Fovea centralis

Fovea centralis
Schematic diagram of the human eye, with the fovea at the bottom. It shows a horizontal section through the right eye
Latin	fovea centralis

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, 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.

Description

The diagram shows the relative acuity of the left human eye (horizontal section) in degrees from the fovea.

Fovea (or Fovea centralis) is the 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. Fovea also grossly corresponds to the retinal avascular zone (which means 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 foveal pit is not located exactly on the optical axis, but is displaced about 4 to 8 degrees temporal to it.

The center of the fovea is the foveola – about 0.2 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 hexagonal pattern). Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.

The high spatial density of cones along with the absence of blood vessels and non photosensitive neurons accounts for the high visual acuity capability at the fovea.

In the primate fovea (presumably including human) the ratio 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 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.

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.^{[citation needed]}

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 that of most other mammals.^{[citation needed]}

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 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 to subsequently bring different portions of the image into the fovea (as in reading).

Since the fovea does not have rods, it is not sensitive to dim lights. Hence, in order to observe dim stars, astronomers 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 and zeaxanthin. 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. 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.

Entopic effects in the Fovea

The presence of the pigment in the radially arranged axons of the Henle fiber layer causes it to be dichroic and birefringent 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 who discovered it.

The fovea in 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 and early postnatal life. Other foveae may show only a reduced thickness in the inner cell layers, rather than an almost complete absence.

Additional Images

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  Structures of the eye labeled

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  This image shows another labeled view of the structures of the eye

See also

• Eye movement
• Eye movement in language reading
• Eye movement in music reading
• Gaze-contingency paradigm
• Macular degeneration
• Foveated imaging