Trichromacy or trichromaticism is the condition of possessing three
independent channels for conveying color information, derived from the three
different cone types. Organisms with trichromacy are called trichromats.
The normal explanation of trichromacy is that the organism's retina contains
three types of color receptors with different absorption spectra. In
actuality the number of such receptor types may be greater than three, since
different types may be active at different light intensities. In
vertebrates with three types of cone cells, at low light intensities the rod
cells may contribute to color vision. Humans and other animals that are
trichromats Humans and some other mammals have
evolved trichromacy based partly on pigments inherited from early
vertebrates. In fish and birds, for example, four pigments are used for
vision. These extra cone receptor visual pigments detect energy of other
wavelengths, including sometimes ultraviolet. Eventually two of these
pigments were lost and another was gained, resulting in trichromacy among
some primates. Humans and closely related primates are usually
trichromats, as are some of the females of most species of New World monkeys,
and both male and female howler monkeys. Recent research suggests that
trichromacy may also be quite general among marsupials. A study conducted
regarding trichromacy in Australian marsupials suggests the medium
wavelength sensitivity, MWS, cones of the honey possum and the fat-tailed
dunnart are features coming from the inherited reptilian retinal arrangement.
The possibility of trichromacy in marsupials potentially has another
evolutionary basis than that of primates. Further biological and
behavioural tests may verify if trichromacy is a common characteristic
of marsupials. Most other mammals are currently thought
to be dichromats, with only two types of cone. Most studies of carnivores, as of
other mammals, reveal dichromacy, examples including the domestic dog, the
ferret, and the Spotted Hyena. Some species of insects are also trichromats,
being sensitive to ultraviolet, blue and green instead of blue, green and red.
Research indicates that trichromacy allows animals to distinguish red fruit
and young leaves from other vegetation that is not beneficial to their
survival. Another theory is that detecting skin flushing and thereby mood
may have influenced the development of primate trichromate vision. The color
red also has other effects on primate and human behavior as discussed in the
color psychology article. Types of cones specifically found in
primates Primates are the only known placental
mammalian trichromats. Their eyes include three different kinds of cones,
each containing a different photopigment. Their peak sensitivities
lie in the blue, green and yellow-green regions of the color spectrum.. S cones
make up 5–10% of the cones and form a regular mosaic. Special bipolar and
ganglion cells pass those signals from S cones and there is evidence that they
have a separate signal pathway through the thalamus to the visual cortex as
well. On the other hand, the L and M cones are hard to distinguish by their
shapes or other anatomical means – their opsins differ in only 15 out of 363
amino acids, so nobody has yet succeeded in producing specific antibodies to
them. But Mollon and Bowmaker did find that L cones and M cones are randomly
distributed and are in equal numbers. Mechanism of trichromatic color vision
Trichromatic color vision is the ability of humans and some other animals to see
different colors, mediated by interactions among three types of
color-sensing cone cells. The trichromatic color theory began in the
18th century, when Thomas Young proposed that color vision was a result of three
different photoreceptor cells. Hermann von Helmholtz later expanded on Young's
ideas using color-matching experiments which showed that people with normal
vision needed three wavelengths to create the normal range of colors.
Physiological evidence for trichromatic theory was later given by Gunnar
Svaetichin. Each of the three types of cones in the
retina of the eye contains a different type of photosensitive pigment, which is
composed of a transmembrane protein called opsin and a light-sensitive
molecule called 11-cis retinal. Each different pigment is especially
sensitive to a certain wavelength of light. The three types of cones are L,
M, and S, which have pigments that respond best to light of long, medium,
and short wavelengths respectively. Since the likelihood of response of a
given cone varies not only with the wavelength of the light that hits it but
also with its intensity, the brain would not be able to discriminate different
colors if it had input from only one type of cone. Thus, interaction between
at least two types of cone is necessary to produce the ability to perceive
color. With at least two types of cones, the brain can compare the signals from
each type and determine both the intensity and color of the light. For
example, moderate stimulation of a medium-wavelength cone cell could mean
that it is being stimulated by very bright red light, or by not very intense
yellowish-green light. But very bright red light would produce a stronger
response from L cones than from M cones, while not very intense yellowish light
would produce a stronger response from M cones than from other cones. Thus
trichromatic color vision is accomplished by using combinations of
cell responses. It is estimated that the average human
can distinguish up to seven million different colors.
See also Monochromacy
Dichromacy Tetrachromacy
Pentachromacy Evolution of color vision in primates
Young–Helmholtz theory LMS color space
References External links
The Straight Dope: "Are cats and dogs really color-blind? How do they know?"
Svaetichin, Gunnar; MacNichol, Edward F.. "Retinal mechanisms for chromatic
and achromatic vision.". Annals of the New York Academy of Sciences 74:
385–404. doi:10.1111/j.1749-6632.1958.tb39560.x.
PMID 13627867.