216
Chapter 7
band of wavelengths. In other words, they receive input from
all three types of cones, and they signal not specifi c color but
general brightness. Ganglion cells of a second type code spe-
cifi c colors. These latter cells are also called
opponent color
cells
because they have an excitatory input from one type
of cone receptor and an inhibitory input from another. For
example, the cell in
Figure 7–31
increases its rate of fi ring
when viewing a blue light but decreases it when a red light
replaces the blue. The cell gives a weak response when stimu-
lated with a white light because the light contains both blue
and red wavelengths. Other more complicated patterns also
exist. The output from these cells is recorded by multiple—
and as yet unclear—strategies in visual centers of the brain.
Our ability to discriminate color also depends on the
inten-
sity
of light striking the retina. In brightly lit conditions,
the differential response of the cones allows for good color
vision. In dim light, however, only the highly sensitive rods
are able to respond. Though rods are activated over a range
of wavelengths that overlap with those that activate the cones
(see Figure 7–30), there is no mechanism for distinguishing
between frequencies. Thus, objects that appear vividly colored
in bright daylight are perceived in shades of gray at night.
Eye Movement
The fovea centralis of the retina (see Figure 7–22) is special-
ized in several ways to provide the highest visual acuity. It is
comprised of densely packed cones with minimal convergence
through the bipolar and ganglion cell layers. In addition, light
rays are scattered less on the way to the fovea than in other
retinal regions, because the interneuron layers and the blood
vessels are displaced to the edges, forming a shallow pit. To
focus the most important point in the visual image (the fi xa-
tion point) on the fovea and keep it there, the eyeball must be
able to move. Six skeletal muscles attached to the outside of
each eyeball (identifi ed in
Figure 7–32
) control its movement.
These muscles perform two basic movements, fast and slow.
The fast movements, called
saccades,
are small, jerking
movements that rapidly bring the eye from one fi xation point
to another to allow a search of the visual fi eld. In addition,
saccades move the visual image over the receptors, thereby
preventing adaptation. Saccades also occur during certain
periods of sleep when dreaming occurs, though these move-
ments are not thought to be involved in “watching” the visual
imagery of dreams.
Slow eye movements are involved both in tracking visual
objects as they move through the visual fi eld and during com-
100
80
60
40
20
Blue
cones
420 nm
Rods
500 nm
Green
cones
531 nm
Red
cones
558 nm
400
500
Wavelength (nm)
Percent of maximum response
600
700
(a)
(b)
Figure 7–30
The sensitivities of the photopigments in the normal human retina.
(a) The frequency of action potentials in the optic nerve is directly
related to a photopigment’s absorption of light. Under bright
lighting conditions, the three types of cones respond over different
frequency ranges. In dim light, only the rods respond.
(b) Demonstration of cone cell fatigue and after-image. Hold very
still and stare at the triangle inside the yellow circle for 30 seconds.
Then, shift your gaze to the square and wait for the image to appear
around it.
Figure 7–30b
physiological
inquiry
What color was the image you saw while you stared at the square?
Why did you perceive that particular color?
Answer can be found at end of chapter.
Light off
Blue light
Red light
White light
Time
(a)
Light off
Light on
(b)
(c)
Figure 7–31
Response of a single opponent color ganglion cell to blue, red, and
white lights.
Redrawn from Hubel and Wiesel.
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