View source for Medicine:Fovea centralis

{{short description|Small pit in the retina of the eye responsible for all central vision}}
{{Hatnote|Not to be confused with the [[Medicine:Optic disc|optic disc]], a nearby structure that also carries signals to the optic nerve.}}
{{Infobox anatomy
| Name        = Fovea centralis
| Latin       = fovea centralis
| Image       = Schematic diagram of the human eye en.svg
| Caption     = Schematic diagram of the [[Biology:Human eye|human eye]], with the fovea at the bottom. It shows a horizontal section through the right eye.
| Width       = 
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}}
The '''fovea centralis''' is a small, central pit composed of closely packed [[Biology:Cone cell|cones]] in the [[Biology:Eye|eye]]. It is located in the center of the macula lutea of the [[Biology:Retina|retina]].<ref name=WebvisionRetina>{{cite web |url=http://webvision.med.utah.edu/sretina.html |title=Simple Anatomy of the Retina |work=Webvision |publisher=University of Utah |access-date=2011-09-28 |archive-url=https://web.archive.org/web/20110315083230/http://webvision.med.utah.edu/sretina.html |archive-date=2011-03-15 |url-status=dead }}</ref><ref name=IOVSnomen>{{cite journal |pmid=3793399 |url=http://iovs.arvojournals.org/article.aspx?volume=27&page=1698 |year=1986 |last1=Iwasaki |first1=M |title=Relation between superficial capillaries and foveal structures in the human retina |journal=Investigative Ophthalmology & Visual Science |volume=27 |issue=12 |pages=1698–705 |last2=Inomata |first2=H }}</ref>

The fovea is responsible for sharp central [[Philosophy:Visual perception|vision]] (also called foveal vision), which is necessary in humans for activities for which visual detail is of primary importance, such as reading and driving. The fovea is surrounded by the ''parafovea'' belt and the ''perifovea'' outer region.<ref name=IOVSnomen/>

The parafovea is the intermediate belt, where the [[Biology:Retinal ganglion cell|ganglion cell]] layer is composed of more than five layers 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 layers 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. That, in turn, is surrounded by a larger [[Peripheral vision|peripheral]] area, which delivers highly compressed information of low resolution following the pattern of compression in [[Foveated imaging|foveated imaging]].{{citation needed|date=August 2018}}

Approximately half the [[Biology:Nerve|nerve]] fibers in the [[Biology:Optic nerve|optic nerve]] carry information from the fovea, while the remaining half carry information from the rest of the retina. The [[Biology:Parafovea|parafovea]] extends to a radius of 1.25&nbsp;mm from the central fovea, and the perifovea is found at a 2.75&nbsp;mm radius from the fovea centralis.<ref>"eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD</ref>

The term fovea comes {{ety|la|fovea|pit}}.

Fovea centralis was named by German [[Biology:Histology|histologist]] Carl Bergmann.<ref name="LT">{{cite journal |last1=Thibos |first1=Larry |last2=Lenner |first2=Katharina |last3=Thibos |first3=Cameron |title=Carl Bergmann (1814–1865) and the discovery of the anatomical site in the retina where vision is initiated |journal=Journal of the History of the Neurosciences |date=18 Dec 2023 |pages=1–24 |doi=10.1080/0964704X.2023.2286991 |pmid=38109332 |s2cid=266361309 |url=https://www.tandfonline.com/doi/full/10.1080/0964704X.2023.2286991?src=}}</ref>

==Structure==
The fovea is a depression in the inner retinal surface, about 1.5&nbsp;mm wide, the photoreceptor layer of which is entirely cones and which is specialized for maximum visual acuity. Within the fovea is a region of 0.5mm diameter called the [[Medicine:Foveal avascular zone|foveal avascular zone]] (an area 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.<ref name=vaughan>{{cite book|author1=Emmett T. Cunningham |author2=Paul Riordan-Eva |title=Vaughan & Asbury's general ophthalmology|publisher=McGraw-Hill Medical|page=13|isbn=978-0-07-163420-5|edition=18th|year=2011 }}</ref>

The fovea is located in a small avascular zone and receives most of its [[Chemistry:Oxygen|oxygen]] from the vessels in the [[Biology:Choroid|choroid]], which is across the [[Medicine:Retinal pigment epithelium|retinal pigment epithelium]] and [[Medicine:Bruch's membrane|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.<ref name=pmid23500068>{{cite journal |doi=10.1016/j.preteyeres.2013.01.005 |pmid=23500068 |pmc=3658155 |title=Adaptation of the central retina for high acuity vision: Cones, the fovea and the avascular zone |journal=Progress in Retinal and Eye Research |volume=35 |pages=63–81 |year=2013 |last1=Provis |first1=Jan M |last2=Dubis |first2=Adam M |last3=Maddess |first3=Ted |last4=Carroll |first4=Joseph }}</ref>

The center of the fovea is the [[Biology:Foveola|foveola]] – about 0.35&nbsp;mm in diameter – or central pit where only cone photoreceptors are present and there are virtually no [[Biology:Rod cell|rods]].<ref name=WebvisionRetina/> 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 [[Hexagon|hexagon]]al pattern). Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.

In 2018 the anatomy of the foveola was reinvestigated, and it was discovered that outer segments from the central foveolar cones of monkeys are not straight and twice as long as those from the parafovea.<ref name=pmid29576957>{{cite journal |doi=10.7717/peerj.4482 |pmid=29576957 |pmc=5853608 |title=The anatomy of the foveola reinvestigated |journal=PeerJ |volume=6 |pages=e4482 |year=2018 |last1=Tschulakow |first1=Alexander V |last2=Oltrup |first2=Theo |last3=Bende |first3=Thomas |last4=Schmelzle |first4=Sebastian |last5=Schraermeyer |first5=Ulrich |doi-access=free }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

===Size===
The size of the fovea is relatively small with regard to the rest of the retina. However, it is the only area in the retina where [[Biology:Visual acuity#Expression|20/20 vision]] is attainable, and is the area where fine detail and colour can be distinguished.<ref>{{cite web |url=http://webvision.med.utah.edu/book/part-xii-cell-biology-of-retinal-degenerations/age-related-macular-degeneration-amd/ |title=Age-Related Macular Degeneration (AMD) |author=Gregory S. Hageman |access-date=December 11, 2013}}</ref><ref>{{cite web |url=http://www.eyesight.org/Macular_Degeneration/FAQ/faq.html |title=Macular Degeneration Frequently Asked Questions |access-date=December 11, 2013 |archive-url=https://web.archive.org/web/20181215190942/http://www.eyesight.org/Macular_Degeneration/FAQ/faq.html |archive-date=December 15, 2018 |url-status=dead }}</ref>

===Properties===
[[File:Retina-OCT800.png|thumb|Time-domain OCT of the macular area of a retina at 800 nm, axial resolution 3 µm]]
[[File:SD-OCT Macula Cross-Section.png|thumb|Spectral-domain OCT macula cross-section scan]]
[[File:Macula Histology OCT.jpg|alt=macula histology (OCT)|thumb|Macula histology (OCT)]]
[[Image:AcuityHumanEye.svg|270px|thumb |right| Diagram showing the relative acuity of the left human eye (horizontal section) in degrees from the fovea]]
[[File:Macula.svg|thumb|Photograph of the retina of the human eye, with overlay diagrams showing the positions and sizes of the macula, fovea, and optic disc]]

* Anatomical macula / macula lutea / area centralis (clinical: [[Medicine:Posterior pole|posterior pole]]):
** Diameter = 5.5mm (~3.5 disc-diameters) (about 18 deg of [[Biology:Visual field|VF]])
** Demarcated by the superior and inferior temporal arterial arcades.
** Has an [[Ellipse|elliptical]] shape horizontally.
** Histologically the only region of the retina where [[Medicine:Ganglion cell layer|GCL]] has >1 layer of ganglion cells
** Yellowish appearance = luteal pigments ([[Chemistry:Xanthophyll|xanthophyll]] and beta-carotenoid (''beta-carotene'') in the outer nuclear layers inward.
* Anatomical [[Biology:Perifovea|perifovea]]: 
** Region between [[Biology:Parafovea|parafovea]] (2.5mm) and edge of macula
** [[Medicine:Ganglion cell layer|GCL]] has 2–4 layers of cells.
** 12 cones / 100&nbsp;um
* Anatomical [[Biology:Parafovea|parafovea]]: 
** Diameter = 2.5mm. 
** [[Medicine:Ganglion cell layer|GCL]] has >5 layers of cells, and highest density of cones
* Anatomical fovea / fovea centralis (clinical: macula)
** Area of depression in the centre of the macula lutea. 
** Diameter = 1.5mm (~1 disc-diameter) (about 5 deg of [[Biology:Visual field|VF]])
* [[Medicine:Foveal avascular zone|Foveal avascular zone]] (FAZ)
** Diameter = 0.5mm (about 1.5 deg of [[Biology:Visual field|VF]])
** Approximately equal to the [[Biology:Foveola|foveola]]
* Anatomical [[Biology:Foveola|foveola]] (clinical: fovea)
** Diameter = 0.35mm (about 1 deg of [[Biology:Visual field|VF]])
** the central floor of depression of fovea centralis
** 50 cones / 100&nbsp;um
** Highest visual acuity
* Anatomical umbo
** Represents the precise center of the macula<ref>Yanoff M, Duker JS. 2014. Ophthalmology. In: Schubert HD, editor. Part 6 Retina and Vitreous, Section 1 Anatomy. 4th ed. China: Elsevier Saunders. p. 420.</ref>
** Diameter = 0.15mm
** Corresponds to the clinical light reflex

==Function==
[[File:ConeMosaics.jpg|thumb|300px|right|Illustration of the distribution of cone cells in the fovea of an 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 [[Biology:Primate|primate]] fovea (including humans) the ratios of [[Biology:Retinal ganglion cell|ganglion cell]]s to [[Biology:Photoreceptor cell|photoreceptor]]s is about 2.5; almost every ganglion cell receives data from a single cone, and each cone feeds onto between one and 3 ganglion cells.<ref name=pmid12916740>{{cite journal |doi=10.1017/s0952523803202091 |pmid=12916740 |title=Cell density ratios in a foveal patch in macaque retina |journal=Visual Neuroscience |volume=20 |issue=2 |pages=189–209 |year=2003 |last1=Ahmad |first1=Kareem M |last2=Klug |first2=Karl |last3=Herr |first3=Steve |last4=Sterling |first4=Peter |last5=Schein |first5=Stan |url=http://retina.anatomy.upenn.edu/pdfiles/6189.pdf |citeseerx=10.1.1.61.2917 |s2cid=2894449 }}</ref> 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.<ref>Smithsonian/The National Academies, ''Light:Student Guide and Source Book.'' [[Company:Carolina Biological Supply Company|Carolina Biological Supply Company]], 2002. {{ISBN|0-89278-892-5}}.</ref> Cones in the central fovea express [[Biology:Opsin|opsin]]s 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 [[Biology:Visual cortex|visual cortex]] in the brain.<ref>{{cite book|last=Krantz|first=John H.|title=Experiencing Sensation and Perception|url=http://psych.hanover.edu/classes/sensation/|access-date=6 April 2012|year=2012|publisher=Pearson Education|isbn=978-0-13-097793-9|oclc=711948862|chapter=Chapter 3: The Stimulus and Anatomy of the Visual System|chapter-url=http://psych.hanover.edu/classes/sensation/chapters/Chapter%203.pdf}}</ref>  The fovea sees only the central two degrees of the visual field, (approximately twice the width of your thumbnail at arm's length).<ref>Fairchild, Mark. (1998), ''Color Appearance Models''. Reading, Mass.: Addison, Wesley, & Longman, p. 7. {{ISBN|0-201-63464-3}}</ref><ref>O’Shea, R. P. (1991). Thumb’s rule tested: Visual angle of thumb’s width is about 2 deg. Perception, 20, 415-418. https://doi.org/10.1068/p200415</ref> If an object is large and thus covers a large angle, the eyes must constantly shift their [[Philosophy:Gaze|gaze]] to subsequently bring different portions of the image into the fovea (as in reading). Foveal fixation is also considered as a overt form of attention which allows to focus sensory processing resources on the most relevant sources of information.<ref>{{Citation|last=Yarbus|first=Alfred L.|title=Methods|date=1967|url=http://dx.doi.org/10.1007/978-1-4899-5379-7_2|work=Eye Movements and Vision|pages=5–58|place=Boston, MA|publisher=Springer US|doi=10.1007/978-1-4899-5379-7_2 |isbn=978-1-4899-5381-0|access-date=2022-01-30}}</ref><ref>{{Cite journal|last1=Borji|first1=Ali|last2=Itti|first2=Laurent|date=2013|title=State-of-the-Art in Visual Attention Modeling|url=http://dx.doi.org/10.1109/tpami.2012.89|journal=IEEE Transactions on Pattern Analysis and Machine Intelligence|volume=35|issue=1|pages=185–207|doi=10.1109/tpami.2012.89|pmid=22487985 |s2cid=641747 |issn=0162-8828}}</ref><ref>{{Cite journal|last1=Tatler|first1=B. W.|last2=Hayhoe|first2=M. M.|last3=Land|first3=M. F.|last4=Ballard|first4=D. H.|date=2011-05-27|title=Eye guidance in natural vision: Reinterpreting salience|url=http://dx.doi.org/10.1167/11.5.5|journal=Journal of Vision|volume=11|issue=5|pages=5|doi=10.1167/11.5.5|pmid=21622729 |pmc=3134223 |issn=1534-7362}}</ref><ref>{{Cite journal|last1=Foulsham|first1=Tom|last2=Walker|first2=Esther|last3=Kingstone|first3=Alan|date=2011|title=The where, what and when of gaze allocation in the lab and the natural environment|journal=Vision Research|volume=51|issue=17|pages=1920–1931|doi=10.1016/j.visres.2011.07.002|pmid=21784095 |s2cid=17511680 |issn=0042-6989|doi-access=free}}</ref> Also, foveated vision may allow speeding up learning of specific visual tasks by disregarding not relevant context and focusing on the relevant information only with lower dimensionality.<ref>{{Cite journal|last=Sailer|first=U.|date=2005-09-28|title=Eye-Hand Coordination during Learning of a Novel Visuomotor Task|url=http://dx.doi.org/10.1523/jneurosci.2658-05.2005|journal=Journal of Neuroscience|volume=25|issue=39|pages=8833–8842|doi=10.1523/jneurosci.2658-05.2005|pmid=16192373 |pmc=6725583 |issn=0270-6474}}</ref><ref>{{Cite journal|last1=Ognibene|first1=Dimitri|last2=Baldassare|first2=Gianluca|date=2014|title=Ecological Active Vision: Four Bioinspired Principles to Integrate Bottom–Up and Adaptive Top–Down Attention Tested With a Simple Camera-Arm Robot|journal=IEEE Transactions on Autonomous Mental Development|volume=7|issue=1|pages=3–25|doi=10.1109/tamd.2014.2341351|s2cid=1197651 |issn=1943-0604|doi-access=free|hdl=10281/301362|hdl-access=free}}</ref>

[[File:Human photoreceptor distribution.svg|thumb|250px|Distribution of rods and cones along a line passing through the fovea and the [[Biology:Blind spot (vision)|blind spot]] of a human eye<ref>[https://stanford.edu/group/vista/cgi-bin/FOV/chapter-3-the-photoreceptor-mosaic Foundations of Vision] {{Webarchive|url=https://web.archive.org/web/20131203022748/http://stanford.edu/group/vista/cgi-bin/FOV/chapter-3-the-photoreceptor-mosaic |date=2013-12-03 }}, Brian A. Wandell</ref>]]
Since the fovea does not have rods, it is not sensitive to dim lighting. Hence, in order to observe dim stars, [[Astronomy:Astronomer|astronomer]]s use [[Astronomy:Averted vision|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 [[Biology:Carotenoid|carotenoid]] pigments [[Chemistry:Lutein|lutein]] and [[Chemistry:Zeaxanthin|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.<ref name="Krinsky2003">{{cite journal|last1=Krinsky|first1=Norman I|last2=Landrum|first2=John T|last3=Bone|first3=Richard A|year=2003|title=Biologic Mechanisms of the Protective Role of Lutein and Zeaxanthin in the Eye|journal=Annual Review of Nutrition|volume=23|pages=171–201|doi=10.1146/annurev.nutr.23.011702.073307|pmid=12626691}}</ref><ref name=Landrum2001>{{cite journal |doi=10.1006/abbi.2000.2171 |pmid=11361022 |title=Lutein, Zeaxanthin, and the Macular Pigment |journal=Archives of Biochemistry and Biophysics |volume=385 |issue=1 |pages=28–40 |year=2001 |last1=Landrum |first1=John T |last2=Bone |first2=Richard A }}</ref> 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 [[Physics:Chromatic aberration|chromatic aberration]].<ref name=Beatty1999>{{cite journal |doi=10.1136/bjo.83.7.867 |pmid=10381676 |title=Macular pigment and age related macular degeneration |journal=British Journal of Ophthalmology |volume=83 |issue=7 |pages=867–877 |year=1999 |last1=Beatty |first1=S |last2=Boulton |first2=M |last3=Henson |first3=D |last4=Koh |first4=H-H |last5=Murray |first5=I J |pmc=1723114 }}</ref> This is also accompanied by a lower density of blue cones at the center of the fovea.<ref name=Curcio1991>{{cite journal |doi=10.1002/cne.903120411 |pmid=1722224 |title=Distribution and morphology of human cone photoreceptors stained with anti-blue opsin |journal=The Journal of Comparative Neurology |volume=312 |issue=4 |pages=610–624 |year=1991 |last1=Curcio |first1=Christine A |last2=Allen |first2=Kimberly A |last3=Sloan |first3=Kenneth R |last4=Lerea |first4=Connie L |last5=Hurley |first5=James B |last6=Klock |first6=Ingrid B |last7=Milam |first7=Ann H |s2cid=1947541 }}</ref> 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.<ref name=Curcio1991/>

===Angular size of foveal cones===
On average, each square millimeter (mm) of the fovea contains approximately 147,000 cone cells,<ref>{{cite book|url=https://books.google.com/books?id=mngaB6jFVS0C&pg=PA97|title=An Eye on Numbers: A Ready Reckoner in Ophthalmology|page=97|author=Shroff, Anand|isbn=978-81-921123-1-2|year=2011|publisher=Postscript Media Pvt }}</ref> or {{#expr: 147000^0.5 round 0}} cones per millimeter. The average [[Focal length|focal length]] of the eye, i.e. the distance between the lens and fovea, is 17.1&nbsp;mm.<ref>{{cite book|title=Optical Processes in Microparticles and Nanostructures: A Festschrift Dedicated to Richard Kounai Chang on His Retirement from Yale University|url=https://books.google.com/books?id=_yYrIBT42BkC&pg=PA414|author1=Serpenguzel, Ali |author2=Serpengüzel, Ali |author3=Poon, Andrew W. |isbn=978-981-4295-77-2|year=2011| publisher=World Scientific }}</ref> From these values, one can calculate the average [[Angle of view|angle of view]] of a single sensor (cone cell), which is approximately {{#expr: 2*asin((1/147000^0.5)/(2*17.1))*180*60*60/pi round 2}} arc seconds.

The following is a table of [[Pixel density|pixel densities]] required at various distances so that there is one pixel per 31.5 arc seconds:
{|class="wikitable"
!Example object!!Distance from eye assumed!![[Physics:Pixels per inch|PPI]] (PPCM) to match <br />avg. foveal cone density <br /> (20/10.5 vision)
|-
|Phone or tablet||{{convert|10|in|cm|sigfig=3}}||{{convert|655.6|cm|in|abbr=values}}
|-
|Laptop screen||{{convert|2|ft|cm|sigfig=2}}||{{convert|273.2|cm|in|abbr=values}}
|-
|42" ({{convert|42|in|m|sigfig=3|disp=output only}}) 16:9 HDTV, [[Engineering:Optimum HDTV viewing distance|{{formatnum:30}}° view]]||{{convert|5.69|ft|m}}||{{convert|96.0|cm|in|abbr=values}}
|}

Peak cone density varies highly between individuals, such that peak values below 100,000 cones/mm{{sup|2}} and above 324,000 cones/mm{{sup|2}} are not uncommon.<ref name=pmid2324310>{{cite journal |doi=10.1002/cne.902920402 |pmid=2324310 |title=Human photoreceptor topography |journal=The Journal of Comparative Neurology |volume=292 |issue=4 |pages=497–523 |year=1990 |last1=Curcio |first1=Christine A |last2=Sloan |first2=Kenneth R |last3=Kalina |first3=Robert E |last4=Hendrickson |first4=Anita E |s2cid=24649779 }}</ref> Assuming average focal lengths, this suggests that individuals with both high cone densities and perfect optics may resolve pixels with an angular size of {{#expr: 2*asin((1/324000^0.5)/(2*17.1))*180*60*60/pi round 1}} arc seconds, requiring PPI values at least {{#expr: 31.5/21.2 round 1}} times those shown above in order for images not to appear pixelated.

It is worth noting that individuals with [[Biology:Visual acuity|20/20]] (6/6 m) vision, defined as the ability to discern a 5x5 pixel letter that has an angular size of 5 arc minutes, cannot see pixels smaller than 60 arc seconds. In order to resolve a pixel the size of 31.5 and 21.2 arc seconds, an individual would need 20/10.5 (6/3.1 m) and 20/7.1 (6/2.1 m) vision, respectively. To find the PPI values discernible at 20/20, simply divide the values in the above table by the visual acuity ratio (e.g. 96 PPI / (20/10.5 vision) = 50.4 PPI for 20/20 vision).

===Entoptic 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<ref name=VanNasdale2009>{{cite journal |doi=10.1167/9.3.21 |pmid=19757960 |pmc=2970516 |title=Determination of foveal location using scanning laser polarimetry |journal=Journal of Vision |volume=9 |issue=3 |pages=21.1–17 |year=2009 |last1=Vannasdale |first1=D. A |last2=Elsner |first2=A. E |last3=Weber |first3=A |last4=Miura |first4=M |last5=Haggerty |first5=B. P }}</ref> to blue light. This effect is visible through the [[Physics:Haidinger's brush|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.<ref name=Magnussen2001>{{cite journal |doi=10.1016/S0042-6989(01)00178-X |pmid=11704235 |title=Filling-in of the foveal blue scotoma |journal=Vision Research |volume=41 |issue=23 |pages=2961–2967 |year=2001 |last1=Magnussen |first1=Svein |last2=Spillmann |first2=Lothar |last3=Stürzel |first3=Frank |last4=Werner |first4=John S |pmc=2715890 }}</ref> 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.<ref name=Magnussen2001/><ref name=pmid13235884>{{cite journal |doi=10.1038/175306a0 |title=Functional Structure of the Retinal Fovea and Maxwell's Spot |journal=Nature |volume=175 |issue=4450 |pages=306–307 |year=1955 |last1=Isobe |first1=Kosaku |last2=Motokawa |first2=Koiti |pmid=13235884 |bibcode=1955Natur.175..306I |s2cid=4181434 }}</ref> This is called the '''Maxwell's spot''' after [[Biography:James Clerk Maxwell|James Clerk Maxwell]]<ref>{{cite journal |doi=10.1001/archopht.1961.00960010262018 |title=Centricity of Maxwell's Spot in Strabismus and Amblyopia |journal=Archives of Ophthalmology |volume=66 |issue=2 |pages=260–268 |year=1961 |last1=Flom |first1=M. C |last2=Weymouth |first2=F. W |pmid=13700314 }}</ref> who discovered it.

===Bifoveal fixation===
In [[Binocular vision|binocular vision]], the two eyes converge to enable bifoveal fixation, which is necessary for achieving high stereoacuity.

In contrast, in a condition known as anomalous retinal correspondence, the brain associates the fovea of one eye with an extrafoveal area of the other eye.


==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 [[Biology:Simian|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 [[Biology:Fetus|fetal]] and early postnatal life. Other foveae may show only a reduced thickness in the inner cell layers, rather than an almost complete absence.

Most birds have a single fovea, but hawks, swallows, and hummingbirds have a double fovea. The second is called the temporal fovea, which enables them to track slow movements.<ref>{{cite web | url=http://archives.evergreen.edu/webpages/curricular/2011-2012/m2o1112/web/birds.html | title= Birds Comparative Physiology of Vision | access-date=December 29, 2019 }}</ref> The density of cones in a typical bird's fovea has 400,000 cones per square millimeter, but some birds can reach a density of 1,000,000 cones per square millimeter (e.g., Common Buzzard).<ref>{{cite web | url=https://campus.murraystate.edu/faculty/tderting/anatomyatlas/salee_shaw/hawktunics.html | title=Avian Eye Tunics | access-date=December 29, 2019 }}</ref>

==Additional images==
<gallery>
File:Blausen 0389 EyeAnatomy 02.png|Illustration showing main structures of the eye including the fovea
File:Three Main Layers of the Eye.png|Structures of the eye labeled
File:Three Internal chambers of the Eye.svg|This image shows another labeled view of the structures of the eye
File:Macula lutea.svg|Schematic diagram of the macula lutea of the retina, showing perifovea, parafovea, fovea, and clinical macula
File:Retinography.jpg|A fundus photograph showing the macula as a spot to the left. The optic disc is the area on the right where blood vessels converge. The grey, more diffuse spot in the centre is a shadow [[Visual artifact|artifact]].
</gallery>

==See also==
{{Anatomy-terms}}
*[[Biology:Eye movement|Eye movement]]
*[[Biology:Gaze-contingency paradigm|Gaze-contingency paradigm]]
*[[Medicine:Macular degeneration|Macular degeneration]]
*[[Foveated imaging]]

==References==
{{Reflist}}

{{Eye anatomy}}

{{DEFAULTSORT:Fovea Centralis}}
[[Category:Human eye anatomy]]

{{Sourceattribution|Fovea centralis}}