Monochromacy, also known as "total color blindness", is a complete inability to distinguish colors. This is distinguished from more common forms of
color blindness Color blindness, or colour blindness, a color vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish. It is most often of genetic nature, but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals...
, in which the affected individual
can perceive color differences, but cannot make the same distinctions between colors as can an unaffected person.
Organisms with monochromacy are called monochromats. The perceptual effect of any arbitrarily chosen light from the
visible spectrumThe visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm...
can be matched by any pure spectral light.
Causes
In vertebrates there are typically two kinds of receptors;
rodsRod 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 photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
, which primarily distinguish between levels of illumination, and
conesCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
, which are responsible for perception of
colorColor or colour is the visual perceptual property corresponding in humans to the categories called red, yellow, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
. There are different types of cones; each perceives only a single color. The normal explanation of monochromacy is that the organism's
retinaThe 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 only a single kind of light receptor cell, or at least that only one kind is active at any particular level of illumination. Monochromacy is caused by either a defect or the complete absence of the retinal
coneCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
s.
Some individuals possess diseases or injuries that lead to
nyctalopiaNyctalopia is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases...
, or night blindness, where rod cells stop responding properly to light.
Types
There are two known types of monochromacy, and a third, theoretical type that has never been identified.
- Rod monochromacy is the condition of having only rods in the retina. A rod monochromat is truly unable to see any color and can see only shades of grey.
- Cone monochromacy is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. In humans, who have three different types of cones, there are three differing forms of cone monochromacy. There are three types named according to the single functioning cone class:
- Blue cone monochromacy, also known as S-cone monochromacy
- Green cone monochromacy, also known as M-cone monochromacy
- Red cone monochromacy, also known as L-cone monochromacy
- Cone monochromacy, type II, if its existence were established, would be the case in which the retina contains no rods, and only a single type of cone. Such an animal would be unable to see at all at lower levels of illumination, and of course would be unable to distinguish hues. In practice it is hard to produce an example of such a retina, at least as the normal condition for a species.
Animals that are monochromats
It used to be confidently claimed that most
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s other than
primateA primate is a member of the biological order Primates , the group that contains lemurs, lorisids, galagos, tarsiers, monkeys, and apes, with the last category including great apes. With the exception of humans, who inhabit every continent on Earth, most primates live in tropical or subtropical...
s were monochromats. In the last half-century, however, evidence of at least
dichromatDichromacy in humans is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two...
ic color vision in a number of mammalian
ordersIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
has accumulated. Two of the orders of sea mammals, the
pinnipedPinnipeds or fin-footed mammals are a widely distributed and diverse group of semi-aquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae...
s (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. The same is true of the owl monkeys, genus
Aotus.
Both rod and cone monochromacy occur as very rare forms of color blindness in humans. Rod monochromacy, or maskun, is the more common of the two. The majority of people described as
color blind Color blindness, or colour blindness, a color vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish. It is most often of genetic nature, but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals...
, however, are either
dichromatDichromacy in humans is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two...
s or anomalous
trichromatTrichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types...
s.
Monochromats are capable of seeing 100 different colors
According to
Jay NeitzJay Neitz is professor of ophthalmology and a renowned color vision researcher at the University of Washington in Seattle, Washington in the United States....
, a renowned color vision researcher at the
Medical College of WisconsinMedical College of Wisconsin is a private, freestanding medical school and graduate school located in Milwaukee, Wisconsin, United States. It was formerly affiliated with Marquette University....
, each of the three standard color-detecting cones in the retina of
trichromatTrichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types...
s–
blueBlue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
,
greenGreen is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
and
redRed is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked human eye...
-- can pick up about 100 different gradations of color. But the brain can combine those variations exponentially, multiplying each new variety of cone by 100, so that the average human can distinguish about one million different hues. Therefore, a monochromat would be able to distinguish about 100 different colors.