tutorial: Opponent
Processes
SEEING
Aspect of
Sensory System
Elements
Key Characteristics
Energy
Visible light:
electromagnetic radiation
with a wavelength of about
400 nm to about 750 nm
The intensity, wavelength,
and complexity of light
waves determine the
brightness, hue, and
saturation of visual
sensations.
Accessory structures
of the eye
Cornea, pupil, iris, lens
Light rays are bent to
focus on the retina.
Conversion of
visual stimuli
to neural activity
Photoreceptors (rods and
cones) in the retina
Rods are more sensitive
to light than cones, but
cones discriminate among
colors. Sensations of color
depend first on the cones,
which respond differently
to different light
wavelengths, and then on
processing by ganglion
cells.
Pathway to the
brain
Optic nerve to optic chiasm
to thalamus to primary
visual cortex
Neurons in the brain
respond to particular
aspects of the visual
stimulus, such as shape.
?
FIGURE
3.12
Are You Colorblind?
At the upper left is a photo as
it appears to people whose
by
cones have all three types of
color-sensitive chemicals. The other photos
simulate how colors appear to people who
are missing chemicals for short wavelengths (lower left), long wavelengths (upper right), or medium wavelengths (lower
right). If any of these photos look to you
just like the one at the upper left, you may
have a form of colorblindness.
doing
2
learn
1. The ability to see in very dim light depends on photoreceptors called
2. Color afterimages are best explained by the
theory of color vision.
3. Nearsightedness and farsightedness occur when images are not focused on
the eye’s
.
.
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