The ''fovea centralis'' is a small, central pit composed of closely packed cones in the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

. It is located in the center of the ''

macula lutea The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal av ...

'' of the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

. The ''fovea'' is responsible for sharp central

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...

(also called foveal vision), which is necessary in humans for activities for which visual detail is of primary importance, such as

reading Reading is the process of taking in the sense or meaning of symbols, often specifically those of a written language, by means of Visual perception, sight or Somatosensory system, touch. For educators and researchers, reading is a multifacete ...

and driving. 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 layers 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 layers 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''. That, in turn, is surrounded by a larger

peripheral A peripheral device, or simply peripheral, is an auxiliary hardware device that a computer uses to transfer information externally. A peripheral is a hardware component that is accessible to and controlled by a computer but is not a core compo ...

area, which delivers highly compressed information of low resolution following the pattern of compression in foveated imaging. Approximately half the

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the Electrochemistry, electrochemical nerv ...

fibers in the

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...

carry information from the ''fovea'', while the remaining half carry information from the rest of the retina. The ''parafovea'' extends to a radius of 1.25 mm from the central fovea, and the ''perifovea'' is found at a 2.75 mm radius from the ''fovea centralis''. The term ''fovea'' comes The ''fovea centralis'' was named by German histologist Carl Bergmann.

Structure

The fovea is a depression in the inner retinal surface, about 1.5 mm wide, the photoreceptor layer of which is entirely cones and which is specialized for maximum visual acuity. Within the fovea is a region of 0.5mm diameter called the

foveal avascular zone The foveal avascular zone (FAZ) is a region within the fovea centralis at the centre of the retina of the human eye that is devoid of retinal blood vessels. The geometric centre of the FAZ is often taken to be the centre of the macula and thus the ...

(an area without any blood vessels). This allows the light to be sensed without any dispersion or loss. This anatomy is responsible for the depression in the center of the fovea. The foveal pit is surrounded by the foveal rim that contains the neurons displaced from the pit. This is the thickest part of the retina. The fovea is located in a small avascular zone and receives most of its

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

from the vessels in the

choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye. It contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear o ...

, which is across the

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), ...

and Bruch's membrane. The high spatial density of cones along with the absence of blood vessels at the fovea accounts for the high visual acuity capability at the fovea. The center of the fovea is the

foveola The foveola is located within a region called the macula, a yellowish, cone photoreceptor filled portion of the human retina. Approximately 0.35 mm in diameter, the foveola lies in the center of the fovea and contains only cone cell C ...

– about 0.35 mm in diameter – or central pit 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. These cones are very densely packed (in a

hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is de ...

al pattern). Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases. In 2018, the anatomy of the foveola was reinvestigated, and it was discovered that outer segments from the central foveolar cones of monkeys are not straight and twice as long as those from the parafovea. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License

Size

The size of the fovea is relatively small with regard to the rest of the retina. However, it is the only area in the retina where

20/20 vision Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye influence the sha ...

is attainable, and is the area where fine detail and colour can be distinguished.

Properties

* Anatomical macula / macula lutea / area centralis (clinical: posterior pole): ** Diameter = 5.5 mm (~3.5 disc-diameters) (about 18 deg of VF) ** Demarcated by the superior and inferior temporal arterial arcades. ** Has an elliptical shape horizontally. ** Histologically the only region of the retina where GCL has >1 layer of ganglion cells ** Yellowish appearance = luteal pigments (

xanthophyll Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek: (), meaning "yellow", an ...

and beta-carotenoid ('' beta-carotene'') in the outer nuclear layers inward. * Anatomical perifovea: ** Region between parafovea (2.5 mm) and edge of macula ** GCL has 2–4 layers of cells. ** 12 cones / 100 μm * Anatomical parafovea: ** Diameter = 2.5 mm. ** GCL has >5 layers of cells, and highest density of cones * Anatomical fovea / fovea centralis (clinical: macula) ** Area of depression in the centre of the macula lutea. ** Diameter = 1.5 mm (~1 disc-diameter) (about 5 deg of VF) *

Foveal avascular zone The foveal avascular zone (FAZ) is a region within the fovea centralis at the centre of the retina of the human eye that is devoid of retinal blood vessels. The geometric centre of the FAZ is often taken to be the centre of the macula and thus the ...

(FAZ) ** Diameter = 0.5 mm (about 1.5 deg of VF) ** Approximately equal to the

* Anatomical

(clinical: fovea) ** Diameter = 0.35 mm (about 1 deg of VF) ** the central floor of depression of fovea centralis ** 50 cones / 100 μm ** Highest visual acuity * Anatomical umbo ** Represents the precise center of the macula ** Diameter = 0.15 mm ** Corresponds to the clinical light reflex

Function

In the

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

fovea (including humans) the ratios of ganglion cells to photoreceptors is about 2.5; almost every ganglion cell receives data from a single cone, and each cone feeds onto between one 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. Cones in the central fovea express

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...

s that are sensitive to green and red light. These cones are the 'midget' pathways that also underpin high acuity functions of the fovea. The fovea is employed for accurate vision in the direction where it is pointed. It comprises less than 1% of retinal size but takes up over 50% of the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalam ...

in the brain. The fovea sees only the central two degrees of the visual field, (approximately twice the width of your thumbnail at arm's length). If an object is large and thus covers a large angle, the eyes must constantly shift their

gaze In critical theory, philosophy, sociology, and psychoanalysis, the gaze (French: ''le regard''), in the figurative sense, is an individual's (or a group's) awareness and perception of other individuals, other groups, or oneself. Since the 20th ...

to subsequently bring different portions of the image into the fovea (as in reading). Foveal fixation is also considered as a overt form of attention which allows to focus sensory processing resources on the most relevant sources of information. Also, foveated vision may allow speeding up learning of specific visual tasks by disregarding not relevant context and focusing on the relevant information only with lower dimensionality. Human photoreceptor distribution

Since the fovea does not have rods, it is not sensitive to dim lighting. Hence, in order to observe dim stars,

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

s use averted vision, looking out of the side of their eyes where the density of rods is greater, and hence dim objects are more easily visible. The fovea has a high concentration of the yellow

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

pigments

lutein Lutein (;"Lutein"

zeaxanthin Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji ( wolfberries) ...

. They are concentrated in the ''Henle fiber layer'' (photoreceptor axons that go radially outward from the fovea) and to a lesser extent in the cones. They are believed to play a protective role against the effects of high intensities of blue light which can damage the sensitive cones. The pigments also enhance the acuity of the fovea by reducing the sensitivity of the fovea to short wavelengths and counteracting the effect of

chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...

. This is also accompanied by a lower density of blue cones at the center of the fovea. The maximum density of blue cones occurs in a ring about the fovea. Consequently, the maximum acuity for blue light is lower than that of other colours and occurs approximately 1° off center.

Angular size of foveal cones

On average, each square millimeter (mm) of the fovea contains approximately 147,000 cone cells, or cones per millimeter. The average

focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...

of the eye, i.e. the distance between the lens and fovea, is 17.1 mm. From these values, one can calculate the average

angle of view In photography, angle of view (AOV) describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term '' field of view''. It is important to distinguish the angle of view from the ...

of a single sensor (cone cell), which is approximately

arc seconds A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

. The following is a table of pixel densities required at various distances so that there is one pixel per 31.5 arc seconds: Peak cone density varies highly between individuals, such that peak values below 100,000 cones/mm and above 324,000 cones/mm are not uncommon. Assuming average focal lengths, this suggests that individuals with both high cone densities and perfect optics may resolve pixels with an angular size of arc seconds, requiring PPI values at least times those shown above in order for images not to appear pixelated. It is worth noting that individuals with 20/20 (6/6 m) vision, defined as the ability to discern a 5x5 pixel letter that has an angular size of 5 arc minutes, cannot see pixels smaller than 60 arc seconds. In order to resolve a pixel the size of 31.5 and 21.2 arc seconds, an individual would need 20/10.5 (6/3.1 m) and 20/7.1 (6/2.1 m) vision, respectively. To find the PPI values discernible at 20/20, simply divide the values in the above table by the visual acuity ratio (e.g. 96 PPI / (20/10.5 vision) = 50.4 PPI for 20/20 vision).

Entoptic effects in the fovea

The presence of the pigment in the radially arranged axons of the Henle fiber layer causes it to be

dichroic In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ab ...

and

birefringent Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefring ...

to blue light. This effect is visible through the Haidinger's brush when the fovea is pointed to a polarized light source. The combined effects of the macular pigment and the distribution of short wavelength cones results in the fovea having a lower sensitivity to blue light (blue light scotoma). Though this is not visible under normal circumstances due to "filling in" of information by the brain, under certain patterns of blue light illumination, a dark spot is visible at the point of focus. Also, if mixture of red and blue light is viewed (by viewing white light through a dichroic filter), the point of foveal focus will have a central red spot surrounded by a few red fringes. This is called the Maxwell's spot after

James Clerk Maxwell James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish physicist and mathematician who was responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism an ...

who discovered it.

Bifoveal fixation

binocular vision Binocular vision is seeing with two eyes. The Field_of_view, field of view that can be surveyed with two eyes is greater than with one eye. To the extent that the visual fields of the two eyes overlap, #Depth, binocular depth can be perceived. Th ...

, the two eyes converge to enable bifoveal fixation, which is necessary for achieving high stereoacuity. In contrast, in a condition known as anomalous retinal correspondence, the brain associates the fovea of one eye with an extrafoveal area of the other eye.

Other animals

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 in its most developed form only in Haplorhine primates, although a more rudimentary fovea-like structure exists in some diurnal

lemurs Lemurs ( ; from Latin ) are wet-nosed primates of the superfamily Lemuroidea ( ), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madagascar. Most existing lemurs are s ...

. 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 A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...

and early postnatal life. Other foveae may show only a reduced thickness in the inner cell layers, rather than an almost complete absence. Most birds have a single fovea, but hawks, swallows, and hummingbirds have a double fovea. The second is called the temporal fovea, which enables them to track slow movements. The density of cones in a typical bird's fovea has 400,000 cones per square millimeter, but some birds can reach a density of 1,000,000 cones per square millimeter (e.g., Common Buzzard).

Additional images

File:Blausen 0389 EyeAnatomy 02.png, Illustration showing main structures of the eye including the fovea File:Three Main Layers of the Eye.png, Structures of the eye labeled File:Three Internal chambers of the Eye.svg, This image shows another labeled view of the structures of the eye File:Macula lutea.svg, Schematic diagram of the macula lutea of the retina, showing perifovea, parafovea, fovea, and clinical macula File:Retinography.jpg, A fundus photograph showing the macula as a spot to the left. The optic disc is the area on the right where blood vessels converge. The grey, more diffuse spot in the centre is a shadow artifact.

References

{{DEFAULTSORT:Fovea Centralis Human eye anatomy