The term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur The femur, or thigh bone, is the most proximal bone of the leg in vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in the rear legs.

Fovea of the eye

The fovea centralis, also generally known as the fovea, is a part of the eye Eyes are organs that detect light, and convert it to electro-chemical impulses in neurons. The simplest photoreceptors connect light to movement . In higher organisms complex neural pathways exist that connect the eye, via the optic nerve to the visual cortex and other areas of the brain. Complex optical systems with resolving power have come in, located in the center of the macula The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells. Near its center is the fovea, a small pit that contains the largest concentration of cone cells in the eye and region of the retina The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These. ^{[1] [2]} The fovea is responsible for sharp central vision Visual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision . The various physiological components involved in vision are referred to collectively as the visual system, and are the focus of much research in psychology, cognitive (also called foveal The foveal system of the human eye is the only part of the retina that permits 100% visual acuity. The line-of-sight is a virtual line connecting the fovea with a fixation point in the outside world vision), which is necessary in humans for reading Reading is a complex cognitive process of decoding symbols for the intention of deriving meaning and/or constructing meaning. Written information is received by the retina, processed by the primary visual cortex, and interpreted in Wernicke's area, watching television Television is a widely used telecommunication medium for transmitting and receiving moving images, either monochromatic ("black and white") or color, usually accompanied by sound. "Television" may also refer specifically to a television set, television programming or television transmission. The word is derived from mixed Latin or movies, driving, and any activity where visual detail is of primary importance. The fovea is surrounded by the parafovea belt, and the perifovea outer region:^[2] the parafovea is the intermediate belt where the ganglion In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells cell layer is composed of more than five rows of cells; as well as the highest density of cones; the perifovea is the outermost region where the ganglion cell layer contains two to four rows of cells, and is where visual acuity is below the optimum. The perifovea contains an even more diminished density of cones, having 12 per 100 micrometres versus 50 per 100 micrometres in the most central fovea. This, in turn, is surrounded by a larger peripheral Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision. "Far peripheral" vision exists at the edges of the field of view, "mid-peripheral" vision exists in the middle of the field of area that delivers highly compressed information of low resolution. Approximately 50% of the nerve fibers in the optic nerve The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. Consequently, the fibres are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system carry information from the fovea, while the other 50% carry information from the rest of the retina. The parafovea extends to a distance of 1¼ mm from the central fovea, and the perifovea is found 2¾ mm away from the fovea centralis.^[3]

Description

In the human eye the term fovea (or fovea centralis) denotes the pit in the retina The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These which allows for maximum acuity of vision.

In the human fovea the ratio of ganglion cells A ganglion cell is a cell found in a ganglion. The term is also sometimes used to refer specifically to a retinal ganglion cell found in the ganglion cell layer of the retina to photoreceptors A photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina that is capable of phototransduction. The great biological importance of photoreceptors is that as cells they convert light (electromagnetic radiation) into the beginning of a chain of biological processes. More specifically, the photoreceptor absorbs is close to one; almost every photoreceptor has one ganglion cell receiving data from it. That is why it has little loss of sensory data, thus it is the area of the eye where most details can be seen.^[5]

The human fovea has a diameter of about 1.0 mm with a high concentration of cone photoreceptors Cone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina. The centre of the fovea is the foveola - about 0.2 mm in diameter - where only cone photoreceptors are present and there are virtually no rods Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 90 million rod cells in the human retina.^[1] The central fovea consists of very compact cones, thinner and more rod-like than cones elsewhere. Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.

Compared to the rest of the retina The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These, the cones in the foveal pit have a smaller diameter and can therefore be more densely packed (in a hexagonal In geometry, a hexagon is a polygon with six edges and six vertices. A regular hexagon has Schläfli symbol {6}. The total of the internal angles of any hexagon is 720 degrees pattern). The high spatial density of cones accounts for the high visual acuity capability at the fovea. This is enhanced by the local absence of retinal blood vessels from the fovea, which, if present, would interfere with the passage of light Light is electromagnetic radiation of a wavelength that is visible to the human eye . In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not striking the foveal cone mosaic. The absence of inner retinal cells from the foveae of primates is assumed to contribute further to the high acuity function of the fovea.

The fovea centralis is a central pit, near the optic axis. It eliminates the necessity of passing through the inner, non-sensitive neurons and allow direct passage to the receptors. It is employed for accurate vision in the direction where it is pointed. If the object is large and taking up a large angle, the eyes must constantly shift their gaze to subsequently bring images into the fovea (as in reading).

Since the macula does not have a blood supply The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pH to maintain homeostasis, the fovea must receive oxygen Oxygen (pronounced /ˈɒksɨdʒɨn/, OK-si-jin, from the Greek roots ὀξύς (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter), is the element with atomic number 8 and represented by the symbol O. It is a member of the chalcogen group on the periodic table, and is a highly from the vessels in the choroid The choroid, also known as the choroidea or choroid coat, is the vascular layer containing connective tissue, of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina . Along with the ciliary body and iris, the choroid forms the uveal, which is across the retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells and Bruch's membrane. This blood supply alone does not satisfy the metabolic needs of the fovea under conditions of bright light, and the fovea thus exists in a state of hypoxia Hypoxia is a pathological condition in which the body as a whole or a region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise. A mismatch between oxygen supply and its demand at the cellular level when under bright illumination.

Since cones contain the pigmented opsins Opsins are a group of light-sensitive 35-55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction that allow humans to discriminate color, the fovea is largely responsible for the color vision Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit. The nervous system derives color by comparing the responses to light from the several types of cone photoreceptors in the eye. These cone photoreceptors are sensitive to different portions of the in humans Humans are a species of animal known taxonomically as Homo sapiens , and are the only extant member of the Homo genus of bipedal primates in Hominidae, the great ape family. However, in some cases "human" is used to refer to any member of the genus Homo which is superior to that of most other mammals Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not^{[citation needed]}.

The fovea comprises less than 1% of retinal size but takes up over 50% of the visual cortex The term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. The extrastriate cortical areas consist of Brodmann area 18 and Brodmann area 19. There is a visual cortex for each hemisphere of the brain in the brain. ^[6] The foveal pit is not located exactly on the optical axis In optics, the term optical axis is used to define a direction along which there is some degree of rotational symmetry. It can be used in several contexts:, but is displaced about 4 to 8 degrees temporal to it. The fovea sees only the central two degrees of the visual field, which is roughly equivalent to twice the width of your thumbnail at arm's length.^[7]

Surrounding the foveal pit is the foveal rim, where the neurons displaced from the pit are located. This is the thickest part of the retina.

Since the fovea does not have rods, it is not sensitive to dim lights. Astronomers An astronomer is a scientist who studies celestial bodies such as planets, stars, and galaxies know this: in order to observe a dim star, they use averted vision, looking out of "the side of their eyes".

The fovea is covered in a yellow pigment called xanthophyll Xanthophylls are yellow pigments from the carotenoid group. The name is from Greek xanthos (ξανθος, "yellow") + phyllon (φύλλον, "leaf"), due to their contribution to the yellow band in early chromatography of leaf pigments. Their molecular structure is based on carotenes, with additional oxidation. Thus, they are,^[1] with the carotenoids Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some types of fungus some bacteria and at least one species of aphid. Carotenoids are generally not manufactured by species in the animal kingdom, although one species of zeaxanthin Zeaxanthin is one of the two carotenoids contained within the retina of the eye. Within the central macula, zeaxanthin is the dominant component, whereas in the peripheral retina, lutein predominates and lutein Lutein is a xanthophyll and one of 600 known naturally-occurring carotenoids. Found in green leafy vegetables such as spinach and kale, lutein is employed by organisms as an antioxidant and for blue light absorption. Lutein is also found in egg yolks, animal fats, and the retina (zeaxanthin predominates at the macula lutea while lutein (Balashov and Bernstein, 1998), present in the cone axons An axon or nerve fiber is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma of the Henle fibre layer.^[1] The pigment area absorbs blue light and is probably an evolutionary Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. Ultimately, life is descended from a common ancestory through a long series of these speciation events, adaptation to the problem of chromatic aberration In optics, chromatic aberration is the failure of a lens to focus all colors to the same point. It occurs because lenses have a different refractive index for different wavelengths of light (the dispersion of the lens). The refractive index decreases with increasing wavelength.

The fovea is also a pit in the surface of the retinas of many types of fish, reptiles and birds. Among mammals it is found only in simian The simians are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, (together being the catarrhines), and the New World monkeys or platyrrhines. Simians tend to be larger than the "lower primates" or prosimians primates A primate is a member of the biological order Primates (/praɪˈmeɪtiːz/ prī·mā′·tēz; Latin: "prime, first rank"), the group that contains prosimians (including lemurs, lorises, galagos and tarsiers ) and simians (monkeys and apes). With the exception of humans, who inhabit every continent on Earth,[a] most primates live in. The retinal fovea takes slightly different forms in different types of animals. For example, in primates, cone photoreceptors line the base of the foveal pit, the cells which elsewhere in the retina form more superficial layers having been displaced away from the foveal region during late fetal A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth and early postnatal Postnatal is the period beginning immediately after the birth of a child and extending for about six weeks. Another term would be postpartum period, as it refers to the mother (whereas postnatal refers to the infant). Less frequently used is puerperium life. Other foveae may show only a reduced thickness in the inner cell layers, rather than an almost complete absence.

The only photo-receptors located in the fovea of most humans are three kinds of cone photo receptors. The red, blue and green allows the eye to see the colours our species needs for survival; however some organisms are known to possess four independent channels for conveying color information, or possessing four different types of cone cells in the eye, a characteristic called tetrachromacy. Organisms with tetrachromacy are called tetrachromats Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four different types of cone cells in the eye. Organisms with tetrachromacy are called tetrachromats. The rods are located on the fovea's periphery. This assists the eye to see in the dark.

See also

• Eye movement Eye movement is the voluntary or involuntary movement of the eyes, helping in acquiring, fixating and tracking visual stimuli. In addition, rapid eye movement occurs during REM sleep
• Eye movement in language reading The study of eye movement in language reading stretches back almost a hundred years. Eye movement during reading were first described by the French ophthalmologist Louis Émile Javal in the late 19th century. He reported that eyes do not move continuously along a line of text, but make short rapid movement intermingled with short stops (fixations)
• Eye movement in music reading Eye movement in music reading is the scanning of a musical score by a musician's eyes. This usually occurs as the music is read during performance, although musicians sometimes scan music silently to study it, and sometimes perform from memory without score. The phenomenon has been studied by researchers from a range of backgrounds, including
• Gaze-contingency paradigm The gaze-contingency paradigm is a technique for studying eye movement, a field of study in psychology that has revealed some of the details of cognitive function. Over the past century, the way the eyes move in human activities as diverse as playing sport, viewing works of art, piloting aircraft, and reading language and music, has revealed some
• Macular degeneration Age related macular degeneration is a medical condition which usually affects older adults that results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of visual impairment in older adults (>50 years). Macular degeneration can make it
• Foveated imaging

References

1. ^ ^{a b c d} "Webvision: Simple Anatomy of the Retina" (definition of terms), University of Utah, Webvision: The Organization of the Retina and Visual System, September 2005, Webvision.med.utah.edu webpage: Med-UtahEdu-retina.
2. ^ ^{a b} "Relation Between Superficial Capillaries and Foveal Structures in the Human Retina" (with nomenclature of fovea terms), Masayuki Iwasaki and Hajime Inomara, Investigative Ophthalmology & Visual Science (journal), volume 27, pages 1698-1705, 1986, IOVS.org, webpage: IOVS-fovea-capillaries.
3. ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
4. ^ Hans-Werner Hunziker, (2006) Im Auge des Lesers: foveale und periphere Wahrnehmung - vom Buchstabieren zur Lesefreude [ The function of the fovea is to catch detailed visual information 3 to 4 times per second at different parts of the visual field The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical objects and light sources in the. The brain integrates these informations within the framework of the condensed peripheral vision Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision. "Far peripheral" vision exists at the edges of the field of view, "mid-peripheral" vision exists in the middle of the field of (extra-foveal information). the eye of the reader: foveal and peripheral perception - from letter recognition to the joy of reading] Transmedia Stäubli Verlag Zürich 2006 ISBN 978-3-7266-0068-6
5. ^ Smithsonian/The National Academies, Light:Student Guide and Source Book. Carolina Biological Supply Company, 2002. ISBN 0-89278-892-5.
6. ^ "The Stimulus and Anatomy of the Visual System" (with fovea description), Hanover College, Psychology Department, HanoverCollege-Fovea-PDF-as-HTML.
7. ^ Fairchild, Mark. (1998), Color Appearance Models. Reading, Mass.: Addison, Wesley, & Longman, p.7. ISBN 0-201-63464-3

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