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]

Bibliography[link]

• Ali, Mohamed Ather; Klyne, M. A. (1985). Vision in Vertebrates. New York: Plenum Press. ISBN 0-306-42065-1. 

External links[link]

Wikimedia Commons has media related to: Eyes

• Evolution of the eye
• Diagram of the eye
• Webvision. The organisation of the retina and visual system. An in-depth treatment of retinal function, open to all but geared most toward graduate students.
• Eye strips images of all but bare essentials before sending visual information to brain, UC Berkeley research shows
• A collection of resources for eye anatomy.
• Anatomy of the eye - flash animated interactive.
• Eyes - Eyes are organs that detect light, and send signals along the optic nerve to the visual and other areas of the brain.
• Detection of weak optical signals by the human visual system : Perspectives in Neuroscience and in Quantum Physics
• Are Your Eyes Right February 1944 Popular Science

v t e Sensory system – visual system – globe of eye (TA A15.2.1–6, TH 3.11.08.0-5, GA 10.1005) Fibrous tunic (outer) Sclera Episcleral layer • Schlemm's canal • Trabecular meshwork Cornea Limbus • layers (Epithelium, Bowman's, Stroma, Descemet's, Endothelium) Uvea/vascular tunic (middle) Choroid Capillary lamina of choroid • Bruch's membrane • Sattler's layer Ciliary body Ciliary processes • Ciliary muscle Iris Stroma • Pupil • Iris dilator muscle • Iris sphincter muscle Retina (inner) Layers Inner limiting membrane • Nerve fiber layer • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer Outer plexiform layer • Outer nuclear layer External limiting membrane • Layer of rods and cones • Retinal pigment epithelium Cells Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retina ganglion cell (Midget cell, Parasol cell, Bistratified cell, Giant retina ganglion cells, Photosensitive ganglion cell) → Diencephalon: P cell, M cell, K cell Muller glia Other Macula (Foveola, Fovea centralis) • Optic disc (Optic cup) Anterior segment Anterior chamber • Aqueous humour • Posterior chamber • Lens (Capsule of lens, Zonule of Zinn) Posterior segment Vitreous humour Other Ocular immune system • Tapetum lucidum • Keratocytes M: EYE anat(g/a/p)/phys/devp/prot noco/cong/tumr, epon proc, drug(S1A/1E/1F/1L)

This article may contain wording that merely promotes the subject without imparting verifiable information. Please remove or replace such wording, unless you can cite independent sources that support the characterization. (April 2011)

This article is an orphan, as few or no other articles link to it. Please introduce links to this page from related articles; suggestions may be available. (December 2010)

Charlotte Tilbury is one of the world's leading make-up artists, known for her fiery red hair, her penchant for high heels and her “boho” chic style. She has earned her iconic status in the fashion/beauty world through her unparalleled artistic prowess and an inimitable eye for colour that helps her turn 'mere beauties into glossy goddesses.' ^[1]

Early life[link]

Charlotte Tilbury grew up on the island of Ibiza where she developed her sense of creativity and fresh outlook on life. She was influenced by her artist father and the free-thinking environment that encouraged artistic expression. At age 11, Charlotte met the make-up artist Mary Greenwell who mentored her throughout her career as she rose through the ranks in the fashion and beauty industry.

Career/Celebrity Clients[link]

The world’s most glamorous celebs call on Charlotte to create iconic, red-carpet looks. She counts Kate Moss, Gwen Stefani, Gwyneth Paltrow, Kate Hudson, Jennifer Lopez and countless others as clients and friends. She has worked with designers such as Cavalli, Givenchy, Armani and Moschino as well as working on editorial shoots for magazines like Vogue, W Magazine and Love. And each year Charlotte works with the world’s top designers to create the cosmetic colour palettes for their fashion shows. In the UK "Charlotte has become as recognizable as her celebrity clients".^[1] She was one of a select few asked to design a T-shirt for Gap's RED campaign. Charlotte has been working as a professional make-up artist for 17 years over during which time she has served as creative director for Helena Rubenstein Cosmetics. Charlotte is represented by Art Partner

Personal life[link]

Charlotte married writer and actor Charles Forbes in Ibiza in 2006. Her wedding has often been referred to as "the biggest party since Studio 54". Some of the celebrities on the guest list included Kate Moss and Mario Testino.^[2] They have a son.

References[link]

Jon Hopkins (born 1979) is a London-based producer and musician who writes and performs his own melodic electronica and dance music. After starting his career performing keyboard for Imogen Heap, he's produced or contributed to albums by Brian Eno, Coldplay, David Holmes, and others. Hopkins composed the soundtrack for the 2010 film Monsters,^[2][3] which was nominated for an Ivor Novello Award for Best Original Score. His third solo album, Insides, reached #15 on the Dance/Electronic Album Chart in 2009,.^[4] His collaborations on Small Craft on a Milk Sea with Brian Eno and Leo Abrahams and Diamond Mine with King Creosote both reached #82 on the UK Albums Chart.^[5][6] In 2011 Diamond Mine was nominated for a Mercury Prize, which is annually awarded for best album from the United Kingdom and Ireland.^[7]

Early life[link]

Jon Hopkins was born in 1979 in Wimbledon in the South of London, where he was raised. He first became aware of electronic music after hearing early house music on the radio at the age of seven or eight, and also became an avid fan of Depeche Mode and the Pet Shop Boys.^[8] These records inspired an early fascination with synths.^[9]

At the age of 12 Hopkins began studying piano at the Junior Department of the Royal College of Music in London, where he continued until age 17.^[10] The composers that were greatly influential to him whilst studying were Ravel and Stravinsky, and he eventually won a competition to perform a concert of Ravel's Piano Concerto in G with an orchestra.^[11] For a time Hopkins considered becoming a professional pianist, only to decide classical performance was too formal and unnerving to pursue full time.^[10]

As a teenager he also listened avidly to acid house, early hardcore, grunge,^[12] as well as electronica artists such as Acen, Seefeel, and Plaid.^[13] When Hopkins was 14 he got his first computer, an Amiga 500, and started programming midi material.^[14] By the age of 15 he had saved up enough money from winning piano competitions to buy a low-level professional Roland synth,^[9] and on this he began creating his first full-length electronic compositions.^[15]

Early career[link]

After finishing his final high school exams at age 17, Hopkins accompanied his friend Leo Abrahams to an audition for Imogen Heap's backing band. Hopkins decided to audition as well, and was hired to handle both keyboard and samples, while Abrahams was hired as guitarist.^[11][14] He toured with the new band for the entirety of 1998.^[10]

Just Music[link]

Opalescent

In 1999 Hopkins signed with boutique London label Just Music as a solo artist, and began recording his debut album Opalescent. At the time he was also working part-time as a studio session musician.^[10][15] Opalescent attracted positive press attention upon its release, and several tracks were licensed to Sex and the City.^[11]

The Guardian reviewed it as "Opalescent is a beautifully realized debut. Using synth oozes, phased and echoed guitars and pianos and chilled beats, his wonderful tunes drift from calm to eerie power like a restless sea. It will delight any lovers of beautiful music." DJ Magazine gave it 4/5 stars, and stated "Piano, guitar strings and slow beats blend like the clouds at sunset (or an opiate smoothy) filtering in and out like elegantly wasted beauty. Darker drums add a further depth."^[16]

Contact Note

Hopkins released his second album, Contact Note, on Just Music in 2004 while still working as a studio musician. The album slowly gained an underground following but failed to take off, and led Hopkins to become disillusioned with his solo career, and take a break from writing in order to learn how to become a producer.^[11]

Brian Eno[link]

By 2004 Leo Abrahams, a friend of Hopkins and former guitarist for Imogen Heap, had been collaborating for some time with ambient musician and producer Brian Eno. Abrahams played some of Hopkins' second record for Eno, and Eno invited him to join them for a jam session. On the first day of their collaboration they came up with some of the music for Eno's upcoming album Another Day on Earth,^[10] and Hopkins became a long-term collaborator.^[13][15]

Shortly afterwards Hopkins also produced King Creosote's album Bombshell, which initiated his relationship with the Fence Collective. He also co-wrote tracks with DJ and composer David Holmes for Holmes' Holy Pictures album, and remixed for James Yorkston.^[13]

Coldplay[link]

Production

In early 2007 Hopkins was invited by Brian Eno, who was producing Coldplay's upcoming album Viva la Vida or Death and All His Friends, to join the band in the studio for a day.^[17] Hopkins ended up staying and contributing to the album for the next year, co-producing^[15] several tracks and playing organs, harmoniums, and other keyboard instruments on others. The intro to the track "Violet Hill" came from an improvisation with Hopkins and Davide Rossi, the album's string arranger. Throughout this time period Hopkins was periodically creating his own solo tracks, and his song "Light Through the Veins" was adapted to serve as the introduction to the album's first track "Life In Technicolor." "Light Through the Veins" was also picked by the band to serve as the backing for the track "The Escapist", which is hidden at the end of the album.^[17] Viva la Vida was released in 2008, and won Best Rock Album at the 2009 Grammy Awards and became the best selling album of 2008. Various people working upon the band's next album, Mylo Xyloto, have also mentioned that he is involved with the songwriting of the album.

Touring

After the album release Coldplay asked Hopkins to serve as the pre-show DJ and opening act for their 2008 World Tour.^[1][13] Hopkins toured with the band for six months through England, the United States, and Japan.^[18] He performed at venues as varied as Madison Square Garden^[14] and the London O2 Arena,^[17] often to crowds as large as 20,000 people.^[14]

Later career[link]

Hopkins also has co-writing or producing credits on albums by artists such as^[15] David Holmes^[1] and Dan Arborise.^[17] He is also known for remixing a variety of artists, including Wild Beasts, Nosaj Thing, Imogen Heap, Four Tet,^[13] and James Yorkston.^[13] He was also one of the few producers chosen by Radio 1's Rob Da Bank to remix film director David Lynch's first electronica release, "Good Day Today" / "I Know", which was released on Sunday Best Records.

In 2008 Hopkins was commissioned by choreographer Wayne McGregor to compose music for Entity, a production of McGregor's "Random Dance" group. Entity was performed live at Sadler's Wells in April 2008 to critical acclaim. A world tour followed throughout 2008 and 2009.^[13]

[edit] Insides (2009)

Hopkins signed to Domino Records in late 2008.^[6] Hopkins's third album, Insides, was released by Domino Records on May 5, 2009.^[1] It included the track "Light Through the Veins", which had previously been used on the Coldplay album. Some of the tracks had been written by Hopkins sporadically since his last release, while others were based on the music he had composed for the Entity production.^[13]

Hopkins developed an intense live show to support the release, resulting in club and festival performances across Europe and the United States. He secured supporting slots with The xx, Röyksopp, and Four Tet.^[13] Many of his live shows utilized background visuals featuring the animations of Vince Collins.^[10]

Reception

Insides charted at #15 on the Billboard Dance/Electronic Albums chart.^[4][13] PopMatters listed the album as one of the top ten electronic albums of 2009.^[9]

According to reviews, the album "takes its cues from ambient electronica, but uses strings and piano, along with some very tasty beats and dubstep-influenced bass on some tracks."^[10] TinyMixTapes stated the album "strikes me as his single most aggressive release yet. His sense of timing, the clarity of his production, and the variety of effects he employs draw you into the story that each instrumental tells. Jon Hopkins is not a button-pushing man of presets; he is a bona fide composer and a trained pianist. Craftsmanship sets him apart, and allows Insides to be as incredibly moving as it is and always will be. It will easily be one of the best electronic albums of 2009."^[1]

Paul Clarke of the BBC wrote that "Hopkins [is] capable of producing music as epic, soaring and emotional as any power ballad in his own way. Take "Light Through The Veins" for example...a close relative of Ulrich Schnauss' "In All The Wrong Places", it's a majestic piece of widescreen shoegazing which grows ever more expansive throughout its entire ten minute duration...no amount of reflected glory could ever fully illuminate Insides' mysterious depths."^[19]

Pure Scenius[link]

In June 2009 Hopkins was invited by Brian Eno to play some solo shows at the Luminous Festival at the Sydney Opera House. A few weeks prior to leaving, Brian Eno asked Hopkins to join with himself, Underworld's Karl Hyde, Leo Abrahams, and the Sydney-based improv trio The Necks in the group "Just Scenius", the planned finale for the Luminous Festival.^[13] They then improvised music based on pre-planned themes, putting on three 1.5 hour shows in the Opera House with Hyde on vocals. Pure Scenius was repeated a year later in Brighton, when Eno was curating the Brighton Festival.^[14]

Also in 2009, Hopkins collaborated with Tunng on the EP Seven Gulps of Air, which was commissioned by designer Christopher Kelly for London Fashion Week.^[13] Seven Gulps of Air was listed as one of Drowned in Sound's singles of the year.^[20]

Film scores[link]

In 2009 Hopkins collaborated with Brian Eno and Leo Abrahams to score the Peter Jackson film The Lovely Bones.^[13] In early 2010 Hopkins composed the score for the short film Rob and Valentyna in Scotland directed by Eric Lynne, which won an honorable mention for the short film-making award at Sundance.^[21]

In 2010 Hopkins was commissioned to create the soundtrack for the British science fiction film Monsters, which was directed by Gareth Edwards.^[2] To create the score, Hopkins partly utilized string parts performed by arranger Davide Rossi and guitar by Leo Abrahams.^[22] The soundtrack album was released on November 29, 2010 on Domino Records.^[3] In 2011 the score was nominated for an Ivor Novello Award for Best Original Score.^[22]

Collaborations[link]

Small Craft on a Milk Sea (2010)

In 2010 Hopkins collaborated with Leo Abrahams and Brian Eno to create the album Small Craft on a Milk Sea. Released on Warp Records in late 2010,^[13] the album is based on a three-week session of improvisation wherein the artists recorded about six hours of material a day.^[9]

Diamond Mine (2011)

In 2011 Hopkins collaborated with Scottish musician King Creosote to create the album Diamond Mine, which featured lyrics and vocals by Creosote sung over musical backdrops arranged and recorded by Hopkins. The album was a culmination of about seven weeks of work spread over seven years of recording and collaboration,^[8] from whenever the two artists had the opportunity to get together. The album was released on March 28, 2011^[6] to positive acclaim,^[23] which included a glowing review from NPR. On July 19, 2011, Hopkins and Anderson were announced as nominees for the 2011 Barclaycard Mercury Prize, which is annually awarded for best album from the United Kingdom and Ireland.^[7]