The fovea centralis is the name given to a small depression in the center of the yellow spot of the human retina. It is the region of sharpest vision because the fovea centralis exclusively contains three different types of cones (photoreceptors) for color vision in the wavelength ranges for red, green, and blue. The more light-sensitive rods are located outside the fovea centralis.
What is the fovea centralis?
The fovea centralis embodies the zone of sharpest color vision and is centrally located within the so-called yellow spot (macula lutea) on the retina, which reaches a diameter of 3 to 5 millimeters. The fovea centralis has a diameter of about 1.5 millimeters and is densely packed with three different color receptors, the S, M and L cones, which cover the spectral range blue through green to red. The much more light-sensitive rod-shaped photoreceptors are located outside the fovea centralis and also mostly outside the yellow spot. In the zone of sharpest vision, as the fovea centralis is also called, each individual cone is connected to a bipolar ganglion cell. This allows the visual center of the brain to accurately locate the incident light pulses and produce a sharp, virtual color image. The 1:1 interconnection of the photoreceptors achieves the highest possible biological resolution. In the central area of the fovea centralis, a small area with a diameter of about 0.33 millimeters, called foveola, can be identified. The foveola contains exclusively the M and L cones, which are particularly slender and densely packed in this area, and whose highest light sensitivity is in the green to red wavelength range.
Anatomy and structure
The fovea centralis, the region of sharpest color vision in the retina, is anatomically designed so that necessary support structures are largely displaced to the periphery to achieve the densest possible packing of cone-shaped color receptors. There are up to 6 million color receptors within the yellow spot. This means that there are on average about 240,000 color receptors per square millimeter. In the foveola, the “packing density” is much higher with M and L receptors. The foveola is surrounded by an area about 0.5 millimeters thick called the parafovea. In the parafovea, the light-strong rod-shaped photoreceptors are already mixed with the cones in a ratio of 1:1. The ring-shaped parafovea is adjoined on the outside by the perifovea, which has a ring width of 1.5 or 3 millimeters, depending on the author and definition. The outer border of the perifovea is also the outer border of the macula lutea. The cone density decreases considerably in this area, whereas the rod density increases strongly. In healthy humans, the visual axis runs through the fovea centralis, and the oculomotor muscles, the tiny positioning muscles of the eyeball, orient themselves along this axis.
Function and tasks
The main task and function of the fovea centralis is to provide the visual centers in the brain with the most accurate local information possible about incident light impulses, including their wave spectrum. From the nerve impulses received, the brain can construct a virtual image that is as sharp and colorful as possible under the lighting conditions of daylight to bright twilight. This is indeed a virtual image, since there is no real projected image on the retina or anywhere in the brain. Particularly helpful in producing a sharp image is the 1:1 interconnection of photoreceptors with bipolar photoreceptors, each having only one axon and one dendrite. Evolution relies entirely on daylight conditions for foveal vision because the fovea centralis contains almost exclusively the low-light cones as photoreceptors. The partially unconscious oculomotor system, which always strives to be able to detect “objects worth seeing” via the fovea centralis, is counterproductive in dark twilight and in darkness because there are practically no light-sensitive rods within the fovea centralis and the cones are not sufficiently sensitive for excitation. To be able to “see” an object in dark twilight, it is recommended to consciously look past the object, because then there is a chance to be able to detect the object with peripheral vision.
Diseases
Diseases and conditions related to the fovea centralis mostly involve degeneration of the retina in the area of the macula and thus also in the area of the fovea centralis or retinal detachments. The most common form of macular degeneration is age-related macular degeneration (AMD), which initially leads to a functional impairment of the so-called Bruch’s membrane. This triggers a small cascade of other problems that ultimately lead to a loss of function of the photoreceptors in the area of the macula lutea. Both men and women are equally affected by AMD. The reduction in vision caused by AMD affects only central foveal vision. Blurred, monochromatic peripheral vision is preserved. The exact causes that lead to the triggering of AMD are not (yet) sufficiently known. Strikingly, familial clusters are observed, so that very likely genetic dispositions also contribute to the onset of AMD. In rare cases, macular degeneration also occurs at a young age as in the very rare Stargardt’s disease, in the course of which there are conspicuous deposits in the pigment epithelium of the retina. In the area of the macula or fovea centralis, edema can form, accumulations of tissue fluid, which can be attributed to various causes. The fluid accumulations can lead to impaired vision, which in many cases is reversible if the cause of the edema has been corrected and the edema itself has been eliminated.
