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CHAPTER TWELVE
Nervous System III
and green light pigments are sensitive to orange light waves.
On the other hand, red pigment absorbs orange light waves
more effectively.
The color perceived depends upon which sets of cones
the light in a given image stimulates. If all three types of sets
of cones are stimulated, the light is perceived as white, and
if none are stimulated, it is seen as black.
Examination of the retinas of different people reveals
that individuals have unique patterns of cone types, all
apparently able to provide color vision. Some parts of the
retina are even normally devoid of one particular type, yet
the brain integrates information from all over to “F
ll in the
gaps,” creating a continuous overall image. People who lack
a cone type due to a mutation are colorblind.
As primates, we humans enjoy a more multicolored world than many
other mammals. This is because the visual systems of nonprimate
mammals funnel input from groups of photoreceptor cells into the
CNS. That is, several photoreceptors signal the same bipolar neurons,
which, in turn, pool their input to ganglion cells. Primates are the
only mammals to have three types of cones (others have two), and
it appears that primates excel in color vision because the cones con-
nect individually to neural pathways to the brain.
Stereoscopic Vision
Stereoscopic vision
(stereopsis) simultaneously perceives
distance, depth, height, and width of objects. Such vision
is possible because the pupils are 6–7 centimeters apart.
Consequently, close objects (less than 20 feet away) produce
slightly different retinal images. That is, the right eye sees a
little more of one side of an object, while the left eye sees a
little more of the other side. The visual cortex superimposes
and interprets the two images. The result is the perception of
a single object in three dimensions
(f g. 12.40)
.
Stereoscopic vision requires vision with two eyes (bin-
ocular vision), so a one-eyed person is less able to judge dis-
tance and depth accurately. To compensate, a person with
one eye can use the relative sizes and positions of familiar
objects as visual clues.
A woman had a stroke that damaged part of her visual cortex, so
that she could no longer integrate images to perceive motion. She
saw movement as a series of separate, static images. Her deF
cit had
profound e±
ects on her life. She could not pour a drink, because she
could not tell when the cup would over²
ow. She could not cross a
street because she could not detect cars moving toward her.
Visual Nerve Pathways
As mentioned in chapter 11 (p. 414), the axons of the gan-
glion cells in the retina leave the eyes to form the
optic
nerves.
Just anterior to the pituitary gland, these nerves give
rise to the X-shaped
optic chiasma,
and in the chiasma, some
channels close, and the receptor cell membrane hyperpolar-
izes (see chapter 10, p. 368). The degree of hyperpolarization
is directly proportional to the intensity of the light stimulat-
ing the receptor cells.
The hyperpolarization reaches the synaptic end of the
cell, inhibiting release of neurotransmitter. Through a com-
plex mechanism, decreased release of neurotransmitter
by photoreceptor cells either stimulates or inhibits nerve
impulses (action potentials) in nearby retinal neurons.
Consequently, complex patterns of nerve impulses travel
away from the retina, through the optic nerve, and into the
brain, where they are interpreted as vision.
In bright light, nearly all of the rhodopsin in the rods
decomposes, sharply reducing the sensitivity of these recep-
tors (the rhodopsin loses its purplish color as a result, and
is said to have “bleached”). The cones continue to function,
however, and in bright light, we therefore see in color. In
dim light, rhodopsin can be regenerated from opsin and reti-
nal faster than it is broken down. This regeneration requires
cellular energy, which ATP provides (see chapter 4, p. 119).
Under these conditions, the rods continue to function and
the cones remain unstimulated. Hence, we see only shades
of gray in dim light.
The light sensitivity of an eye whose rods have con-
verted the available opsin and retinal to rhodopsin increases
about 100,000 times, and the eye is said to be
dark adapted.
A person needs a dark-adapted eye to see in dim light. ±or
example, when going from daylight into a darkened theater,
it may be difF
cult to see well enough to locate a seat, but
soon the eyes adapt to the dim light, and vision improves.
Later, leaving the theater and entering the sunlight may
cause discomfort or even pain. This occurs at the moment
that most of the rhodopsin decomposes in response to the
bright light. At the same time, the light sensitivity of the eyes
decreases greatly, and they become
light adapted.
Too little vitamin A in the diet reduces the amount of retinal, impair-
ing rhodopsin production and sensitivity of the rods. The result is
poor vision in dim light, called nightblindness.
The light-sensitive pigments of cones, called
iodopsins,
are similar to rhodopsin in that they are composed of retinal
combined with a protein; the protein, however, differs from
the protein in the rods. The three sets of cones in the retina
all contain an abundance of one of three different visual pig-
ments.
The wavelength of a particular type of light determines
the color perceived from it. ±or example, the shortest wave-
lengths of visible light are perceived as violet, whereas the
longest wavelengths of visible light are seen as red. One
type of cone pigment (erythrolabe) is most sensitive to red
light waves, another (chlorolabe) to green light waves, and
a third (cyanolabe) to blue light waves. The sensitivities of
these pigments overlap somewhat. ±or example, both red
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