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Schematic diagram of the human eye en

Schematic diagram of the human eye, with the fovea at the bottom.

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. [1] [2] The fovea is responsible for sharp central 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:[2] the parafovea is the intermediate belt where the ganglion cell layer is composed of more than five rows of cells; 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. This, in turn, is surrounded by a larger peripheral area that delivers information of low resolution.

Description

At the center of the fovea there is a pit (termed the "foveal pit") with a diameter of about 0.2 mm, containing a high concentration of cone cells and virtually no rods. [1] Compared to the rest of the retina, the cones in the foveal pit are smaller and more densely packed (in a hexagonal pattern), and they are not obscured by a layer of nerve cells or blood vessels; all of this together accounts for the sharp vision associated with them.

Due to the lack of a vitreal blood supply, the fovea must receive its 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 opsins that allow humans to discriminate color, the fovea is largely responsible for the color vision in humans which is superior to most other mammals'.

The foveal pit is not located exactly on the optical axis, but is displaced about 4 to 8 degrees temporal to it.

The fovea is less than 1% of the retina but takes up over 50% of the visual cortex in the brain. [3] 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. [4]

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 peripheral vision, looking out of "the side of their eyes".

The fovea is covered in a yellow pigment called xanthophyll,[1] with the carotenoids zeaxanthin and lutein (Balashov and Bernstein, 1998), present in the cone axons of the Henle fibre layer.[1] The pigment area absorbs blue light and is probably an evolutionary adaptation to the problem of chromatic aberration.

See also

Additional images

References

  1. 1.0 1.1 1.2 1.3 "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. 2.0 2.1 "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. "The Stimulus and Anatomy of the Visual System" (with fovea description), Hanover College, Psychology Department, HanoverCollege-Fovea-PDF-as-HTML.
  4. Fairchild, Mark. (1998), Color Appearance Models. Reading, Mass.: Addison, Wesley, & Longman, p.7.

External links

Sensory system - Visual system - Eye - edit
Anterior chamber | Aqueous humour | Blind spot | Choroid | Ciliary body | Conjunctiva | Cornea | Iris | Lens | Macula | Optic disc | Optic fovea | Posterior chamber | Pupil | Retina | Schlemm's canal | Sclera | Tapetum lucidum | Trabecular meshwork | Vitreous humour
Sensory system - Visual system - edit
Eye | Optic nerve | Optic chiasm | Optic tract | Lateral geniculate nucleus | Optic radiation | Visual cortex


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