Color blindness in humans is
the inability to perceive differences between some or all colors that other
people can distinguish. It is most often of genetic nature, but might also
occur because of eye, nerve, or brain damage, or due to exposure to certain
chemicals. The English chemist John Dalton in 1794 published the first
scientific paper on the subject, "Extraordinary facts relating to the
vision of colors", after the realization of his own color blindness.
Color blindness is usually
labeled as a disability; however, in select situations colorblind people
have advantages over people with a full color range. Color blind hunters
are better at picking out prey against a confusing background, and the
military have found that color blind soldiers can sometimes see through
camouflage that fools everyone else.
Monochromats may have a minor
advantage in dark vision, but only in the first five minutes of dark
Rates of occurrence
Although exact numbers vary
in various populations, color blindness affects a significant proportion of
people. Among Americans, approximately 10% of males suffer from some form
of color perception deficiency. Isolated communities with a restricted gene
pool sometimes produce high proportions of color blindness, including the
less usual types: examples include rural Finland and some of the Scottish
Causes of color blindness
There are many types of
color blindness. The most common varieties are hereditary (genetic)
photoreceptor disorders. Higher brain areas implicated in color processing
include the parvocellular pathway of the lateral geniculate nucleus of the
thalamus, and visual area V4 of the visual cortex. Acquired color blindness
is generally unlike the more typical genetic disorders. For example, it is
possible to acquire color blindness only in a portion of the visual field
but maintain normal color vision elsewhere. Some forms of acquired color
blindness are reversible. In order to understand retinal color blindness,
it is necessary to know that the normal human retina contains two kinds of
light sensitive cells, the rod cells (active in low light) and the cone
cells (active in normal daylight). There are three kinds of cones, each
containing a different pigment. The cones are activated when the pigments
absorb light. The sensitivity of normal color vision actually depends on
the overlap between the absorption spectra of these three pigments:
different colors are recognized when the different types of cone are
stimulated to different extents.
The different kinds of color
blindness result from one or more of the different cone systems either not
functioning at all, or functioning in an unusual way. When one cone system
is compromised, dichromacy results. The most frequent forms of color
blindness result from problems with either the middle or long wavelength
cone systems, and involve difficulties in discriminating reds, yellows, and
greens from one another. They are collectively referred to as
"red-green color blindness", though the term is an
over-simplification and somewhat misleading. Other forms of color blindness
are rare. They include problems in discriminating blues from yellows, and
complete color blindness, or monochromacy, where one cannot distinguish any
color from gray.
Red-green color blindness
Types of red-green
There are several types of
red-green color blindness:
· Protanopia: lacking
the long-wavelength sensitive retinal cones, those with this condition are
unable to distinguish between colors in the green-yellow-red section of the
spectrum. They have a neutral point at a wavelength of 492 nanometers that
is they cannot discriminate light of this wavelength from white. Their
sensitivity to light in the orange and red part of the spectrum is also
reduced. These unilateral dichromats report that with only their protanopic
eye open, they see wavelengths below the neutral point as blue and those
above it as yellow. This is a rare form of colorblindness.
lacking the medium-wavelength cones, those affected are again unable to
distinguish between colors in the green-yellow-red section of the spectrum.
Their neutral point is at a slightly longer wavelength, 498 nanometers.
This is one of the rare forms of colorblindness making up about 1% of the
male population. Deuteranopic unilateral dichromats report that with only
their deuteranopic eye open, they see wavelengths below the neutral point
as blue and those above it as yellow.
· Protanomaly: having
a mutated form of the long-wavelength pigment, whose peak sensitivity is at
a shorter wavelength than in the normal retina, protanomalous individuals
are less sensitive to red light than normal. This means that they are less
well able to discriminate colors, and they do not see mixed lights as
having the same colors as normal observers. They also suffer from a
darkening of the red end of the spectrum. This causes reds to reduce in
intensity to the point where they can be mistaken for black. Protoanomaly
is a fairly rare form of colorblindness making up about 1% of the male
having a mutated form of the medium-wavelength pigment. The
medium-wavelength pigment is shifted towards the red end of the spectrum
resulting in a reduction in sensitivity to the green area of the spectrum.
Unlike protanomaly the intensity of colors is unchanged. This is the most
common form of colorblindness making up about 8% of the male population.
red-green color blindness
Genetic red-green color
blindness affects men much more often than women, because the genes for the
red and green color receptors are located on the X chromosome, of which men
have only one and women have two. Such a trait is called sex-linked.
Genetic females (46, XX) are red-green colorblind, only if both their X
chromosomes are defective with a similar deficiency, whereas genetic males
(46, XY) are color blind if their only X chromosome is defective.
Blue-yellow color blindness
Color blindness involving the
inactivation of the short-wavelength sensitive cone system (whose
absorption spectrum peaks in the bluish-violet) is called tritanopia or,
loosely, blue-yellow color blindness. It is equally distributed among males
and females, because the gene coding for the short-wavelength receptor is
not sex-linked (being located on chromosome 7).
This is either Autosomal
Dominant or X linked Dominant.
Complete inability to
distinguish any colors is called monochromacy. It occurs in two forms: cone
monochromacy, where only a single cone system appears to be functioning, so
that no colors can be distinguished, but vision is otherwise more or less
normal; and achromatopsia, or maskun, or rod monochromacy where the retina
contains no cone cells, so that in addition to the absence of color
discrimination, vision in lights of normal intensity is difficult.
While normally rare, complete
color blindness (maskun) is very common in Pohnpei: about 1/12 of the
population there has maskun.