Fovea



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, 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 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
The term 'fovea' comes from the Latin, meaning 'pit'or 'arm pit'. As an anatomical term there are several foveae around the body, including in the head of the femur.

The fovea centralis is a pit in the surface of the retinas of many types of fish, reptiles and birds and amongst mammals 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 0.2mm in diameter - where only cone photoreceptors are present and there are virtually no rods. Compared to the rest of the retina, the cones in the foveal pit have a smaller diameter and can therefore be more densely packed (in a hexagonal pattern). The high spatial density of cones accounts for the high visual acuity capability at the fovea. This is enhanced by the local absence of retinal blood vessels from the fovea - which if present would interfer with the passage of light striking the foveal cone mosaic. The absence of inner retinal cells from the foveae of primates is assumed also to contribute to the high acuity function of the fovea although there is no convincing evidence to this effect, since the refractive index of the retina is not significantly different from that of the vitreous filling the pit.

Due to the lack of a retinal 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.

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. 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.