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

Monochromacy (fromGreekmono, meaning "one" andchromo, meaning "color") is the ability of organisms to perceive only light intensity without respect tospectral composition. Organisms with monochromacy lackcolor vision and can only see in shades of grey ranging from black to white. Organisms with monochromacy are called monochromats. Many mammals, such ascetaceans, theowl monkey and theAustralian sea lion are monochromats. Inhumans, monochromacy is one among several other symptoms of severe inherited or acquired diseases, includingachromatopsia orblue cone monochromacy, together affecting about 1 in 30,000 people.

Human vision relies on aduplex retina, comprising two types ofphotoreceptor cells.Rods are primarily responsible for dim-lightscotopic vision andcones are primarily responsible for day-lightphotopic vision. For all knownvertebrates, 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 enablescolor vision during daytime conditions.

Most humans have three classes of cones, each with a different class ofopsin. These three opsins have differentspectral sensitivities, which is a prerequisite fortrichromacy. An alteration of any of these three cone opsins can lead tocolorblindness.

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-yellowopponent 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 (RM), also calledcongenital complete achromatopsia or total color blindness, is a rare and extremely severe form of anautosomal 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 andnystagmus. 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 (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 hybridopsin 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 thefovea. 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 toconditional dichromacy. Inmesopic conditions, both rods and cones are active andopponent interactions between the cones and rods can afford slight color vision.^[7]

According toJay Neitz, a color vision researcher at theUniversity of Washington, each of the three standard color-detecting cones in the retina oftrichromats 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]

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

Two mammalian orders containingmarine mammals exhibit monochromatic vision:

• Pinnipeds (including seals, sea lions and walruses)
• Cetaceans (including dolphins and whales)

Unlike thetrichromacy exhibited in most primates,Owl monkeys (genusAotus) are also monochromats^{[citation needed]}. Several members of the familyProcyonidae (raccoon,crab-eating raccoon andkinkajou) and a fewrodents 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 orburrowing mammals, which experience less light, have lessevolutionary pressure to preserve dichromacy, so often evolve monochromacy.^{[citation needed]}

A recent study using through PCR analysis of genesOPN1SW,OPN1LW, andPDE6C determined that all mammals in the cohortXenarthra (representing sloths, anteaters and armadillos) developed rod monochromacy through a stem ancestor.^[11]

• Achromatopsia
• Blue cone monochromacy
• Cone dystrophy
• Dichromacy
• Trichromacy
• Tetrachromacy

• Rossi, Ethan (February 2013)."Visual Function and Cortical Organization in Carriers of Blue Cone Monochromacy".PLOS ONE.8 (2): e57956.Bibcode:2013PLoSO...857956R.doi:10.1371/journal.pone.0057956.PMC 3585243.PMID 23469117.
• Weleber, Richard (June 2002). "Infantile and childhood retinal blindness: A molecular perspective (TheFranceschetti Lecture)".Ophthalmic Genetics.23 (2):71–98.doi:10.1076/opge.23.2.71.2214.PMID 12187427.S2CID 30741530.

• Visual disturbances and blindness
• Color vision

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