Page 626 - Fundamentals of anatomy physiology

source of carotene, leading to the old adage that carrots are Chapter 17 The Special Senses 613
good for your eyes.
Figure 17–18 A Standard Test for Color Vision. People who
Light and Dark Adaptation
lack one or more types of cones cannot see the number 12 in this
The sensitivity of your visual system varies with the intensity of pattern.
illumination. After 30 minutes or more in the dark, almost all
visual pigments will have recovered from photobleaching. They 17types of cones. For example, you perceive yellow from a com-
are fully receptive to stimulation. This is the dark-adapted
state. When dark-adapted, the visual system is extremely sensi- bination of inputs from highly stimulated green cones, less
tive. For example, a single rod will hyperpolarize in response strongly stimulated red cones, and relatively unaffected blue
to a single photon of light. Even more remarkable, if as few cones (Figure 17–15, p. 609). If all three cone populations are
as seven rods absorb photons at one time, you will see a flash stimulated, we perceive the color as white. We also perceive
of light. white if rods, rather than cones, are stimulated. For this reason,
everything appears black-and-white when you enter dimly lit
When the lights come on, at first they seem almost un- surroundings or walk by starlight.
bearably bright, but over the next few minutes your sensitivity
decreases as bleaching occurs. Eventually, the rate of bleaching Persons who are unable to distinguish certain colors have
is balanced by the rate at which the visual pigments reassemble. a form of color blindness. The standard tests for color vision
This condition is the light-adapted state. If you moved from involve picking numbers or letters out of a complex colored
the depths of a cave to the full sunlight of midday, your receptor picture such as the one in Figure 17–18. Color blindness oc-
sensitivity would decrease by a factor of 25,000. curs when one or more types of cones are nonfunctional. The
cones may be absent, or they may be present but unable to
A variety of central responses further adjust light sensi- manufacture the necessary visual pigments. In the most com-
tivity. The pupils constrict, by the pupillary constrictor reflex, mon type of color blindness (red–green color blindness), the
reducing the amount of light entering your eye to one-thirtieth red cones are missing, so the individual cannot distinguish red
the maximum dark-adapted level. Dilating the pupil fully can light from green light. Inherited color blindness involving one
produce a thirtyfold increase in the amount of light entering or two cone pigments is not unusual. Ten percent of all males
the eye, and facilitating some of the synapses along the visual show some color blindness, but only about 0.67 percent of all
pathway can perhaps triple its sensitivity. In these ways, the females have color blindness. Total color blindness is extremely
sensitivity of the entire system may increase by a factor of more rare. Only 1 person in 300,000 does not manufacture any cone
than 1 million. pigments. We consider the inheritance of color blindness in
Chapter 29.
Retinitis pigmentosa (RP) is an inherited disease char-
acterized by progressive retinal degeneration. As the visual The Visual Pathways
­receptors gradually deteriorate, blindness eventually results.
The mutations that are responsible change the structure of The visual pathways begin at the photoreceptors and end at
the photoreceptors—specifically, the visual pigments of the the visual cortex of the cerebral hemispheres. In other sensory
­membrane discs. We do not know how the altered pigments pathways we have examined, only one synapse lies between
lead to the destruction of photoreceptor cells.

Color Vision

An ordinary lightbulb or the sun emits photons of all wave-
lengths. These photons stimulate both rods and cones. Your
eyes also detect photons that reach your retina after they bounce
off objects around you. If photons of all colors bounce off an
object, the object will appear white to you. If the object absorbs
all the photons (so that none reaches the retina), the object will
appear black.

An object appears to have a particular color when it reflects
(or transmits) photons from one portion of the visible spec-
trum and absorbs the rest. Color discrimination takes place
through the integration of information arriving from all three