Close up of a human iris

Eye color is a polygenic phenotypic character and is determined by two distinct factors: the pigmentation of the eye's iris^[1][2] and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.^[3]

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma.^[4] The appearance of blue, green, as well as hazel eyes results from the Rayleigh scattering of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky. Neither blue nor green pigments are ever present in the human iris or ocular fluid.^[3][5] Eye color is thus an instance of structural color and varies depending on the lighting conditions, especially for lighter-colored eyes.

The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids.^[6] Humans and other animals have many phenotypic variations in eye color.^[7] The genetics of eye color are complicated, and color is determined by multiple genes. So far, as many as 15 different genes have been associated with eye color inheritance. Some of the eye-color genes include OCA2 and HERC2.^[8] The once-held view that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur.^[9][10] However, OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.^[11]

Genetic determination of eye color[link]

Eye colors can range from the most common color, brown, to the least common, green. Eye color is an inherited trait influenced by more than one gene.^[12][13] These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single-nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is currently unknown, but there are a few likely candidates. A study in Rotterdam (2009) found that it was possible to predict the color of eyes with more than 90% accuracy for brown and blue, using just six SNPs.^[14] Finally, there is evidence that as much as 16 different genes could be responsible for eye color in humans, however the main two genes associated with eye color variation are OCA2 and HERC2, both localized in Chromosome 15.^[8]

The gene OCA2 (OMIM: 203200), when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation.^[11] A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes.^[15] Other genes implicated in eye color variation are: SLC24A4^[16] and TYR.^[16]

Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome. As Eiberg said: "The SNP rs12913832 [of the Herc2 gene] is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots."  

Classification of color[link]

Iris color can provide a large amount of information about a person, and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals.^[17] Various classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison.^[17] Others have attempted to set objective standards of color comparison.^[18]

As the perception of color depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.^[19]

Eye colors range from the darkest shades of brown to the lightest tints of blue.^[12] To meet the need for standardized classification, at once simple yet detailed enough for research purposes, Seddon et al. developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present.^[20] There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris: brown, yellow, and blue. Green irides, for example, have blue and some yellow. Brown irides contain mostly brown. Eye color in animals other than Homo sapiens is differently regulated. For example, instead of blue as in humans, autosomal recessive eye color in the skink species Corucia zebrata is black, and the autosomal dominant color is yellow-green.^[21]

Changes in eye color[link]

Most babies who have European ancestry have light-colored eyes before the age of one. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Most eye changes happen when the infant is around one year old, although it can happen up to three years of age.^[22] Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn green or brown as the infant ages.

Changes (lightening or darkening) of eye colors during puberty, early childhood, pregnancy, and sometimes after serious trauma (like heterochromia) do represent cause for plausible argument to state that some eyes can or do change, based on chemical reactions and hormonal changes within the body.

Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye-color changes have been observed or reported in the Caucasian population with hazel eyes.^[23]

Eye color chart (Martin–Schultz scale)[link]

Carleton Coon created this chart by the Martin–Schultz scale often used in physical anthropology.

1. Light eyes

   Eyes light and light mixed are 16–12 in Martin scale.

   Light

   15: Blue, Gray only. 16: Blue exclusive; 16-1a: pure light blue.

   Light-mixed

   14–12 in Martin scale.

   1. Very light-mixed (blue with gray or green or green with gray)
   2. Light-mixed (light or very light-mixed with small admixture of brown pigment)
2. Mixed eyes

   Mixed

   12–6 in Martin scale. Mixture of light eyes (blue, gray or green) with brown pigment when light and brown pigment are the same level.

3. Dark eyes

   Dark-mixed

   6–4 in Martin scale. Brown with small admixture of light pigment.

   Dark

   4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown (almost black).

Amber[link]

Amber eyes in sunlight – displaying an orange color rather than brown

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This might be due to the deposition of the yellow pigment called lipochrome in the iris (which is also found in green eyes).^[24][25] Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid gold hue. Even though amber is considered to be like gold, some people have russet or copper colored amber eyes which many people mistake for hazel, though hazel tends to be duller and contains green with red/gold flecks, as mentioned above. Amber eyes may also contain amounts of very light gold-ish gray.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines.^[26] The bright yellow eyes of the Great Horned Owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma.^[27] In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.^[28] Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common color, whereas in humans this color occurs less frequently.

Blue[link]

A blue iris

There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin.^[29] Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma.^[4] This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky.^[3][5] The result is a "Tyndall blue" structural color that varies with external lighting conditions.

Distribution[link]

Distribution of light-eyed people in Europe

Blue eyes are most common in the Baltic Sea area and Northern Europe,^[30] and are also found in Eastern, Central, and Southern Europe. Blue eyes are also found in parts of the Western Asia, most notably in Afghanistan, Lebanon, Iran and Iraq,^[31][32][33] as well as in the Levant, especially amongst the Jewish population of Israel. Many modern Israeli Jews are of European Ashkenazi origin, among whom this trait is common (53.7% of Ukrainian Jews have blue eyes^[34]).

Y-Chromosome DNA testing performed on ancient Scythian skeletons dating to the Bronze and Iron Ages in the Siberian Krasnoyarsk region found that all but one of 11 subjects carried Y-DNA R1a1, with blue or green eye color and light hair common, suggesting mostly European origin of that particular population.^[35]

In Estonia, 99% of people have blue eyes, stated Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen.^[36] In Denmark 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In Germany, about 75% have blue eyes.^[36]

A 2002 study found that the prevalence of blue eye color among Caucasians in the United States to be 33.8 percent for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905.^[13] Blue eyes are continuing to become less common among American children, with only one out of every six or 16.6% of the total population, and 22.3% of the white population (including Hispanics) having blue eyes.^[37][38]

Worldwide, only 150 million people (some 2.2%) have blue eyes.^[39]

Genetics[link]

In humans, the inheritance pattern followed by blue eyes is considered similar to that of a recessive trait (in general, eye color inheritance is considered a polygenic trait, meaning that it is controlled by the interactions of several genes, not just one).^[13] In 2008, new research suggested that people with blue eyes have a single common ancestor. Scientists tracked down a genetic mutation that leads to blue eyes. "Originally, we all had brown eyes," said Eiberg.^[40] Eiberg and colleagues showed in a study published in Human Genetics that a mutation in the 86th intron of the HERC2 gene, which is hypothesized to interact with the OCA2 gene promoter, reduced expression of OCA2 with subsequent reduction in melanin production.^[41] The authors concluded that the mutation may have arisen in a single individual probably living in the northwestern part of the Black Sea region (around modern Romania) 6,000–10,000 years ago during the Neolithic revolution.^[40][41][42] Eiberg stated, "A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch,' which literally 'turned off' the ability to produce brown eyes."

The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue. If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as albinism.^[40]

File:Karasu2a.JPG

Blue-eyed Siamese cat and Jungle Crow. The Australian Wildlife Experience has the first blue-eyed koala known to be born in captivity in the world.^[43]

Blue eyes are rare in mammals; one example is the quite recently-discovered Blue-eyed Spotted Cuscus (Spilocuscus wilsoni). The trait is hitherto known only from a single primate other than humans – Sclater's Lemur (Eulemur flavifrons) of Madagascar. While some cats and dogs have blue eyes, this is usually due to another mutation which is associated with deafness. But in cats alone, there are four identified gene mutations that produce blue eyes, some of which are associated with congenital neurological disorders. The mutation found in the Siamese cats is associated with strabismus (crossed eyes). The mutation found in blue-eyed solid white cats (where the coat colour is caused by the gene for "epistatic white") is associated with deafness. However, there are phenotypically identical, but genotypically different, blue-eyed white cats (where the coat colour is caused by the gene for white spotting) where the coat colour is not strongly associated with deafness. In the blue-eyed Ojos Azules breed, there may be other neurological defects. Blue-eyed non-white cats of unknown genotype also occur at random in the cat population.

Brown[link]

Dark brown iris is most common in East Asia, Southeast Asia, South Asia.

Light Brown iris or sometimes referred to as "honey eyes". Is most common in West Asia, Eastern Europe and among the Americas.

Dark brown eyes are dominant in humans^[44] and in many parts of the world, it is nearly the only iris color present.^[45] Dark pigment of brown eyes is most common in East Asia, Southeast Asia, South Asia, West Asia, Oceania, Africa, Americas, etc. as well as parts of Eastern Europe and Southern Europe.^[16][46]

Dark brown is also found in the semitic people seen in a fair percentage among Jewish and Arab populations in the Middle East.

Light or medium-pigmented brown eyes are common in Europe, Afghanistan, Pakistan and Northern India, as well as some parts of the Middle East. Light-pigmented brown eyes are sometimes referred to as "honey eyes".

In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.^[3]

Gray[link]

Gray Eye With Yellow Ring

True gray eyes

Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in the appearance of gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma, so that the light that is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering off large water droplets when the sky is cloudy.^[47] Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.^[47]

Gray eyes are most common in Northern and Eastern Europe.^[48] Gray eyes can also be found in parts of North West Africa (Aurès Mountains) among the Algerian Shawia people^[49] and in the Middle East, most notably in Iran, Afghanistan (the Indo-Iranian Nuristani people), and Pakistan (the Kalasha, another Indo-Iranian people).^[31][32][33] Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

A gray iris may indicate the presence of a uveitis. However, other visual signs make a uveitis obvious. Gray iris color, as well as blue, are at increased risk of uveal melanoma.^[50]

Green[link]

A light green iris with small amounts of brown

Olive green eyes

Green is the least common eye color and as in the case of blue eyes, the color of green eyes does not result simply from the pigmentation of the iris. Rather, their appearance is caused by the combination of an amber or light brown pigmentation of the stroma, given by a low or moderate concentration of melanin, with the blue tone imparted by the Rayleigh scattering of the reflected light.^[3]

Green eyes probably result from the interaction of multiple variants within the OCA2 and other genes. They were present in south Siberia during the Bronze Age.^[51] They are most common in Northern and Central Europe.^[52][53] They can also be found in Southern Europe^[48] and parts of South Asia (Afghanistan, northern Pakistan and northwestern India), South America and North Africa. In Iceland, 89% of women and 87% of men have either blue or green eye color.^[54] A study of Icelandic and Dutch adults found green eyes to be much more prevalent in women than in men.^{[citation needed]} Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry, about 16%.^[55]

Hazel[link]

Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer.^[4][28] Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is how many people mistake hazel eyes to be amber and vice versa.^{[56][57][58][59][60][61][62]} This can sometimes produce a multicolored iris, i.e., an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa) when observed in sunlight.

Definitions of the eye color hazel vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell.^{[56][58][61][63]}

Red and violet[link]

"Red" albino eyes

The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin,^[64] allowing the blood vessels to show through. In addition, flash photography can sometimes cause a "red-eye effect", in which the very bright light from a flash reflects off the back of the eyeball, which is abundantly vascular, causing the pupil to appear red in the photograph.^[65]

Although the deep blue eyes of some people such as Elizabeth Taylor can appear violet at certain times, true violet-colored eyes occur only due to albinism.^[66]

Spectrum of eye color[link]

Medical implications[link]

Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color;^[60] lighter eye color is also associated with an increased risk of ARMD progression.^[67] An increased risk of uveal melanoma has been found in those with blue, green or gray iris color.^[50][54] However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.^[68]

Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the whites of the eyes is associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria. Yellowing of the whites of the eyes in people with darker pigmented skin is often due to melanin being present in the whites of the eyes. However, any sudden changes in the color of the whites of the eyes should be addressed by a medical professional.

Wilson's disease[link]

A Kayser–Fleischer ring in a patient with Wilson's disease

Wilson's disease involves a mutation of the gene coding for the enzyme ATPase7B, which prevents copper within the liver from entering the Golgi apparatus in cells. Instead, the copper accumulates in the liver and in other tissues, including the iris of the eye. This results in the formation of Kayser-Fleischer rings, which are dark rings that encircle the periphery of the iris.^[69]

Anomalous conditions[link]

Aniridia[link]

Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.^[70]

Ocular albinism and eye color[link]

Normally, there is a thick layer of melanin on the back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism, the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation.^[71] The ocular albino also lacks normal amounts of melanin in the retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.

Heterochromia[link]

An example of complete heterochromia. The subject has one brown eye and one hazel/green eye.

An example of sectoral heterochromia. The subject has a blue iris with a brown section.

Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury.^[72] This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetic, such as chimerism, Horners Syndrome and Waardenburg syndrome.

An example of sectoral heterochromia. A green eye with a brown section.

A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if the DNA difference happens to be in an eye-color gene.

There are many other possible reasons for having two different-colored eyes. For example, David Bowie has the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated. Another idea about how this can happen is if an early viral infection while in the womb turns an eye color gene on or off in just one eye. Occasionally it can be a sign of a serious disease.

A common cause in females with heterochromia is X-inactivation, which can result in a number of heterochromatic traits, such as calico cats. Trauma and certain medications, such as some prostaglandin analogues, can also cause increased pigmentation in one eye.^[73] On occasion, the condition of having two different-colored eyes is caused by blood staining the iris after sustaining injury.

See also[link]

• Limbal ring
• Iridology
• Hair color
• Human skin color
• Xanthophore
• List of Mendelian traits in humans

References[link]

External links[link]

Wikimedia Commons has media related to: Eyes by color

• Genetics of eye color
• Eye Color and Human Diseases

Eye
Schematic diagram of the vertebrate eye.
Compound eye of Antarctic krill

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.^[1] Image-resolving eyes are present in molluscs, chordates and arthropods.^[2]

The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms.^{[citation needed]} From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.

Overview[link]

Eye of the wisent,
the European bison

Complex eyes can distinguish shapes and colours. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception. In other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision.

The first proto-eyes evolved among animals 600 million years ago about the time of the Cambrian explosion.^[3] The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the thirty-plus^[4] main phyla.^[1] In most vertebrates and some molluscs, the eye works by allowing light to enter and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye,^[5] and reducing aberrations when there is enough light.^[6]

The eyes of most cephalopods, fish, amphibians and snakes have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.^[7]

Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360° field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images.

Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the world's most complex colour vision system.^[8] Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.

In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents–in this way the bearers can spot hot springs and avoid being boiled alive.^[9]

Evolution[link]

Evolution of the eye

Photoreception is phylogenetically very old, with various theories of phylogenesis.^[10] The common origin (monophyly) of all animal eyes is now widely accepted as fact. This is based upon the shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago,^[11][12][13] and the PAX6 gene is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race".^[14] Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel.

Eyes in various animals show adaptation to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eye in, for example, vertebrates and molluscs are often cited as examples of parallel evolution, despite their distant common ancestry.

The very earliest "eyes", called eyespots, were simple patches of photoreceptor protein in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source.^[1]

Through gradual change, as the eyespot depressed into a shallow "cup" shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes.^[15]

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into a transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.^[16]

The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the cornea and iris. Separation of the forward layer again formed a humour, the aqueous humour. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.^[16]

Types of eye[link]

There are ten different eye layouts—indeed every way of capturing an optical image commonly used by human beings, with the exceptions of zoom and Fresnel lenses. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface.^[1] Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures.^[1] Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomeric. These two groups are not monophyletic; the cnidaria also possess cilliated cells, ^[17] and some annelids possess both.^[18]

Non-compound eyes[link]

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustaceans and cubozoa.^[19]

Pit eyes[link]

Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light.^[1] Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eye". They are small, comprising up to about 100 cells covering about 100 µm.^[1] The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.^[1]

Pit vipers have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates.

Spherical lensed eye[link]

The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable.^[1] The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina.^[1] This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration.^[1] Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.^[1]

Heterogeneous eyes have evolved at least eight times: four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.^[1] No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".^[1]

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles.^[1]

The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain.^[20] Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity.^[20] This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).^[20]

Multiple lenses[link]

Some marine organisms bear more than one lens; for instance the copepod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Another copepod, Copilia, has two lenses in each eye, arranged like those in a telescope.^[1] Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.^[1]

Refractive cornea[link]

In the eyes of most mammals, birds, reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air.^[1] In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see Luneburg lens), or with an aspheric shape.^[1] Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens.^[1]

As mentioned above, a refractive cornea is only useful out of water; in water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water–penguins and seals, for example–lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea.^[1]

Reflector eyes[link]

An alternative to a lens is to line the inside of the eye with " mirrors", and reflect the image to focus at a central point.^[1] The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.^[1]

Many small organisms such as rotifers, copepods and platyhelminths use such organs, but these are too small to produce usable images.^[1] Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimetre-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.^[1]

There is at least one vertebrate, the spookfish, whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of guanine crystals.^[21]

[edit] Compound eyes

An image of a house fly compound eye surface by using scanning electron microscope

Anatomy of the compound eye of an insect

Arthropods such as this Calliphora vomitoria fly have compound eyes

A compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarisation of light.^[22] Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their heads.^{[citation needed]}

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.^[23] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.^[24]

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.^[25]

Apposition eyes[link]

Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum.^[1] Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point.^[1] (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information.

The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels.^[1]

Superposition eyes[link]

The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition compound eye type, seen in arthropods such as mayflies, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also have a transparent gap but use corner mirrors instead of lenses.

Parabolic superposition[link]

This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.^[9]

Other[link]

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialised zones of ommatidia organised into a fovea area which gives acute vision. In the acute zone the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution.

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp Dioptromysis paucispinosa. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organised in a way that resembles a true compound eye.

The body of Ophiocoma wendtii, a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons. The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect the directionality of light by the shadow cast by its opaque body.^[26]

Nutrients[link]

The ciliary body is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle.

The vitreous is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.^[27] It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98-99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.

Relationship to life requirements[link]

Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.

Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition.^[1]:23-4 Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia.^[28] Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example if the animal is on a slope.^[29] An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey.^[28] In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipods are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether.^[28] Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.^[9]

Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop.^[28] On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.^[28]

It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.^[28]

Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.^[9]

Visual acuity[link]

The eye of a red-tailed hawk

Visual acuity, or resolving power, is "the ability to distinguish fine detail" and is the property of cone cells.^[30] It is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white/black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white/black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a grey block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD^[31] (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.^[32] A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region.^{[citation needed]}

Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair.^[33] A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.

However, in the compound eye, the resolution is related to the size of individual ommatidia and the distance between neighbouring ommatidia. Physically these cannot be reduced in size to achieve the acuity seen with single lensed eyes as in mammals. Compound eyes have a much lower acuity than mammalian eyes.^[34]

Perception of colors[link]

"Color vision is the faculty of the organism to distinguish lights of different spectral qualities."^[35] All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between wavelengths of 400 and 700 nm.^[36] This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.^[37]

The most sensitive pigment, rhodopsin, has a peak response at 500 nm.^[38] Small changes to the genes coding for this protein can tweak the peak response by a few nm;^[2] pigments in the lens can also filter incoming light, changing the peak response.^[2] Many organisms are unable to discriminate between colours, seeing instead in shades of grey; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of cone cells, from which the more sensitive rod cells evolved.^[38] Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced.^[2]

Most organisms with color vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.^[38]

Rods and cones[link]

The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones.

Rods cannot distinguish colours, but are responsible for low-light (scotopic) monochrome (black-and-white) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for colour vision. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibres to the brain.^[38]

Pigmentation[link]

The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related – although problems of convergence do exist.^[38]

Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.^[2]

There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells.^[39] The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some ganglion cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes.^[39] Likewise, c-opsins have been found to be expressed in the brain of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form.^[39] C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins.^[39] Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group.^[39] Incidentally, the melanin produced in the raticate is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.^[39]

See also[link]

	Book: Eye
Wikipedia books are collections of articles that can be downloaded or ordered in print.

• Arthropod eye
• Cephalopod eye
• Emission theory (vision)
• Eye colour
• Eyelid
• Eye movement (sensory)
• Human eye
• Mammalian eye
• Mollusc eye
• Nictitating membrane
• Simple eye in invertebrates
• Tapetum lucidum
• Tears

References[link]

Notes[link]