The fovea centralis is a small, central pit composed of closely packed

cones A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines conn ...

in the eye. It is located in the center of the macula lutea of the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. 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 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 {{stack, A ganglion cell is a cell found in a ganglion. Examples of ganglion cells include: * Retinal ganglion cell (RGC) found in the ganglion cell layer of the retina * Cells that reside in the adrenal medulla, where they are involved in the ...

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 area, which delivers highly compressed information of low resolution following the pattern of compression in foveated imaging. Approximately half the nerve 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 information from the retina to the brain. In humans, the optic nerve is derived fro ...

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 from the .

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 the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

from the vessels in the choroid, which is across the

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) 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 ce ...

and

Bruch's membrane Bruch's membrane is the innermost layer of the choroid of the eye. It is also called the ''vitreous lamina'' or ''Membrane vitriae'', because of its glassy microscopic appearance. It is 2–4 μm thick. Layers Bruch's membrane consists of five l ...

. 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 – 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'' has ...

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. CC-BY icon

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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 examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...

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

Properties

* Anatomical macula / macula lutea / area centralis (clinical:

posterior pole In ophthalmology, the posterior pole is the back of the eye, usually referring to the retina between the optic disc and the macula.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company, 1990. ...

): ** Diameter = 5.5mm (~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 (, "yellow") and (, "lea ...

and beta-carotenoid ('' beta-carotene'') in the outer nuclear layers inward. * Anatomical perifovea: ** Region between parafovea (2.5mm) and edge of macula ** GCL has 2–4 layers of cells. ** 12 cones / 100 um * Anatomical parafovea: ** Diameter = 2.5mm. ** 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 = 1.5mm (~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 = 0.5mm (about 1.5 deg of VF) ** Approximately equal to the foveola * Anatomical foveola (clinical: fovea) ** Diameter = 0.35mm (about 1 deg of VF) ** the central floor of depression of fovea centralis ** 50 cones / 100 um ** Highest visual acuity * Anatomical umbo ** Represents the precise center of the macula ** Diameter = 0.15mm ** Corresponds to the clinical light reflex

Function

In the

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

fovea (including humans) the ratios of

s 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 ...

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 thalamus and ...

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, sociology, and psychoanalysis, the gaze (French ''le regard''), in the philosophical and figurative sense, is an individual's (or a group's) awareness and perception of other individuals, other groups, or oneself. The concept ...

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 their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...

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 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 and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...

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

and

birefringent Birefringence 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 said to be birefringent (or birefractive). The 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 mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and li ...

who discovered it.

Bifoveal fixation

binocular vision In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...

, 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 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 A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...

and early

postnatal The postpartum (or postnatal) period begins after childbirth and is typically considered to end within 6 weeks as the mother's body, including hormone levels and uterus size, returns to a non-pregnant state. The terms puerperium, puerperal perio ...

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 Fundus photography involves photographing the rear of an eye, also known as the fundus. Specialized fundus cameras consisting of an intricate microscope attached to a flash enabled camera are used in fundus photography. The main structures that ...

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