| 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. [1] [2] 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.[2] 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.[3]
In the human eye the term fovea (or fovea centralis) denotes the pit in the retina, which allows for maximum acuity of vision.
The human fovea has a diameter of about 1.0 mm with a high concentration of cone photoreceptors. The center of the fovea is the foveola – about 0.2 mm in diameter – where only cone photoreceptors are present and there are virtually no rods.[1] The central fovea consists of very compact cones, thinner and more rod-like in appearance than cones elsewhere. Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.
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.[5] 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.[6]
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 interfere with the passage of light striking the foveal cone mosaic. The absence of inner retinal cells from the foveae of primates is assumed to contribute further to the high acuity function of the fovea.
The fovea centralis is a central pit, near the optic axis. It eliminates the necessity to pass through the inner, non-sensitive neurons and allows direct passage to the receptors. It is employed for accurate vision in the direction where it is pointed. If an object is large and thus covering 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 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.
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.
The fovea comprises less than 1% of retinal size but takes up over 50% of the visual cortex in the brain. [7] 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.[8]
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,[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.
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.
The only photo-receptors located in the fovea of most humans are three kinds of cone photo receptors. The red, blue, and green allow the eye to see the colours that humans need for survival; however, some organisms are known to possess four independent channels for conveying color information, or possessing four different types of cone cells in the eye, a characteristic called tetrachromacy. Organisms with tetrachromacy are called tetrachromats. The rods are located on the fovea's periphery. This assists the eye in seeing in the dark.
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