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 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.

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Humans

Human vision relies on a duplex retina, comprising two types of photoreceptor cells. Rods are primarily responsible for dim-light scotopic vision and cones are primarily responsible for day-light photopic vision. For all known vertebrates, scotopic vision is monochromatic, since there is typically only one class of rod cell. However, the presence of multiple cone classes contributing to photopic vision enables color vision during daytime conditions.

Most humans have three classes of cones, each with a different class of opsin. These three opsins have different spectral sensitivities, which is a prerequisite for trichromacy. An alteration of any of these three cone opsins can lead to colorblindness.

  1. Anomalous trichromacy, when all three cones are functional, but one or more is altered in its spectral sensitivity.
  2. Dichromacy, when one of the cones is non-functional and one of the red-green or blue-yellow opponent channels are fully disabled.
  3. Cone monochromacy, when two of the cones are non-functional and both chromatic opponent channels are disabled. Vision is reduced to blacks, whites, and greys.
  4. Rod monochromacy (Achromatopsia), when all three of the cones are non-functional and therefore photopic vision (and therefore color vision) is disabled.

Monochromacy of photopic vision is a symptom of both Cone Monochromacy and Rod Monochromacy, so these two conditions are typically referred to collectively as monochromacy. [1] [2]

Rod monochromacy

Rod monochromacy (RM), also called congenital complete achromatopsia or total color blindness, is a rare and extremely severe form of an autosomal recessively inherited retinal disorder resulting in severe visual handicap. People with RM have a reduced visual acuity, (usually about 0.1 or 20/200), have total color blindness, photo-aversion and nystagmus. The nystagmus and photo-aversion usually are present during the first months of life, and the prevalence of the disease is estimated to be 1 in 30,000 worldwide. [3] Since patients with RM have no cone function, they lack photopic vision, relying entirely on their rods and scotopic vision, [3] which is necessarily monochromatic. They therefore cannot see any color but only shades of grey.

Cone monochromacy

Cone monochromacy (CM) is a condition defined by the exhibition of only one class of cones. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues.

As humans typically exhibit three classes of cones, cone monochromats can hypothetically derive their photopic vision from any one of them, leading to three categories of cone monochromats: [4]

  1. Blue cone monochromacy (BCM), also known as S-cone monochromacy, is an X-linked cone disease. [5] 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. [6] 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]
  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 estimated to be 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, the prevalence of RCM is also estimated at less than 1 in 1 million.

Cone Monochromats with normal rod function can sometimes exhibit mild color vision due to conditional dichromacy. In mesopic conditions, both rods and cones are active and opponent interactions between the cones and rods can afford slight color vision. [7]

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. [8] Therefore, a monochromat would be able to distinguish about 100 colors. [9]

Mammals

Until the 1960s, popular belief held that most mammals outside of primates were monochromats. In the last half-century,[ when? ] however, a focus on behavioral and genetic testing of mammals has accumulated extensive evidence of at least dichromatic color vision in a number of mammalian orders. Mammals are now usually assumed to be dichromats (possessing S- and L-cones), with monochromats viewed as the exceptions.

Two mammalian orders containing marine mammals exhibit monochromatic vision:

Unlike the trichromacy exhibited in most primates, Owl monkeys (genus Aotus) are also monochromats[ citation needed ]. Several members of the family Procyonidae (raccoon, crab-eating raccoon and kinkajou) and a few rodents have been demonstrated as cone monochromats, having lost functionality of the S-cone (retaining the L-cone). [10]

The light available in an animal's habitat is a significant determiner of a mammal's color vision. Marine, nocturnal or burrowing mammals, which experience less light, have less evolutionary pressure to preserve dichromacy, so often evolve monochromacy.[ citation needed ]

A recent study using through PCR analysis of genes OPN1SW, OPN1LW, and PDE6C determined that all mammals in the cohort Xenarthra (representing sloths, anteaters and armadillos) developed rod monochromacy through a stem ancestor. [11]

See also

Related Research Articles

<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.

Achromatopsia, also known as rod monochromacy, is a medical syndrome that exhibits symptoms relating to five conditions, most notably monochromacy. Historically, the name referred to monochromacy in general, but now typically refers only to an autosomal recessive congenital color vision condition. The term is also used to describe cerebral achromatopsia, though monochromacy is usually the only common symptom. The conditions include: monochromatic color blindness, poor visual acuity, and day-blindness. The syndrome is also present in an incomplete form that exhibits milder symptoms, including residual color vision. Achromatopsia is estimated to affect 1 in 30,000 live births worldwide.

<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">Trichromacy</span> Possessing of three independent channels for conveying color information

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. Organisms with trichromacy are called trichromats.

<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">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.

<span class="mw-page-title-main">Duplex retina</span>

A duplex retina is a retina consisting of both rod cells and cone cells, which are the photoreceptor cells for two parallel but mostly separate visual systems. The rods enable the scotopic visual system, which is active in dim light. The cones enable the photopic visual system, which is active in bright light. While one is active, the other is generally inactive; either the rods are photobleached, or oversaturated, in bright light, or the cones are not sensitive enough to hyperpolarize, or instigate the phototransduction cascade, in dim light. However, at mesopic (twilight) conditions, both visual systems are active. In this region of overlap, both systems are active and combine to contribute to mesopic vision.

Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light conditions. Mesopic levels range approximately from 0.01 to 3.0 cd/m2 in luminance. Most nighttime outdoor and street lighting conditions are in the mesopic range.

<span class="mw-page-title-main">OPN1MW</span> 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.

<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.

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.

Blue cone monochromacy (BCM) is an inherited eye disease that causes severe color blindness, poor visual acuity, nystagmus, hemeralopia, and photophobia due to the absence of functional red (L) and green (M) cone photoreceptor cells in the retina. BCM is a recessive X-linked disease and almost exclusively affects XY karyotypes.

<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. ↑ Alpern M (Sep 1974). "What is it that confines in a world without color?" (PDF). Invest Ophthalmol. 13 (9): 648–74. PMID   4605446.
  2. ↑ Hansen E (Apr 1979). "Typical and atypical monochromacy studied by specific quantitative perimetry". Acta Ophthalmol (Copenh). 57 (2): 211–24. doi:10.1111/j.1755-3768.1979.tb00485.x. PMID   313135. S2CID   40750790.
  3. 1 2 3 Eksandh L, Kohl S, Wissinger B (June 2002). "Clinical features of achromatopsia in Swedish patients with defined genotypes". Ophthalmic Genet. 23 (2): 109–20. doi:10.1076/opge.23.2.109.2210. PMID   12187429. S2CID   25718360.
  4. ↑ Nathans, J; Thomas, D; Hogness, D S (1986). "Molecular genetics of human color vision: the genes encoding blue, green, and red pigments". Science. 232 (4747): 193–202. Bibcode:1986Sci...232..193N. CiteSeerX   10.1.1.461.5915 . doi:10.1126/science.2937147. PMID   2937147.
  5. ↑ Weleber RG (June 2002). "Infantile and childhood retinal blindness: a molecular perspective (The Franceschetti Lecture)". Ophthalmic Genet. 23 (2): 71–97. doi:10.1076/opge.23.2.71.2214. PMID   12187427. S2CID   30741530.
  6. ↑ Michaelides M, Johnson S, Simunovic MP, Bradshaw K, Holder G, Mollon JD, Moore AT, Hunt DM (January 2005). "Blue cone monochromatism: a phenotype and genotype assessment with evidence of progressive loss of cone function in older individuals". Eye (Lond). 19 (1): 2–10. doi: 10.1038/sj.eye.6701391 . PMID   15094734.
  7. ↑ Reitner, A; Sharpe, L T; Zrenner, E (1991). "Is colour vision possible with only rods and blue-sensitive cones?". Nature. 352 (6338): 798–800. Bibcode:1991Natur.352..798R. doi:10.1038/352798a0. PMID   1881435. S2CID   4328439.
  8. ↑ Mark Roth (September 13, 2006). "Some women who are tetrachromats may see 100,000,000 colors, thanks to their genes". Pittsburgh Post-Gazette.
  9. ↑ Neitz J, Carroll J, Neitz M (2001). "Color Vision: Almost Reason Enough for Having Eyes". Optics and Photonics News. 12 (1): 26. Bibcode:2001OptPN..12...26N. doi:10.1364/OPN.12.1.000026. ISSN   1047-6938.
  10. ↑ Peichl, Leo; Behrmann, Gunther; Kroger, Ronald H. H. (April 2001). "For whales and seals the ocean is not blue: a visual pigment loss in marine mammals". European Journal of Neuroscience. 13 (8): 1520–1528. CiteSeerX   10.1.1.486.616 . doi:10.1046/j.0953-816x.2001.01533.x. PMID   11328346. S2CID   16062564.
  11. ↑ Emerling, Christopher A.; Springer, Mark S. (2015-02-07). "Genomic evidence for rod monochromacy in sloths and armadillos suggests early subterranean history for Xenarthra". Proceedings of the Royal Society B: Biological Sciences. 282 (1800): 20142192. doi:10.1098/rspb.2014.2192. ISSN   0962-8452. PMC   4298209 . PMID   25540280.


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