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What is Defective
Color Vision?
Normal perception of
color is something most of us take for granted. Color labels we attach
to objects such as “blue sky” and “green grass” have specific meaning
only to people with normal color vision.
“Colorblind” is an
exaggerated term handed down over time, referring to individuals who
have abnormal color vision. About eight percent of males and less than
one percent of females have faulty color perception from birth. The
degree to which a person may possess abnormal color vision ranges from
slight difficulty in recognizing shades of color to complete loss of
color vision. If one is reasonably correct in identifying colors, this
slight difference may not be detectable. It is when a person
incorrectly identifies a color that these differences become apparent
to others.
What Causes
Defective Color Vision?
While the perception of
color involves complex activities of the eye and the brain, the causes
of congenital (from birth) defect of color vision are more simply
explained. The retina at the back of the eye (like the film in a
camera) contains types of seeing elements (cells) called rods and
cones. The rods are responsible for night vision and operate under
conditions of dim light. The cones are responsible for color vision
and operate in daylight conditions. There are three types of cones:
blue, green and red, which act together to allow us to perceive a full
range of color from deep indigo to bright red.
The formation of each
cone pigment (blue, green, or red) is controlled by heredity. An
abnormal gene may subtly alter or completely eliminate a specific
pigment. An individual who has an inherited an abnormal gene can have
difficulty distinguishing certain shades such as reds and greens.
Are There Other
Forms of Color Vision Abnormalities?
Many people have color
vision deficiencies that are not hereditary. For example, aging
changes may produce color vision abnormalities. By far the most common
color defect accompanies the normal aging of the lens called
cataracts. In a child, the lens inside the eye is crystal cleat but
thereafter the lens gradually darken making it difficult for some
older people to distinguish dark blues from dark greens, or dark
grays.
Some medications may
affect color vision. Changes in color vision may also accompany
retinal or optic nerve disease. Therefore, if you suspect a color
vision problem you should consult your ophthalmologist.
How Is Color Vision
Tested?
There are several ways
to test color vision. A common method requires recognition of colored
numbers or symbols against a confusing background. If a color test is
taken using a colored filter in front of the eye (usually a red
contact lens) it is possible to fool the test by making the colors in
the colored plates stand out more boldly and achieve a higher score.
However, color-sense outside the test situation is not normal.
Can Anything Be Done
About Defective Color Vision?
Hereditary color vision
abnormalities cannot be cured. It is impossible to restore to the eye
those elements in the retina which nature did not provide at birth.
Some acquired color vision defects may be helped, as with surgical
removal of cataracts. However wearing a colored contact lens can
reduce the sharpness of vision, produce sever distortion of 3-D
perception and is only minimally effective in helping to recognize and
identify most light shades of color with which most people with
abnormal color vision have trouble.
Despite the lack of
treatment for hereditary color vision defect, these patients go on to
have perfectly normal lives and usually have no trouble with the
ordinary tasks of daily life, including driving and reading.
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