Trichromat
Encyclopedia
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 (called cone cell
s in vertebrates) 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 cell
s may contribute to color vision
, giving a small region of tetrachromacy in the color space.
s and closely related primates
are usually trichromats, as are some of the females of most species of New World monkey
s, and both male and female howler monkey
s.
Recent research suggests that trichromacy may also be quite general among marsupial
s. Most other mammals are currently thought to be dichromat
s, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active). 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 (such as honeybees) are also trichromats, being sensitive to ultraviolet
, blue and green instead of blue, green and red.
Trichromatic color vision is the ability of humans and some other animals to see different color
s, mediated by interactions among three types of color-sensing cone cell
s. 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
(1956).
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
(that is, the pigment is most likely to produce a cell
ular response when it is hit by a photon
with the specific wavelength to which that pigment is most sensitive). The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) 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 (long-wavelength) 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 each of the three cone types in the human retina can pick up about 100 different gradations. Assuming the brain can combine those variations exponentially
, the average human should be able to distinguish about one million different colors.
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
information, derived from the three different cone
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
types. Organisms with trichromacy are called trichromats.
The normal explanation of trichromacy is that the organism's retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
contains three types of color receptors (called cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s in vertebrates) with different absorption spectra
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
. 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 cell
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s may contribute to color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
, giving a small region of tetrachromacy in the color space.
Humans and other animals that are trichromats
HumanHuman
Humans are the only living species in the Homo genus...
s and closely related primates
Catarrhini
Catarrhini is one of the two subdivisions of the higher primates . It contains the Old World monkeys and the apes, which in turn are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas, chimpanzees, bonobos, and humans...
are usually trichromats, as are some of the females of most species of New World monkey
New World monkey
New World monkeys are the five families of primates that are found in Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Platyrrhini parvorder and the Ceboidea superfamily, which are essentially synonymous since...
s, and both male and female howler monkey
Howler monkey
Howler monkeys are among the largest of the New World monkeys. Fifteen species are currently recognised. Previously classified in the family Cebidae, they are now placed in the family Atelidae. These monkeys are native to South and Central American forests...
s.
Recent research suggests that trichromacy may also be quite general among marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s. Most other mammals are currently thought to be dichromat
Dichromat
Dichromacy is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights...
s, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active). Most studies of carnivores, as of other mammals, reveal dichromacy, examples including the domestic dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
, the ferret
Ferret
The ferret is a domesticated mammal of the type Mustela putorius furo. Ferrets are sexually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur...
, and the Spotted Hyena
Spotted Hyena
The spotted hyena also known as laughing hyena, is a carnivorous mammal of the family Hyaenidae, of which it is the largest extant member. Though the species' prehistoric range included Eurasia extending from Atlantic Europe to China, it now only occurs in all of Africa south of the Sahara save...
. Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, blue and green instead of blue, green and red.
Types of Cones specifically found in Primates
Primates i.e. Monkeys, Apes and Man are the only known mammalian trichromates. Their eyes consist of 3 different kinds of cones, each containing a different photopigment(opsin). Their peak sensitivities lie in the violet (short wavelength S-cones), green (medium wavelength M-cones) and yellow-green (long wavelength L-cones) regions of the colour spectrum. (Schnapf et all ,1987). S-cones make up 5-10% of the cones and form a regular mosaic. Special bi-polar and ganglion cells pass those siganls from S-cones and their is evidence that they have a separate signal pathway through the thalamus to the visual cortex as well. On the other hand But these L and M cones are hard to get distinguished 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
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
s, mediated by interactions among three types of color-sensing cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s. The trichromatic color theory began in the 18th century, when Thomas Young
Thomas Young (scientist)
Thomas Young was an English polymath. He is famous for having partly deciphered Egyptian hieroglyphics before Jean-François Champollion eventually expanded on his work...
proposed that color vision was a result of three different photoreceptor cells. Hermann von Helmholtz
Hermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...
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
Gunnar Svaetichin (scientist)
Gunnar Svaetichin was a Swedish-Finnish-Venezuelan physiologist who, in 1956, showed by examining the external layers of fish retinas that electroretinograms display particular sensitivity to three different groups of wavelengths in the areas of blue, green and red. This provided the first...
(1956).
Each of the three types of cones in the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
contains a different type of photosensitive pigment, which is composed of a transmembrane protein
Transmembrane protein
A transmembrane protein is a protein that goes from one side of a membrane through to the other side of the membrane. Many TPs function as gateways or "loading docks" to deny or permit the transport of specific substances across the biological membrane, to get into the cell, or out of the cell as...
called opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
and a light-sensitive molecule called 11-cis retinal. Each different pigment is especially sensitive to a certain wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
(that is, the pigment is most likely to produce a cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
ular response when it is hit by a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
with the specific wavelength to which that pigment is most sensitive). The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) 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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
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 (long-wavelength) 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 each of the three cone types in the human retina can pick up about 100 different gradations. Assuming the brain can combine those variations exponentially
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
, the average human should be able to distinguish about one million different colors.
See also
- Monochromacy
- Dichromacy
- Tetrachromacy
- Pentachromacy
- Evolution of color vision in primatesEvolution of color vision in primatesThe evolution of color vision in primates is unique compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs...
External links
- The Straight Dope: "Are cats and dogs really color-blind? How do they know?"
- Scientific Paper: "Retinal mechanisms for chromatic and achromatic vision"