AskDefine | Define fovea

Dictionary Definition

fovea n : area consisting of a small depression in the retina containing cones and where vision is most acute [syn: fovea centralis] [also: foveae (pl)]

User Contributed Dictionary

English

Etymology

From fovea

Pronunciation

Noun

  1. A slight depression or pit in a bone or organ.
  2. The retinal fovea, or fovea centralis, responsible for sharp central vision.

Extensive Definition

The fovea, also known as the fovea centralis, 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 ratio of ganglion cells to photoreceptors is close to one; all or 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 high detail can be seen, for example for reading text.
The fovea centralis is 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 which 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.
At the center of the macula, approximately on the visual axis, there is a pit (termed the "foveal pit") with a diameter of about 1.0 mm, that is associated with a high concentration of cone photoreceptors. 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. 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. Astronomers 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 carotenoids zeaxanthin and lutein (Balashov and Bernstein, 1998), present in the cone axons of the Henle fibre layer. The pigment area absorbs blue light and is probably an evolutionary adaptation to the problem of chromatic aberration. hi

See also

References

fovea in Bengali: ফোভিয়া
fovea in Catalan: Fòvea òptica
fovea in German: Fovea centralis
fovea in Spanish: Fóvea
fovea in French: Fovéa
fovea in Korean: 중심와
fovea in Italian: Fovea
fovea in Hebrew: רשתית#.D7.9E.D7.91.D7.A0.D7.94_.D7.A8.D7.A9.D7.AA.D7.99.D7.AA_.D7.94.D7.90.D7.93.D7.9D
fovea in Dutch: Fovea centralis
fovea in Polish: Dołek środkowy siatkówki oka
fovea in Portuguese: Fóvea
fovea in Slovak: Fovea
fovea in Swedish: Fovea
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