Trichromacy

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Close-up of a trichromatic in-line shadow mask CRT display, which creates most visible colors through combinations and different levels of the three primary colors: red, green and blue Tricolour television close up.jpg
Close-up of a trichromatic in-line shadow mask CRT display, which creates most visible colors through combinations and different levels of the three primary colors: red, green and blue

Trichromacy or trichromatism is the possession of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. [1] Organisms with trichromacy are called trichromats.

Contents

The normal explanation of trichromacy is that the organism's retina contains three types of color receptors (called cone cells 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 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, sometimes including ultraviolet. Eventually two of these pigments were lost (in placental mammals) and another was gained, resulting in trichromacy among some primates. [2] 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. [3]

Recent research suggests that trichromacy may also be quite general among marsupials. [4] A study conducted regarding trichromacy in Australian marsupials suggests the medium wavelength sensitivity (MWS), cones of the honey possum (Tarsipes rostratus) and the fat-tailed dunnart (Sminthopsis crassicaudata) 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. [2]

Most other mammals are currently thought to be dichromats, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active). [5] Most studies of carnivores, as of other mammals, reveal dichromacy; examples include the domestic dog, the ferret, and the spotted hyena. [6] [7] Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet, blue and green instead of blue, green and red. [3]

Research indicates that trichromacy allows animals to distinguish brightly colored fruit and young leaves from other vegetation that is not beneficial to their survival. [8] 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. [9]

Types of cones specifically found in primates

Primates are the only known placental mammalian trichromats. [10] [ failed verification ] Their eyes include three different kinds of cones, each containing a different photopigment (opsin). Their peak sensitivities lie in the blue (short-wavelength S cones), green (medium-wavelength M cones) and yellow-green (long-wavelength L cones) regions of the color spectrum. [11] 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 no one has yet succeeded in producing specific antibodies to them. But Mollon and Bowmaker [12] did find that L cones and M cones are randomly distributed and are in equal numbers. [13]

Mechanism of trichromatic color vision

Normalised responsivity spectra of human cone cells Cones SMJ2 E.svg
Normalised responsivity spectra of human cone cells
Illustration of color metamerism: In column 1, a ball is illuminated by monochromatic light. Multiplying the spectrum by the cones' spectral sensitivity curves gives the response for each cone type. In column 2, metamerism is used to simulate the scene with blue, green and red LEDs, giving a similar response. Metamerism spectrum example.svg
Illustration of color metamerism:
In column 1, a ball is illuminated by monochromatic light. Multiplying the spectrum by the cones' spectral sensitivity curves gives the response for each cone type.
In column 2, metamerism is used to simulate the scene with blue, green and red LEDs, giving a similar response.

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. From the middle of the 19th century, in his Treatise on Physiological Optics, [14] [15] 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). [16]

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 cellular 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. [17] [18]

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 the average human can distinguish up to ten million different colors. [19]

See also

Related Research Articles

<span class="mw-page-title-main">Color</span> Visual perception of the light spectrum

Color or colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra, and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelengths, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.

<span class="mw-page-title-main">Color blindness</span> Decreased ability to see color or color differences

Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception. Color blindness is usually an inherited problem or variation in the functionality of one or more of the three classes of cone cells in the retina, which mediate color vision. The most common form is caused by a genetic condition called congenital red–green color blindness, which affects up to 1 in 12 males (8%) and 1 in 200 females (0.5%). The condition is more prevalent in males, because the opsin genes responsible are located on the X chromosome. Rarer genetic conditions causing color blindness include congenital blue–yellow color blindness, blue cone monochromacy, and achromatopsia. Color blindness can also result from physical or chemical damage to the eye, the optic nerve, parts of the brain, or from medication toxicity. Color vision also naturally degrades in old age.

<span class="mw-page-title-main">Visible spectrum</span> Portion of the electromagnetic spectrum that is visible to the human eye

The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light . The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well, known collectively as optical radiation.

<span class="mw-page-title-main">Color vision</span> Ability to perceive differences in light frequency

Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different frequencies independently of light intensity.

<span class="mw-page-title-main">Photoreceptor cell</span> Type of neuroepithelial cell

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

<span class="mw-page-title-main">Tetrachromacy</span> Type of color vision with four types of cone cells

Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.

<span class="mw-page-title-main">Cone cell</span> Photoreceptor cells responsible for color vision made to function in bright light

Cone cells or cones are photoreceptor cells in the retinas of vertebrates' eyes. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.

In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision as cone cells are only able to function at higher illumination levels. Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds.

Dichromacy is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats require only two primary colors to be able to represent their visible gamut. By comparison, trichromats need three primary colors, and tetrachromats need four. Likewise, every color in a dichromat's gamut can be evoked with monochromatic light. By comparison, every color in a trichromat's gamut can be evoked with a combination of monochromatic light and white light.

<span class="mw-page-title-main">Monochromacy</span> Type of color vision

Monochromacy is the ability of organisms to perceive only light intensity without respect to spectral composition. Organisms with monochromacy lack color vision and can only see in shades of grey ranging from black to white. Organisms with monochromacy are called monochromats. Many mammals, such as cetaceans, the owl monkey and the Australian sea lion are monochromats. In humans, monochromacy is one among several other symptoms of severe inherited or acquired diseases, including achromatopsia or blue cone monochromacy, together affecting about 1 in 30,000 people.

<span class="mw-page-title-main">Melanopsin</span> Mammalian protein found in Homo sapiens

Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene Opn4. In the mammalian retina, there are two additional categories of opsins, both involved in the formation of visual images: rhodopsin and photopsin in the rod and cone photoreceptor cells, respectively.

The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are three opponent channels, each comprising an opposing color pair: red versus green, blue versus yellow, and black versus white (luminance). The theory was first proposed in 1892 by the German physiologist Ewald Hering.

Visual phototransduction is the sensory transduction process of the visual system by which light is detected by photoreceptor cells in the vertebrate retina. A photon is absorbed by a retinal chromophore, which initiates a signal cascade through several intermediate cells, then through the retinal ganglion cells (RGCs) comprising the optic nerve.

<span class="mw-page-title-main">Young–Helmholtz theory</span> Postulated existence of three photoreceptor types in the eye

The Young–Helmholtz theory, also known as the trichromatic theory, is a theory of trichromatic color vision – the manner in which the visual system gives rise to the phenomenological experience of color. In 1802, Young postulated the existence of three types of photoreceptors in the eye, with different but overlapping response to different wavelengths of visible light.

<span class="mw-page-title-main">OPN1LW</span> Protein-coding gene in humans

OPN1LW is a gene on the X chromosome that encodes for long wave sensitive (LWS) opsin, or red cone photopigment. It is responsible for perception of visible light in the yellow-green range on the visible spectrum. The gene contains 6 exons with variability that induces shifts in the spectral range. OPN1LW is subject to homologous recombination with OPN1MW, as the two have very similar sequences. These recombinations can lead to various vision problems, such as red-green colourblindness and blue monochromacy. The protein encoded is a G-protein coupled receptor with embedded 11-cis-retinal, whose light excitation causes a cis-trans conformational change that begins the process of chemical signalling to the brain.

<span class="mw-page-title-main">Evolution of color vision in primates</span> Loss and regain of colour vision during the evolution of primates

The evolution of color vision in primates is highly unusual 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. Most teleost fish, reptiles and birds are therefore tetrachromatic while most mammals are strictly dichromats, the exceptions being some primates and marsupials, who are trichromats, and many marine mammals, who are monochromats.

Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components.

Gene therapy for color blindness is an experimental gene therapy of the human retina aiming to grant typical trichromatic color vision to individuals with congenital color blindness by introducing typical alleles for opsin genes. Animal testing for gene therapy began in 2007 with a 2009 breakthrough in squirrel monkeys suggesting an imminent gene therapy in humans. While the research into gene therapy for red-green colorblindness has lagged since then, successful human trials are ongoing for achromatopsia. Congenital color vision deficiency affects upwards of 200 million people in the world, which represents a large demand for this gene therapy.

<span class="mw-page-title-main">Congenital red–green color blindness</span> Most common genetic condition leading to color blindness

Congenital red–green color blindness is an inherited condition that is the root cause of the majority of cases of color blindness. It has no significant symptoms aside from its minor to moderate effect on color vision. It is caused by variation in the functionality of the red and/or green opsin proteins, which are the photosensitive pigment in the cone cells of the retina, which mediate color vision. Males are more likely to inherit red–green color blindness than females, because the genes for the relevant opsins are on the X chromosome. Screening for congenital red–green color blindness is typically performed with the Ishihara or similar color vision test. It is a lifelong condition, and has no known cure or treatment.

<span class="mw-page-title-main">Vertebrate visual opsin</span>

Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to opsin, as they were the first opsins discovered and are still the most widely studied opsins.

References

  1. ↑ "Color Glossary". Archived from the original on 4 October 2015. Retrieved 8 November 2006.
  2. 1 2 Arrese, Catherine; Thomas, Nathan; Beazley, Lyn; Shand, Julia (2002). "Trichromacy in Australian Marsupials". Current Biology. 12 (8): 657–660. Bibcode:2002CBio...12..657A. doi: 10.1016/S0960-9822(02)00772-8 . PMID   11967153. S2CID   14604695.
  3. 1 2 Rowe, Michael H (2002). "Trichromatic color vision in primates". News in Physiological Sciences. 17 (3): 93–98. doi:10.1152/nips.01376.2001. PMID   12021378. S2CID   15241669.
  4. ↑ Arrese, CA; Oddy, AY; Runham, PB; Hart, NS; Shand, J; Hunt, DM (2005). "Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus)". Proceedings of the Royal Society of London B. 272 (1595): 791–796. doi:10.1098/rspb.2004.3009. PMC   1599861 . PMID   15888411.
  5. ↑ Jacobs, Gerald H.; Nathans, Jeremy (2009). "The Evolution of Primate Color Vision". Scientific American. 300 (4): 56–63. Bibcode:2009SciAm.300d..56J. doi:10.1038/scientificamerican0409-56 (inactive 2 November 2024). ISSN   0036-8733. JSTOR   26001303. PMID   19363921.{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
  6. ↑ Calderone, JB; Jacobs, GH (2003). "Spectral properties and retinal distribution of ferret cones" (PDF). Visual Neuroscience. 20 (1): 11–17. doi:10.1017/s0952523803201024. PMID   12699079. S2CID   10415194.
  7. ↑ Calderone, JB; Reese, BE; Jacobs, GH (2003). "Topography of photoreceptors and retinal ganglion cells in the spotted hyena (Crocuta crocuta)". Brain, Behavior and Evolution. 62 (4): 182–192. doi:10.1159/000073270. PMID   14573992. S2CID   9167855.
  8. ↑ Sharpe LT, de Luca E, Hansen T, Jägle H, Gegenfurtner KR (2006). "Advantages and disadvantages of human dichromacy". Journal of Vision. 6 (3): 213–223. doi: 10.1167/6.3.3 . PMID   16643091.
  9. ↑ Diana Widermann, Robert A. Barton, and Russel A. Hill. Evolutionary perspectives on sport and competition. In Roberts, S. C. (2011). Roberts, S. Craig (ed.). Applied Evolutionary Psychology. Oxford University Press. doi:10.1093/acprof:oso/9780199586073.001.0001. ISBN   9780199586073.
  10. ↑ Ronald G. Boothe (2002). Perception of the visual environment. Springer. p. 219. ISBN   978-0-387-98790-3.
  11. ↑ Schnapf, J. L.; Kraft, T. W.; Baylor, D. A. (January 1987). "Spectral sensitivity of human cone photoreceptors". Nature. 325 (6103): 439–441. Bibcode:1987Natur.325..439S. doi:10.1038/325439a0. PMID   3808045. S2CID   11399054.
  12. ↑ Mollon, J. D.; Bowmaker, J. K. (December 1992). "The spatial arrangement of cones in the primate fovea". Nature. 360 (6405): 677–679. Bibcode:1992Natur.360..677M. doi:10.1038/360677a0. PMID   1465131. S2CID   4234999.
  13. ↑ Wässle, Heinz (11 February 1999). "Colour vision: A patchwork of cones". Nature. 397 (6719): 473–475. Bibcode:1999Natur.397..473W. doi:10.1038/17216. PMID   10028963. S2CID   4431471.
  14. ↑ von Helmholtz, Hermann (1909). Handbuch der Physiologischen Optik (3 ed.). Hamburg ; Leipzig: Leopold Voss. Retrieved 18 February 2020.
  15. ↑ von Helmholtz, Hermann (2013). Treatise on Physiological Optics. Courier Corporation. ISBN   978-0486174709 . Retrieved 18 February 2020.
  16. ↑ Svaetichin, G (1956). "Spectral response curves from single cones". Acta Physiologica Scandinavica. 39 (134): 17–46. PMID   13444020.
  17. ↑ Kandel ER, Schwartz JH, Jessell TM (2000). Principles of Neural Science (4th ed.). New York: McGraw-Hill. pp.  182–185. ISBN   978-0-8385-7701-1.
  18. ↑ Jacobs GH, Nathans J (March 2009). "Color Vision: How Our Eyes Reflect Primate Evolution". Scientific American.
  19. ↑ Leong, Jennifer. "Number of Colors Distinguishable by the Human Eye". hypertextbook. Retrieved 21 February 2013.
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