Fovea centralis

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Fovea centralis
Schematic diagram of the human eye en.svg
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 activities where visual detail is of primary importance, such as reading and driving. 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.[further explanation needed] Approximately half of the nerve fibers in the optic nerve carry information from the fovea, while the remaining half carry information from the rest of the retina. The parafovea extends to a radius of 1.25 mm from the central fovea, and the perifovea is found at a 2.75 mm radius from the fovea centralis.[3]

Structure[edit]

The diagram shows the relative acuity of the left human eye (horizontal section) in degrees from the fovea.[4]

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.[5]

The fovea is located in a small avascular zone and receives most of its oxygen from the vessels in the choroid, which is across the retinal pigment epithelium and Bruch's membrane. The high spatial density of cones along with the absence of blood vessels at the fovea accounts for the high visual acuity capability at the fovea.[6]

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.[1] 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.

Size[edit]

Fovea size is relatively small compared to the rest of retina, but the fovea is the only area of the retina where 20/20 vision is attainable, and very important for seeing fine detail and color.[7][8]

Function[edit]

Illustration of the distribution of cone cells in the fovea of a individual with normal color vision (left), and a color blind (protanopic) retina. Note that the center of the fovea holds very few blue-sensitive cones.

In the primate fovea (including humans) 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.[9] 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.[10] Cones in the central fovea express pigments that are sensitive to green and red light. These cones are the 'midget' pathways that also underpin high acuity functions of the fovea.

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.[11] The fovea sees only the central two degrees of the visual field, (approximately twice the width of your thumbnail at arm's length).[12] 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).

Distribution of rods and cones along a line passing through the fovea and the blind spot of a human eye[13]

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.[14][15] 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.[16] This is also accompanied by a lower density of blue cones at the center of the fovea.[17] 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.[17]

Entoptic effects in the Fovea[edit]

The presence of the pigment in the radially arranged axons of the Henle fiber layer causes it to be dichroic and birefringent[18] 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.[19] 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.[19][20] This is called the Maxwell's spot after James Clerk Maxwell[21] who discovered it.

In other animals[edit]

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[edit]

See also[edit]

This article uses anatomical terminology; for an overview, see anatomical terminology.

References[edit]

  1. ^ a b "Simple Anatomy of the Retina". Webvision. University of Utah. Retrieved 2011-09-28. 
  2. ^ a b "Relation Between Superficial Capillaries and Foveal Structures in the Human Retina" - (with nomenclature of fovea terms), Masayuki Iwasaki and Hajime Inomara, - InvestigativeOphthalmology & Visual Science (journal), - volume 27, pages 1698-1705, 1986, IOVS.org, webpage: - IOVS-fovea-capillaries. -
  3. ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
  4. ^ Hans-Werner Hunziker, (2006) Im Auge des Lesers: foveale und periphere Wahrnehmung - vom Buchstabieren zur Lesefreude [ The function of the fovea is to catch detailed visual information 3 to 4 times per second at different parts of the visual field. The brain integrates this information within the framework of the condensed peripheral vision (extra-foveal information). the eye of the reader: foveal and peripheral perception - from letter recognition to the joy of reading] Transmedia Stäubli Verlag Zürich 2006 ISBN 978-3-7266-0068-6
  5. ^ Emmett T. Cunningham, Paul Riordan-Eva. Vaughan & Asbury's general ophthalmology. (18th ed. ed.). McGraw-Hill Medical. p. 13. ISBN 978-0071634205. 
  6. ^ Provis et al., Prog Retin Eye Res Vol35 pp63-81
  7. ^ "Age-Related Macular Degeneration (AMD) by Gregory S. Hageman". Retrieved December 11, 2013. 
  8. ^ "Macular Degeneration Frequently Asked Questions". Retrieved December 11, 2013. 
  9. ^ Ahmad et al., 2003. Cell density ratios in a foveal patch in macaque retina. Vis. Neurosci. 20:189-209.
  10. ^ Smithsonian/The National Academies, Light:Student Guide and Source Book. Carolina Biological Supply Company, 2002. ISBN 0-89278-892-5.
  11. ^ Krantz, John H. (1 October 2012). "Chapter 3: The Stimulus and Anatomy of the Visual System". Experiencing Sensation and Perception. Pearson Education. ISBN 978-0-13-097793-9. OCLC 711948862. Retrieved 6 April 2012. Available online ahead of publication. 
  12. ^ Fairchild, Mark. (1998), Color Appearance Models. Reading, Mass.: Addison, Wesley, & Longman, p.7. ISBN 0-201-63464-3
  13. ^ Foundations of Vision, Brian A. Wandell
  14. ^ Krinsky, N. I.; Landrum, J. T.; Bone, R. A. (2003). "Biologic mechanisms of the protective role of lutein and zeaxanthin in the eye". Annual Review of Nutrition 23: 171–201. doi:10.1146/annurev.nutr.23.011702.073307. PMID 12626691.  edit
  15. ^ Landrum, J. T.; Bone, R. A. (2001). "Lutein, Zeaxanthin, and the Macular Pigment". Archives of Biochemistry and Biophysics 385 (1): 28–40. doi:10.1006/abbi.2000.2171. PMID 11361022.  edit
  16. ^ Beatty, S.; Boulton, M.; Henson, D.; Koh, H. -H.; Murray, I. J. (1999). "Macular pigment and age related macular degeneration". British Journal of Ophthalmology 83 (7): 867–877. doi:10.1136/bjo.83.7.867. PMC 1723114. PMID 10381676.  edit
  17. ^ a b Curcio, C. A.; Allen, K. A.; Sloan, K. R.; Lerea, C. L.; Hurley, J. B.; Klock, I. B.; Milam, A. H. (1991). "Distribution and morphology of human cone photoreceptors stained with anti-blue opsin". The Journal of Comparative Neurology 312 (4): 610–624. doi:10.1002/cne.903120411. PMID 1722224.  edit
  18. ^ Vannasdale, D. A.; Elsner, A. E.; Weber, A.; Miura, M.; Haggerty, B. P. (2009). "Determination of foveal location using scanning laser polarimetry". Journal of Vision 9 (3): 21.21–17. doi:10.1167/9.3.21. PMC 2970516. PMID 19757960.  edit
  19. ^ a b Magnussen, S.; Spillmann, L.; Stürzel, F.; Werner, J. S. (2001). "Filling-in of the foveal blue scotoma". Vision Research 41 (23): 2961–2967. doi:10.1016/S0042-6989(01)00178-X. PMC 2715890. PMID 11704235.  edit
  20. ^ Isobe, K.; Motokawa, K. (1955). "Functional Structure of the Retinal Fovea and Maxwell's Spot". Nature 175 (4450): 306–307. doi:10.1038/175306a0. PMID 13235884.  edit
  21. ^ Flom, M. C.; Weymouth, F. W. (1961). "Centricity of Maxwell's Spot in Strabismus and Amblyopia". Archives of Ophthalmology 66 (2): 260. doi:10.1001/archopht.1961.00960010262018.  edit