Fovea
Encyclopedia
The fovea centralis, also generally known as the fovea (the term fovea comes from the Latin, meaning pit or pitfall), is a part of the eye
, located in the center of the macula
region of the retina
.
The fovea is responsible for sharp central vision
(also called foveal
vision), which is necessary in humans for reading, watching television
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. The parafovea is the intermediate belt, where the ganglion
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
area that delivers highly compressed information of low resolution. Approximately 50% of the nerve fibers in the optic nerve
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.
, which allows for maximum acuity of vision.
The human fovea has a diameter of about 1.0 mm with a high concentration of cone photoreceptor
s. The center of the fovea is the foveola – about 0.2 mm in diameter – where only cone photoreceptors are present and there are virtually no rods
. The central fovea consists of very compact cones, thinner and more rod-like in appearance than cones elsewhere. Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.
In the primate fovea (presumably including human) the ratio of ganglion cell
s to photoreceptors is about 2.5; almost every ganglion cell receives data from a single cone, and each cone feeds onto between 1 and 3 ganglion cells. Therefore, the acuity of foveal vision is limited only by the density of the cone mosaic, and the fovea is the area of the eye with the highest sensitivity to fine details.
Compared to the rest of the retina
, the cones in the foveal pit have a smaller diameter and can, therefore, be more densely packed (in a hexagonal 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
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 to pass through the inner, non-sensitive neurons and allows direct passage to the receptors. It is employed for accurate vision in the direction where it is pointed. If an object is large and thus covering a large angle, the eyes must constantly shift their gaze to subsequently bring different portions of the image into the fovea (as in reading).
Since the macula does not have a blood supply, the fovea must receive oxygen
from the vessels in the choroid
, which is across the retinal pigment epithelium 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
when under bright illumination.
Since cones contain the pigmented opsin
s that allow humans to discriminate color, the fovea is largely responsible for the color vision
in humans, which is superior to that of most other mammal
s.
The fovea comprises less than 1% of retinal size but takes up over 50% of the visual cortex
in the brain.
The foveal pit is not located exactly on the optical axis
, 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.
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. Astronomer
s 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
, with the carotenoid
s zeaxanthin
and lutein
(Balashov and Bernstein, 1998), present in the cone axon
s of the Henle fibre layer. The pigment area absorbs blue light and is probably an evolution
ary adaptation to the problem of chromatic aberration
.
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
primates. 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 that elsewhere in the retina form more superficial layers having been displaced away from the foveal region during late fetal
and early postnatal
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 allow the eye to see the colours that humans need 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. The rods are located on the fovea's periphery. This assists the eye in seeing in the dark.
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
, located in the center of the macula
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...
region of the retina
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...
.
The fovea is responsible for sharp central vision
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
(also called foveal
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, watching television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
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. The parafovea is the intermediate belt, where the ganglion
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
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...
area that delivers highly compressed information of low resolution. Approximately 50% of the nerve fibers in the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
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.
Description
In the human eye the term fovea (or fovea centralis) denotes the pit in the retinaRetina
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...
, which allows for maximum acuity of vision.
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s. The center 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 cell
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 visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
. The central fovea consists of very compact cones, thinner and more rod-like in appearance than cones elsewhere. Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.
In the primate fovea (presumably including human) the ratio of ganglion cell
Ganglion cell
A retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...
s to photoreceptors is about 2.5; almost every ganglion cell receives data from a single cone, and each cone feeds onto between 1 and 3 ganglion cells. Therefore, the acuity of foveal vision is limited only by the density of the cone mosaic, and the fovea is the area of the eye with the highest sensitivity to fine details.
Compared to the rest of the retina
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...
, the cones in the foveal pit have a smaller diameter and can, therefore, be more densely packed (in a hexagonal 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
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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 to pass through the inner, non-sensitive neurons and allows direct passage to the receptors. It is employed for accurate vision in the direction where it is pointed. If an object is large and thus covering a large angle, the eyes must constantly shift their gaze to subsequently bring different portions of the image into the fovea (as in reading).
Since the macula does not have a blood supply, the fovea must receive oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
from the vessels in the choroid
Choroid
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...
, which is across the retinal pigment epithelium and Bruch's membrane
Bruch's membrane
Bruch's membrane is the innermost layer of the choroid. It is also called the vitreous lamina, because of its glassy microscopic appearance.It is 2–4 μm thick.-Layers:Bruch's membrane consists of five layers :...
. 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 (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
when under bright illumination.
Since cones contain the pigmented opsin
Opsin
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...
s that allow humans to discriminate color, the fovea is largely responsible for the color vision
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...
in humans, which is superior to that of most other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s.
The fovea comprises less than 1% of retinal size but takes up over 50% of the visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
in the brain.
The foveal pit is not located exactly on the optical axis
Optical axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope.The optical axis is an imaginary line that defines the path along which light propagates through the system...
, 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.
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. Astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s know this; in order to observe a dim star, they use averted vision
Averted vision
Averted vision is a technique for viewing faint objects which uses peripheral vision. It involves not looking directly at the object, but looking a little off to the side, while continuing to concentrate on the object. This subject is discussed in the popular astronomy literature but only a few...
, looking out of "the side of their eyes".
The fovea is covered in a yellow pigment called xanthophyll
Xanthophyll
Xanthophylls are yellow pigments that form one of two major divisions of the carotenoid group. The name is from Greek xanthos + phyllon , due to their formation of the yellow band seen in early chromatography of leaf pigments...
, with the carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
s zeaxanthin
Zeaxanthin
Zeaxanthin is one of the most common carotenoid alcohols found in nature. It is important in the xanthophyll cycle. Synthesized in plants & some micro-organisms, it is the pigment that gives paprika , corn, saffron, and many other plants & microbes their characteristic color.The name is derived...
and lutein
Lutein
Lutein is a xanthophyll and one of 600 known naturally occurring carotenoids. Lutein is synthesized only by plants and like other xanthophylls is found in high quantities in green leafy vegetables such as spinach and kale...
(Balashov and Bernstein, 1998), present in the cone axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s of the Henle fibre layer. The pigment area absorbs blue light and is probably an evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary adaptation to the problem of chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
.
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
Simian
The simians are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, , and the New World monkeys or platyrrhines. Simians tend to be larger than the "lower primates" or prosimians.- Classification and evolution :The simians are split into three groups...
primates. 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 that elsewhere in the retina form more superficial layers having been displaced away from the foveal region during late fetal
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
and early postnatal
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...
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 allow the eye to see the colours that humans need 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. The rods are located on the fovea's periphery. This assists the eye in seeing in the dark.
See also
- Eye movement
- Eye movement in language readingEye movement in language readingEye movement in reading involves visual processing of words. This was 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 movements intermingled with short stops...
- Eye movement in music readingEye movement in music readingEye 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...
- Gaze-contingency paradigmGaze-contingency paradigmThe gaze-contingency paradigm is a general term for techniques allowing to change the display on a computer screen in function of where the viewer is looking.Gaze-contingent techniques are part of the eye movement field of study in psychology...
- Macular degenerationMacular degenerationAge-related macular degeneration is a medical condition which usually affects older adults and 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 blindness and visual impairment in older adults...
- Foveated imagingFoveated imagingthumb|16:1 compression. Foveated image with fixation point at Stephen F. Austin statue.Foveated imaging is a digital image processing technique in which the image resolution, or amount of detail, varies across the image according to one or more "fixation points."A fixation point indicates the...