Monochromacy

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Monochromacy
Neophoca cinerea.JPG
Monochromacy is a disease state in human vision but is normal in pinnipeds (such as Neophoca cinerea shown here), cetaceans, owl monkeys and some other animals.
Specialty Ophthalmology

Monochromacy (from Greek mono, meaning "one" and chromo, meaning "color") is the ability of organisms or machines to perceive only light intensity, without respect to spectral composition (color). Organisms with monochromacy are called monochromats.

Contents

For example, about 1 in 30,000 people have monochromatic vision because the color-sensitive cone cells in their eyes do not function. Affected people can distinguish light, dark, and shades of gray but not color.

Many species, such as marine mammals, the owl monkey and the Australian sea lion (pictured at right) are monochromats under normal conditions. In humans, absence of color discrimination or poor color discrimination is one among several other symptoms of severe inherited or acquired diseases, as for example inherited achromatopsia, acquired achromatopsia or inherited blue cone monochromacy.

Humans

Vision in humans is due to a system that includes rod and cone photoreceptors, retinal ganglion cells and the visual cortex in the brain. Color vision is primarily achieved through cone cells. Cone cells are more concentrated in the fovea centralis, which is the central portion of the retina. This allows greater spatial resolution and color discrimination.

Rod cells are concentrated in the periphery of the human retina. Rod cells are more light sensitive than cone cells, and are mainly responsible for scotopic (night) vision. Cones in most humans have three types of opsins with different spectral sensitivities which allow for trichromatic color discrimination, whereas rods all have a similar, broad spectral response which does not allow for color discrimination. Because of the distribution of rods and cones in the human eye, people have good color vision near the fovea (where cones are) but not in the periphery (where the rods are). [1]

These types of color blindness can be inherited, resulting from alterations in cone pigments or in other proteins needed for the process of phototransduction: [2]

  1. Anomalous trichromacy, when one of the three cone pigments is altered in its spectral sensitivity but trichromacy (distinguishing color by both the green-red and blue-yellow distinctions) is not fully impaired.
  2. Dichromacy, when one of the cone pigments is missing and colour is reduced to the green-red distinction only or the blue-yellow distinction only.
  3. Monochromacy, when two of the cones are not functional. Vision reduced to blacks, whites, and greys.
  4. Rod Monochromacy (Achromatopsia.), when all three of the cones are non functional and light perception is achieved only with rod cells. Color vision is heavily or completely impaired, vision reduced to seeing only the level of light coming from an object. Dyschromatopsia is a less severe type of achromatopsia.

Monochromacy is one of the symptoms of diseases that occur when only one kind of light receptor in the human retina is functional at a particular level of illumination. It is one of the symptoms of either acquired or inherited disease as for example acquired achromatopsia, inherited autosomal recessive achromatopsia and recessive X-linked blue cone monochromacy. [3] [4] [5] [6]

There are two basic types of monochromacy. [7] [8] "Animals with monochromatic vision may be either rod monochromats or cone monochromats. These monochromats contain photoreceptors which have a single spectral sensitivity curve." [9]

In humans, who have three types of cones, the short (S, or blue) wavelength sensitive, middle (M, or green) wavelength sensitive and long (L, or red) wavelength sensitive cones [11] have three differing forms of cone monochromacy, named according to the single functioning cone class:

  1. Blue cone monochromacy (BCM), also known as S-cone monochromacy, [3] [4] is an X-linked cone disease. [12] It is a rare congenital stationary cone dysfunction syndrome, affecting less than 1 in 100,000 individuals, and is characterized by the absence of L- and M-cone function. [13] BCM results from mutations in a single red or red–green hybrid opsin gene, mutations in both the red and the green opsin genes or deletions within the adjacent LCR (locus control region) on the X chromosome. [3] [4] [10]
  2. Green cone monochromacy (GCM), also known as M-cone monochromacy, is a condition where the blue and red cones are absent in the fovea. The prevalence of this type of monochromacy is less than 1 in 1 million.
  3. Red cone monochromacy (RCM), also known as L-cone monochromacy, is a condition where the blue and green cones are absent in the fovea. Like GCM, RCM is also present in less than 1 in 1 million people. Animal research studies have shown that the nocturnal wolf and ferret have lower densities of L-cone receptors. [14]

Animals that are monochromats

It used to be confidently claimed that most mammals other than primates were monochromats. In the last half-century, however, evidence of at least dichromatic color vision in a number of mammalian orders has accumulated. While typical mammals are dichromats, with S and L cones, two of the orders of marine mammals, the pinnipeds (which includes the seal, sea lion and walrus) and cetaceans (which includes dolphins and whales) clearly are cone monochromats, since the short-wavelength sensitive cone system is genetically disabled in these animals.[ dubious ] The same is true of the owl monkeys, genus Aotus.

A recent study using through PCR analysis of genes OPN1SW, OPN1LW, and PDE6C determined that all mammals in the order Xenarthra (representing sloths, anteaters and armadillos) developed rod monochromany through a stem ancestor. [15]

Researchers Leo Peichl, Guenther Behrmann and Ronald H. H. Kroeger report that of the many animal species studied, there are three carnivores that are cone monochromats: raccoon, crab-eating raccoon and kinkajou and a few rodents are cone monochromats because they are lacking the S-cone. [14] These researchers also report that the animal's living environment also plays a significant role in the animals' eyesight. They use the example of water depth and the smaller amount of sunlight that is visible as one continues to go down. They explain it as follows, "Depending on the type of water, the wavelengths penetrating deepest may be short (clear, blue ocean water) or long (turbid, brownish coastal or estuarine water.)" [14] Therefore, the variety of visible availability in some animals resulted in them losing their S-cone opsins.

Monochromat capability

According to Jay Neitz, a color vision researcher at the University of Washington, each of the three standard color-detecting cones in the retina of trichromats can detect approximately 100 gradations of color. The brain can process the combinations of these three values so that the average human can distinguish about one million colors. [16] Therefore, a monochromat would be able to distinguish about 100 colors. [17]

See also

Related Research Articles

Color blindness Inability or decreased ability to see colour or colour differences

Color blindness, also known as color vision deficiency, is the decreased ability to see color or differences in color. Simple tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights can be more challenging. Color blindness may also make some educational activities more difficult. However, problems are generally minor, and most people find that they can adapt. People with total color blindness (achromatopsia) may also have decreased visual acuity and be uncomfortable in bright environments.

Color vision ability of an organism or machine to distinguish objects based on wavelengths of light

Color vision is an ability of animals to perceive differences between light composed of different wavelengths independently of light intensity. Color perception is a part of the larger visual system and is mediated by a complex process between neurons that begins with differential stimulation of different types of photoreceptors by light entering the eye. Those photoreceptors then emit outputs that are then propagated through many layers of neurons and then ultimately to the brain. Color vision is found in many animals and is mediated by similar underlying mechanisms with common types of biological molecules and a complex history of evolution in different animal taxa. In primates, color vision may have evolved under selective pressure for a variety of visual tasks including the foraging for nutritious young leaves, ripe fruit, and flowers, as well as detecting predator camouflage and emotional states in other primates.

Photoreceptor cell specialized type of cell found in the retina that is capable of visual phototransduction

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.

Tetrachromacy 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.

Trichromacy type of color vision

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

Cone cell

Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes. They respond differently to light of different wavelengths, and are thus responsible for color vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light. 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. There are about six to seven million cones in a human eye and are most concentrated towards the macula. The commonly cited figure of six million cone cells in the human eye was found by Osterberg in 1935. Oyster's textbook (1998) cites work by Curcio et al. (1990) indicating an average close to 4.5 million cone cells and 90 million rod cells in the human retina.

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 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. By comparison, trichromats require three pure spectral lights to match all colors that they can perceive, and tetrachromats require four.

Fovea centralis

The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.

Opsin

Opsins are a group of proteins, made light-sensitive, via the chromophore retinal 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 signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision.

OPN1SW protein-coding gene in the species Homo sapiens

Blue-sensitive opsin is a protein that in humans is encoded by the OPN1SW gene.

OPN1MW protein-coding gene in the species Homo sapiens

Green-sensitive opsin is a protein that in humans is encoded by the OPN1MW gene. OPN1MW2 is a similar opsin.

OPN1LW protein-coding gene in the species Homo sapiens

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.

Evolution of color vision in primates The loss and regain of colour vision during the evolution of primates

The 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. 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.

Bird vision

Vision is the most important sense for birds, since good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". The avian eye resembles that of a reptile, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.

Mammalian eye eye of a mammal

Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.

G-protein-coupled receptor kinase 7 is a serine/threonine-specific protein kinase involved in phototransduction. This enzyme catalyses the phosphorylation of cone (color) photopsins in retinal cones during high acuity color vision primarily in the fovea.

Gene therapy for color blindness is an experimental gene therapy aiming to convert congenitally colorblind individuals to trichromats by introducing a photopigment gene that they lack. Though partial color blindness is considered only a mild disability, it is a condition that affects many people, particularly males. Complete color blindness, or achromatopsia, is very rare but more severe. While never demonstrated in humans, animal studies have shown that it is possible to confer color vision by injecting a gene of the missing photopigment using gene therapy. As of 2018 there is no medical entity offering this treatment, and no clinical trials available for volunteers.

Evolution of human colour vision over time: humans have developed a trichromatic view of the world in comparison to a majority of other mammals that only see the world from a dichromatic view. Early human ancestors are believed to have viewed the world using UV vision as far back as 90 million years ago. It is thought that the shift to trichromatic vision capabilities and the ability to see blue light have evolved as an adaptive trait over time.

Blue cone monochromacy (BCM) is an inherited eye disease that causes severely impaired color discrimination, low vision, nystagmus and photophobia due to the absence of functionality of red (L) and green (M) cone photoreceptor cells in the retina. This form of retinal disorder is a recessive X-linked disease and manifests its symptoms in early infancy.

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