Editing Visual system

{{Short description|Body parts responsible for vision}}
{{About|the physiological components involved in vision|the ability to interpret the surrounding environment|Visual perception }}
{{Redirect|Visual sensor|electronic visual sensors|Visual sensor network}}
{{Redirect|Visual|the album|Tabú Tek#Discography{{!}}''Visual'' (album)}}

{{Infobox anatomy
| Name        = Visual system
| Latin       = 
| Image       = Human visual pathway.svg
| Caption     = The visual system includes the eyes, the connecting pathways through to the visual cortex and other parts of the brain (human system shown).| Image2 = Human eye with limbal ring, anterior view.jpg
| Caption2    = The [[eye]] is the sensory organ of the visual system. The [[iris (anatomy)|iris]], [[pupil]], and [[sclera]] are visible
}}

The '''visual system''' is the physiological basis of [[visual perception]] (the ability to [[perception|detect and process]] [[light]]). The system detects, [[phototransduction|transduces]] and interprets information concerning [[light]] within the [[visible range]] to construct an [[imaging|image]] and build a [[mental model]] of the surrounding environment. The visual system is associated with the [[eye]] and functionally divided into the [[optics|optical]] system (including [[cornea]] and [[crystalline lens|lens]]) and the [[nervous system|neural]] system (including the [[retina]] and [[visual cortex]]).

The visual system performs a number of complex tasks based on the ''image forming'' functionality of the eye, including the formation of monocular images, the neural mechanisms underlying [[stereopsis]] and assessment of distances to ([[depth perception]]) and between objects, [[motion perception]], [[pattern recognition]], accurate [[motor coordination]] under visual guidance, and [[colour vision]]. Together, these facilitate higher order tasks, such as [[Object recognition (cognitive science)|object identification]]. The [[neuropsychological]] side of visual information processing is known as [[visual perception]], an abnormality of which is called [[visual impairment]], and a complete absence of which is called [[blindness]].  The visual system also has several non-image forming visual functions, independent of visual perception, including the [[pupillary light reflex]] and circadian [[Entrainment (chronobiology)|photoentrainment]].

This article describes the human visual system, which is representative of [[mammalian vision]], and to a lesser extent the [[vertebrate]] visual system.

== System overview ==
[[File:Comprehensive List of Relevant Pathways for the Visual System.png|thumb|This diagram linearly (unless otherwise mentioned) tracks the projections of all known structures that allow for vision to their relevant endpoints in the human brain. Click to enlarge the image.]]
[[File:ERP - optic cabling.jpg|thumb|left|300px|Representation of optic pathways from each of the 4 quadrants of view for both eyes simultaneously]]

=== Optical ===
Together, the [[cornea]] and [[Lens (anatomy)|lens]] refract light into a small image and shine it on the [[retina]]. The retina [[Visual phototransduction|transduces]] this image into electrical pulses using [[Rods (eye)|rods]] and [[Cones (eye)|cones]]. The [[optic nerve]] then carries these pulses through the [[optic canal]]. Upon reaching the [[optic chiasm]] the nerve fibers decussate (left becomes right). The fibers then branch and terminate in three places.<ref>"How the Human Eye Sees." [[WebMD]]. Ed. Alan Kozarsky. WebMD, 3 October 2015. Web. 27 March 2016.</ref><ref>Than, Ker. "How the Human Eye Works." [[Live Science|LiveScience]]. [[TechMedia Network, Inc.|TechMedia Network]], 10 February 2010.   Web. 27 March 2016.</ref><ref>"How the Human Eye Works | Cornea Layers/Role | Light Rays." NKCF. The Gavin Herbert Eye Institute. Web. 27 March 2016.</ref><ref>Albertine, Kurt. Barron's Anatomy Flash Cards</ref><ref>Tillotson, Joanne. McCann, Stephanie. Kaplan's Medical Flashcards. April 2, 2013.</ref><ref>"Optic Chiasma." Optic Chiasm Function, Anatomy & Definition. Healthline Medical Team, 9 March 2015. Web. 27 March 2016.</ref><ref>Jefferey, G., and M. M. Neveu. "Chiasm Formation in Man Is Fundamentally Different from That in the Mouse." [[Nature (journal)|Nature.com]]. [[Nature Publishing Group]], 21 March 2007. Web. 27 March 2016.</ref>

=== Neural ===
Most of the optic nerve fibers end in the [[lateral geniculate nucleus]] (LGN). Before the LGN forwards the pulses to V1 of the visual cortex (primary) it gauges the range of objects and tags every major object with a velocity tag. These tags predict object movement.

The LGN also sends some fibers to V2 and V3.<ref>Card, J. Patrick, and Robert Y. Moore. "Organization of Lateral Geniculate-hypothalamic Connections in the Rat." [[Wiley Online Library]]. 1 June. 1989. Web. 27 March 2016.</ref><ref>{{Cite journal |last1=Murphy |first1=Penelope C. |last2=Duckett |first2=Simon G. |last3=Sillito |first3=Adam M. |date=1999-11-19 |title=Feedback Connections to the Lateral Geniculate Nucleus and Cortical Response Properties |url=http://dx.doi.org/10.1126/science.286.5444.1552 |journal=Science |volume=286 |issue=5444 |pages=1552–1554 |doi=10.1126/science.286.5444.1552 |pmid=10567260 |issn=0036-8075|url-access=subscription }}</ref><ref>{{Cite journal |last1=Schiller |first1=P. H. |last2=Malpeli |first2=J. G. |date=1978-05-01 |title=Functional specificity of lateral geniculate nucleus laminae of the rhesus monkey |url=https://www.physiology.org/doi/10.1152/jn.1978.41.3.788 |journal=Journal of Neurophysiology |language=en |volume=41 |issue=3 |pages=788–797 |doi=10.1152/jn.1978.41.3.788 |pmid=96227 |issn=0022-3077|url-access=subscription }}</ref><ref>{{Cite journal |last1=Schmielau |first1=F. |last2=Singer |first2=W. |date=1977 |title=The role of visual cortex for binocular interactions in the cat lateral geniculate nucleus |url=https://linkinghub.elsevier.com/retrieve/pii/0006899377909143 |journal=Brain Research |language=en |volume=120 |issue=2 |pages=354–361 |doi=10.1016/0006-8993(77)90914-3|pmid=832128 |s2cid=28796357 |url-access=subscription }}</ref><ref>{{Cite journal |last1=Clay Reid |first1=R. |last2=Alonso |first2=Jose-Manuel |date=1995-11-16 |title=Specificity of monosynaptic connections from thalamus to visual cortex |url=http://dx.doi.org/10.1038/378281a0 |journal=Nature |volume=378 |issue=6554 |pages=281–284 |doi=10.1038/378281a0 |pmid=7477347 |bibcode=1995Natur.378..281C |s2cid=4285683 |issn=0028-0836|url-access=subscription }}</ref>

V1 performs edge-detection to understand spatial organization (initially, 40 milliseconds in, focusing on even small spatial and color changes. Then, 100 milliseconds in, upon receiving the translated LGN, V2, and V3 info, also begins focusing on global organization). V1 also creates a bottom-up [[saliency map]] to guide attention or [[gaze shift]].<ref>{{Cite book |last=Zhaoping |first=Li |url=https://academic.oup.com/book/8719 |title=Understanding Vision: Theory, Models, and Data |date=2014-05-08 |publisher=Oxford University Press |isbn=978-0-19-956466-8 |edition=1st |language=en |chapter=The V1 hypothesis—creating a bottom-up saliency map for preattentive selection and segmentation |doi=10.1093/acprof:oso/9780199564668.001.0001}}</ref>

V2 both forwards (direct and via [[Pulvinar nuclei|pulvinar]]) pulses to V1 and receives them. Pulvinar is responsible for [[saccade]] and visual attention. V2 serves much the same function as V1, however, it also handles [[illusory contours]], determining depth by comparing left and right pulses (2D images), and foreground distinguishment. V2 connects to V1 - V5.

V3 helps process '[[global motion]]' (direction and speed) of objects. V3 connects to V1 (weak), V2, and the [[inferior temporal cortex]].<ref>{{Cite journal |last1=Heim |first1=Stefan |last2=Eickhoff |first2=Simon B. |last3=Ischebeck |first3=Anja K. |last4=Friederici |first4=Angela D. |last5=Stephan |first5=Klaas E. |last6=Amunts |first6=Katrin |date=2009 |title=Effective connectivity of the left BA 44, BA 45, and inferior temporal gyrus during lexical and phonological decisions identified with DCM |journal=Human Brain Mapping |language=en |volume=30 |issue=2 |pages=392–402 |doi=10.1002/hbm.20512 |issn=1065-9471 |pmc=6870893 |pmid=18095285}}</ref><ref>Catani, Marco, and Derek K. Jones. "Brain." Occipito‐temporal Connections in the Human Brain. 23 June 2003. Web. 27 March 2016.</ref>

V4 recognizes simple shapes, and gets input from V1 (strong), V2, V3, LGN, and pulvinar.<ref>{{Cite journal |last1=Benevento |first1=Louis A. |last2=Standage |first2=Gregg P. |date=1983-07-01 |title=The organization of projections of the retinorecipient and nonretinorecipient nuclei of the pretectal complex and layers of the superior colliculus to the lateral pulvinar and medial pulvinar in the macaque monkey |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.902170307 |journal=Journal of Comparative Neurology |language=en |volume=217 |issue=3 |pages=307–336 |doi=10.1002/cne.902170307 |pmid=6886056 |s2cid=44794002 |issn=0021-9967|url-access=subscription }}</ref> V5's outputs include V4 and its surrounding area, and eye-movement motor cortices ([[Frontal eye fields|frontal eye-field]] and [[Lateral intraparietal cortex|lateral intraparietal area]]).

V5's functionality is similar to that of the other V's, however, it integrates local object motion into global motion on a complex level. V6 works in conjunction with V5 on motion analysis. V5 analyzes self-motion, whereas V6 analyzes motion of objects relative to the background. V6's primary input is V1, with V5 additions. V6 houses the [[Topographic map|topographical map]] for vision. V6 outputs to the region directly around it (V6A). V6A has direct connections to arm-moving cortices, including the [[premotor cortex]].<ref>{{Cite journal |last1=Hirsch |first1=Ja |last2=Gilbert |first2=Cd |date=1991-06-01 |title=Synaptic physiology of horizontal connections in the cat's visual cortex |journal=The Journal of Neuroscience |language=en |volume=11 |issue=6 |pages=1800–1809 |doi=10.1523/JNEUROSCI.11-06-01800.1991 |issn=0270-6474 |pmc=6575415 |pmid=1675266}}</ref><ref>{{Cite journal |last1=Schall |first1=JD |last2=Morel |first2=A. |last3=King |first3=DJ |last4=Bullier |first4=J. |date=1995-06-01 |title=Topography of visual cortex connections with frontal eye field in macaque: convergence and segregation of processing streams |journal=The Journal of Neuroscience |language=en |volume=15 |issue=6 |pages=4464–4487 |doi=10.1523/JNEUROSCI.15-06-04464.1995 |issn=0270-6474 |pmc=6577698 |pmid=7540675}}</ref>

The [[inferior temporal gyrus]] recognizes complex shapes, objects, and faces or, in conjunction with the [[hippocampus]], creates new [[memories]].<ref>Moser, May-Britt, and Edvard I. Moser. "Functional Differentiation in the Hippocampus." Wiley Online Library. 1998. Web. 27 March 2016.</ref> The [[pretectal area]] is seven unique [[Nucleus (neuroanatomy)|nuclei]]. Anterior, posterior and medial pretectal nuclei inhibit pain (indirectly), aid in [[Rapid eye movement sleep|REM]], and aid the [[accommodation reflex]], respectively.<ref>{{Cite journal |last1=Kanaseki |first1=T. |last2=Sprague |first2=J. M. |date=1974-12-01 |title=Anatomical organization of pretectal nuclei and tectal laminae in the cat |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.901580307 |journal=Journal of Comparative Neurology |language=en |volume=158 |issue=3 |pages=319–337 |doi=10.1002/cne.901580307 |pmid=4436458 |s2cid=38463227 |issn=0021-9967|url-access=subscription }}</ref> The [[Edinger–Westphal nucleus|Edinger-Westphal nucleus]] moderates [[pupil dilation]] and aids (since it provides parasympathetic fibers) in convergence of the eyes and lens adjustment.<ref>Reiner, Anton, and Harvey J. Karten. "Parasympathetic Ocular Control — Functional Subdivisions and Circuitry of the Avian Nucleus of Edinger-Westphal."Science Direct. 1983. Web. 27 March 2016.</ref> Nuclei of the optic tract are involved in smooth pursuit eye movement and the accommodation reflex, as well as REM.

The [[suprachiasmatic nucleus]] is the region of the [[hypothalamus]] that halts production of [[melatonin]] (indirectly) at first light.<ref>{{Cite journal |last1=Welsh |first1=David K |last2=Logothetis |first2=Diomedes E |last3=Meister |first3=Markus |last4=Reppert |first4=Steven M |date=April 1995 |title=Individual neurons dissociated from rat suprachiasmatic nucleus express independently phased circadian firing rhythms |journal=Neuron |language=en |volume=14 |issue=4 |pages=697–706 |doi=10.1016/0896-6273(95)90214-7|pmid=7718233 |doi-access=free }}</ref>

==Structure==
[[File:Schematic diagram of the human eye en.svg|thumb|300px|The [[human eye]] (horizontal section)<br />''The image projected onto the retina is inverted due to the optics of the eye.'']]
* The [[Eye#Eye|eye]], especially the [[#Retina|retina]]
* The [[#Optic nerve|optic nerve]]
* The [[Optic chiasm#Optic chiasm|optic chiasma]]
* The [[#Optic tract|optic tract]]
* The [[Lateral geniculate body#Lateral geniculate body|lateral geniculate body]]
* The [[Optic radiation#Optic radiation|optic radiation]]
* The [[visual cortex]]
* The [[Visual association cortex#V2|visual association cortex]].

These are components of the '''visual pathway''', also called the '''optic pathway''',<ref name="MSD">{{cite web |title=The Optic Pathway - Eye Disorders |url=https://www.msdmanuals.com/en-gb/professional/eye-disorders/optic-nerve-disorders/the-optic-pathway |website=MSD Manual Professional Edition |access-date=18 January 2022}}</ref> that can be divided into [[Anatomical terms of location#Anterior and posterior|anterior and posterior visual pathways]]. The anterior visual pathway refers to structures involved in vision before the [[lateral geniculate nucleus]]. The posterior visual pathway refers to structures after this point.

===Eye===
{{Main|Eye|Anterior segment of eyeball}}
Light entering the eye is [[refracted]] as it passes through the [[cornea]]. It then passes through the [[pupil]] (controlled by the [[Iris (anatomy)|iris]]) and is further refracted by the [[lens (vision)|lens]]. The cornea and lens act together as a compound lens to project an inverted image onto the retina.

[[File:Cajal Retina.jpg|thumb|left|[[S. Ramón y Cajal]], ''Structure of the [[Mammal]]ian Retina, 1900'']]
====Retina====
{{Main|Retina}}
The retina consists of many [[photoreceptor cell]]s which contain particular [[protein]] [[molecule]]s called [[opsin]]s. In humans, two types of opsins are involved in conscious vision: [[Rod cell|rod opsins]] and [[Cone cell|cone opsins]]. (A third type, [[melanopsin]] in some [[Retinal ganglion cell|retinal ganglion cells]] (RGC), part of the [[body clock]] mechanism, is probably not involved in conscious vision, as these RGC do not project to the [[lateral geniculate nucleus]] but to the [[Pretectal area|pretectal olivary nucleus]].<ref>{{Cite journal | last = Güler | first = A.D.  |date= May 2008 | title =  Melanopsin cells are the principal conduits for rod/cone input to non-image forming vision | journal = Nature | volume = 453 | issue = 7191 | pages = 102–5 | pmid =  18432195 | doi = 10.1038/nature06829| bibcode =2008Natur.453..102G | format = Abstract | pmc = 2871301 |display-authors=etal}}</ref>) An opsin absorbs a [[photon]] (a particle of light) and transmits a signal to the [[cell (biology)|cell]] through a [[signal transduction pathway]], resulting in hyper-polarization of the photoreceptor.

Rods and cones differ in function. Rods are found primarily in the periphery of the retina and are used to see at low levels of light. Each human eye contains 120 million rods. Cones are found primarily in the center (or [[Fovea centralis|fovea]]) of the retina.<ref name="HyperPhysics">{{cite web |last1=Nave |first1=R |title=Light and Vision |url=http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html |access-date=2014-11-13 |publisher=[[HyperPhysics]]}}</ref> There are three types of cones that differ in the [[wavelengths]] of light they absorb; they are usually called short or blue, middle or green, and long or red. Cones mediate day vision and can distinguish [[color]] and other features of the visual world at medium and high light levels. Cones are larger and much less numerous than rods (there are 6-7 million of them in each human eye).<ref name="HyperPhysics" />

In the retina, the photoreceptors [[synapse]] directly onto [[bipolar cell of the retina|bipolar cell]]s, which in turn synapse onto [[retinal ganglion cell|ganglion cell]]s of the outermost layer, which then conduct [[action potentials]] to the [[brain]]. A significant amount of [[visual processing]] arises from the patterns of communication between [[neuron]]s in the retina. About 130 million photo-receptors absorb light, yet roughly 1.2 million [[axons]] of ganglion cells transmit information from the retina to the brain. The processing in the retina includes the formation of center-surround [[receptive fields]] of bipolar and ganglion cells in the retina, as well as convergence and divergence from photoreceptor to bipolar cell. In addition, other neurons in the retina, particularly [[Horizontal cell|horizontal]] and [[amacrine cell]]s, transmit information laterally (from a neuron in one layer to an adjacent neuron in the same layer), resulting in more complex receptive fields that can be either indifferent to color and sensitive to [[motion (physics)|motion]] or sensitive to color and indifferent to motion.<ref name="Tov2008">{{harvnb|Tovée|2008}}</ref>

===== Mechanism of generating visual signals =====
The retina adapts to change in light through the use of the rods. In the dark, the [[chromophore]] [[retinal]] has a bent shape called cis-retinal (referring to a ''cis'' conformation in one of the double bonds). When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. This is called bleaching because the purified [[rhodopsin]] changes from violet to colorless in the light. At baseline in the dark, the rhodopsin absorbs no light and releases [[glutamate]], which inhibits the bipolar cell. This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. When there is light present, glutamate secretion ceases, thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected.<ref>Saladin, Kenneth D. ''Anatomy & Physiology: The Unity of Form and Function''. 5th ed. New York: [[McGraw Hill Education|McGraw-Hill]], 2010.</ref><ref>{{Cite web |url=http://webvision.med.utah.edu/GCPHYS1.HTM |title=Webvision: Ganglion cell Physiology |access-date=2018-12-08 |archive-url=https://web.archive.org/web/20110123202041/http://webvision.med.utah.edu/GCPHYS1.HTM |archive-date=2011-01-23 }}</ref>

The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain:<ref name=Tov2008 />
#M cells, with large center-surround receptive fields that are sensitive to [[Depth perception|depth]], indifferent to color, and rapidly adapt to a stimulus;
#P cells, with smaller center-surround receptive fields that are sensitive to color and [[shape]];
#K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
#[[Photosensitive ganglion cell|another population that is intrinsically photosensitive]]; and
#a final population that is used for eye movements.<ref name=Tov2008 />

A 2006 [[University of Pennsylvania]] study calculated the approximate [[Bandwidth (computing)|bandwidth]] of human retinas to be about 8,960 [[Kilobit|kilobits]] per second, whereas [[guinea pig]] retinas transfer at about 875 kilobits.<ref>{{cite web|url=https://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight|title=Calculating the speed of sight}}</ref>

In 2007 Zaidi and co-researchers on both sides of the Atlantic studying patients without rods and cones, discovered that the novel photoreceptive ganglion cell in humans also has a role in conscious and unconscious visual perception.<ref name="Zaidi, 2007">{{cite journal |display-authors=etal |vauthors=Zaidi FH, Hull JT, Peirson SN |date=December 2007 |title=Short-wavelength light sensitivity of circadian, pupillary, and visual awareness in humans lacking an outer retina |journal=[[Curr. Biol.]] |volume=17 |issue=24 |pages=2122–8 |doi=10.1016/j.cub.2007.11.034 |pmc=2151130 |pmid=18082405|bibcode=2007CBio...17.2122Z }}</ref> The peak [[spectral sensitivity]] was 481&nbsp;nm. This shows that there are two pathways for vision in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photo-receptive ganglion cells which act as rudimentary visual brightness detectors.

====Photochemistry====
{{Main|Visual cycle}}
The functioning of a [[camera]] is often compared with the workings of the eye, mostly since both focus light from external objects in the [[field of view]] onto a light-sensitive medium. In the case of the camera, this medium is film or an electronic sensor; in the case of the eye, it is an array of visual receptors. With this simple geometrical similarity, based on the laws of optics, the eye functions as  a [[transducer]], as does a [[Charge-coupled device|CCD camera]].

In the visual system, '''retinal''', technically called ''[[retinene]]''<sub>1</sub> or "retinaldehyde", is a light-sensitive molecule found in the rods and cones of the [[retina]].  Retinal is the fundamental structure involved in the transduction of [[light]] into visual signals, i.e. nerve impulses in the ocular system of the [[central nervous system]].  In the presence of light, the retinal molecule changes configuration and as a result, a [[nerve impulse]] is generated.<ref name=Tov2008 /><!-- look up page from Tovée2008 -->

===Optic nerve===
{{Main|Optic nerve}}
[[File:1543,Vesalius'Fabrica,VisualSystem,V1.jpg|right|thumb|Information flow from the [[eye]]s (top), crossing at the [[optic chiasm]]a, joining left and right eye information in the [[optic tract]], and layering left and right visual stimuli in the [[lateral geniculate nucleus]]. [[Visual cortex#Primary visual cortex (V1)|V1]] in red at bottom of image.
(1543 image from [[Andreas Vesalius]]' ''Fabrica'')]]
The information about the image via the eye is transmitted to the brain along the [[optic nerve]].  Different populations of ganglion cells in the retina send information to the brain through the optic nerve. About 90% of the [[axons]] in the optic nerve go to the [[lateral geniculate nucleus]] in the [[thalamus]]. These axons originate from the M, P, and K ganglion cells in the retina, see above. This [[Parallel processing (psychology)|parallel processing]] is important for reconstructing the visual world; each type of information will go through a different route to [[perception]]. Another population sends information to the [[superior colliculus]] in the [[midbrain]], which assists in controlling eye movements ([[saccades]])<ref name="nolte">{{cite book |author1=Sundsten, John W. |author2=Nolte, John |title=The human brain: an introduction to its functional anatomy |publisher=Mosby |location=St. Louis |year=2001 |pages=410–447 |isbn=978-0-323-01320-8 |oclc=47892833 }}</ref> as well as other motor responses.

A final population of [[photosensitive ganglion cell]]s, containing [[melanopsin]] for [[photosensitivity]], sends information via the [[retinohypothalamic tract]] to the [[pretectum]] ([[pupillary reflex]]), to several structures involved in the control of [[circadian rhythms]] and [[sleep]] such as the [[suprachiasmatic nucleus]] (the biological clock), and to the [[ventrolateral preoptic nucleus]] (a region involved in [[sleep regulation]]).<ref>{{cite journal |vauthors=Lucas RJ, Hattar S, Takao M, Berson DM, Foster RG, Yau KW |title=Diminished pupillary light reflex at high irradiances in melanopsin-knockout mice |journal=Science |volume=299 |issue=5604 |pages=245–7 |date=January 2003 |pmid=12522249 |doi=10.1126/science.1077293 |bibcode=2003Sci...299..245L |citeseerx=10.1.1.1028.8525 |s2cid=46505800 }}</ref> A recently discovered role for photoreceptive ganglion cells is that they mediate conscious and unconscious vision – acting as rudimentary visual brightness detectors as shown in rodless coneless eyes.<ref name="Zaidi, 2007"/>

===Optic chiasm===
{{Main|Optic chiasm}}
The optic nerves from both eyes meet and cross at the optic chiasm,<ref>{{cite book |author=Turner, Howard R. |title=Science in medieval Islam: an illustrated introduction |chapter-url=https://archive.org/details/scienceinmedieva0000turn |chapter-url-access=registration |publisher=University of Texas Press |location=Austin |year=1997 |chapter=Optics |page=[https://archive.org/details/scienceinmedieva0000turn/page/197 197] |isbn=978-0-292-78149-8 |oclc=440896281 }}</ref><!--[http://www.stanford.edu/~kendric/DPC3/medieval_eye_files/medieval_eye.pdf Another link to al-Haytham's sketch of optic chiasm]--><ref>{{harvnb|Vesalius|1543}}</ref> at the base of the [[hypothalamus]] of the brain. At this point, the information coming from both eyes is combined and then splits according to the [[visual field]].  The corresponding halves of the field of view (right and left) are sent to the left and right [[Cerebral hemisphere|halves of the brain]], respectively, to be processed. That is, the right side of [[primary visual cortex]] deals with the left half of the ''field of view'' from both eyes, and similarly for the left brain.<ref name="nolte"/> A small region in the center of the field of view is processed redundantly by both halves of the brain.

===Optic tract===
{{Main|Optic tract}}
Information from the right ''visual field'' (now on the left side of the brain) travels in the left optic tract.  Information from the left ''visual field'' travels in the right optic tract.  Each optic tract terminates in the [[lateral geniculate nucleus]] (LGN) in the thalamus.
[[File:Lateral geniculate nucleus.png|left|thumb|200px|Six layers in the [[Lgn|LGN]]]]

===Lateral geniculate nucleus===
: {{Main|Lateral geniculate nucleus}}

The '''lateral geniculate nucleus''' (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN consists of six layers in [[human]]s and other [[primate]]s starting from [[Catarrhini|catarrhines]], including [[cercopithecidae]] and [[Ape|apes]]. Layers 1, 4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal retina (temporal visual field); layers 2, 3, and 5 correspond to [[information]] from the ipsilateral (uncrossed) fibers of the temporal retina (nasal visual field).

Layer one contains M cells, which correspond to the M ([[Magnocellular cell|magnocellular]]) cells of the optic nerve of the opposite eye and are concerned with depth or motion. Layers four and six of the LGN also connect to the opposite eye, but to the P cells (color and edges) of the optic nerve. By contrast, layers two, three and five of the LGN connect to the M cells and P ([[Parvocellular cell|parvocellular]]) cells of the optic nerve for the same side of the brain as its respective LGN.

Spread out, the six layers of the LGN are the area of a [[credit card]] and about three times its thickness.  The LGN is rolled up into two [[Ellipsoid|ellipsoids]] about the size and shape of two small birds' eggs. In between the six layers are smaller cells that receive information from the K cells (color) in the retina.  The neurons of the LGN then relay the visual image to the [[primary visual cortex]] (V1) which is located at the back of the brain ([[Posterior (anatomy)|posterior end]]) in the [[occipital lobe]] in and close to the [[calcarine sulcus]]. The LGN is not just a simple relay station, but it is also a center for processing; it receives reciprocal input from the [[Cortical layer|cortical]] and subcortical layers and [[reciprocal innervation]] from the visual cortex.<ref name=Tov2008 />

[[File:Lisa analysis.png|left|thumb|200px| Scheme of the [[optic tract]] with image being decomposed on the way, up to simple cortical cells (simplified)]]

===Optic radiation===
{{Main|Optic radiation}}

The '''optic radiations''', one on each side of the brain, carry information from the thalamic [[lateral geniculate nucleus]] to layer 4 of the [[visual cortex]]. The P layer neurons of the LGN relay to V1 layer 4C β. The M layer neurons relay to V1 layer 4C α. The K layer neurons in the LGN relay to large neurons called blobs in layers 2 and 3 of V1.<ref name=Tov2008 />

There is a direct correspondence from an angular position in the [[visual field]] of the eye, all the way through the optic tract to a nerve position in V1 up to V4, i.e. the primary visual areas. After that, the visual pathway is roughly separated into a [[Two-streams hypothesis|ventral and dorsal pathway]].

===Visual cortex===
{{Main|Visual cortex}}
[[File:Brodmann areas 17 18 19.png|thumb|200px|[[Visual cortex]]: <br />V1; V2; V3; V4; V5 (also called MT)]]
The visual cortex <!--is the largest system in the human brain{{citation needed|date=September 2024}} and--> is responsible for processing the visual image. It lies at the rear of the brain (highlighted in the image), above the [[cerebellum]]. The region that receives information directly from the LGN is called the [[Visual cortex#Primary visual cortex (V1)|primary visual cortex]] (also called V1 and striate cortex). It creates a bottom-up saliency map of the visual field to guide attention or eye gaze to salient visual locations.<ref name=":0">{{Cite journal|last=Li|first=Z|date=2002|title=A saliency map in primary visual cortex|url=https://www.sciencedirect.com/science/article/abs/pii/S1364661300018179|journal=Trends in Cognitive Sciences|volume=6|issue=1|pages=9–16|doi=10.1016/s1364-6613(00)01817-9|pmid=11849610|s2cid=13411369|url-access=subscription}}</ref>{{Clarify|date=September 2024|reason=The source says they are proposing something; can it be cited as consolidated knowledge?}} Hence selection of visual input information by attention starts at V1<ref>{{Cite journal|last=Zhaoping|first=L.|date=2019|title=A new framework for understanding vision from the perspective of the primary visual cortex|url=https://www.sciencedirect.com/science/article/abs/pii/S0959438819300042|journal=Current Opinion in Neurobiology|volume=58|pages=1–10|doi=10.1016/j.conb.2019.06.001|pmid=31271931|s2cid=195806018|url-access=subscription}}</ref> along the visual pathway.

Visual information then flows through a cortical hierarchy.  These areas include V2, V3, V4 and area V5/MT. (The exact connectivity depends on the species of the animal.) These secondary visual areas (collectively termed the extrastriate visual cortex) process a wide variety of visual primitives. Neurons in V1 and V2 respond selectively to bars of specific orientations, or combinations of bars.  These are believed to support edge and corner detection.  Similarly, basic information about color and motion is processed here.<ref>{{cite book |author1=Jessell, Thomas M. |author2=Kandel, Eric R. |author3=Schwartz, James H. |title=Principles of neural science |publisher=McGraw-Hill |chapter=27. Central visual pathways |location=New York |year=2000 |pages=[https://archive.org/details/isbn_9780838577011/page/533 533–540] |isbn=978-0-8385-7701-1 |oclc=42073108 |url-access=registration |url=https://archive.org/details/isbn_9780838577011/page/533 }}</ref>

Heider, et al. (2002) found that neurons involving V1, V2, and V3 can detect stereoscopic [[illusory contours]]; they found that stereoscopic stimuli subtending up to 8° can activate these neurons.<ref>[http://watarts.uwaterloo.ca/~cellard/teaching/PSYC771/heideretal%282002%29.pdf Heider, Barbara; Spillmann, Lothar; Peterhans, Esther (2002)  "Stereoscopic Illusory Contours— Cortical Neuron Responses and Human Perception" ''J. Cognitive Neuroscience'' '''14''':7 pp.1018-29] {{Webarchive|url=https://web.archive.org/web/20161011144931/http://watarts.uwaterloo.ca/~cellard/teaching/PSYC771/heideretal%282002%29.pdf |date=2016-10-11 }} accessdate=2014-05-18</ref>

[[File:restingStateModels.jpg|thumb|right|Visual cortex is active even during [[resting state fMRI]]. ]]

===Visual association cortex===
{{Main|Two-streams hypothesis}}
As visual information passes forward through the visual hierarchy, the complexity of the neural representations increases. Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular [[Retinotopy|retinotopic]] location, neurons in the lateral occipital complex respond selectively to a complete object (e.g., a figure drawing), and neurons in the visual association cortex may respond selectively to human faces, or to a particular object.

Along with this increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: the [[dorsal stream]] and the [[ventral stream]] (the [[Two Streams hypothesis]],<ref name=UngerleiderMishkin>{{Cite journal |journal=Behav. Brain Res. |year=1982 |volume=6 |issue=1 |pages=57–77 |title=Contribution of striate inputs to the visuospatial functions of parieto-preoccipital cortex in monkeys |vauthors=Mishkin M, Ungerleider LG |pmid=7126325 |doi=10.1016/0166-4328(82)90081-X |s2cid=33359587 }}</ref> first proposed by Ungerleider and Mishkin in 1982). The dorsal stream, commonly referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates with regions that control eye movements and hand movements.  More recently, this area has been called the "how" stream to emphasize its role in guiding behaviors to spatial locations. The ventral stream, commonly referred to as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli.

[[File:Gray726 intraparietal sulcus.svg|thumb|right| [[Intraparietal sulcus]] (red)]]

However, there is still much debate about the degree of specialization within these two pathways, since they are in fact heavily interconnected.<ref name=Farivar>{{Cite journal|journal=Brain Res. Rev.|year=2009|title=Dorsal-ventral integration in object recognition|author=Farivar R.|doi=10.1016/j.brainresrev.2009.05.006|pmid=19481571|volume=61|issue=2|pages=144–53|s2cid=6817815}}</ref>

[[Horace Barlow]] proposed the ''[[efficient coding hypothesis]]'' in 1961 as a theoretical model of [[sensory neuroscience|sensory coding]] in the [[brain]].<ref>Barlow, H. (1961) "Possible principles underlying the transformation of sensory messages" in ''Sensory Communication'', MIT Press</ref> Limitations in the applicability of this theory in the [http://www.scholarpedia.org/article/Area_V1 primary visual cortex (V1)] motivated the [[V1 Saliency Hypothesis]] that V1 creates a bottom-up saliency map to guide attention exogenously.<ref name=":0" />  With attentional selection as a center stage, vision is seen as composed of encoding, selection, and decoding stages.<ref name=":1">{{Cite book|last=Zhaoping|first=Li|title=Understanding vision: theory, models, and data|publisher=Oxford University Press|year=2014|isbn=978-0-19-882936-2|location=United Kingdom}}</ref>

The [[default mode network]] is a network of brain regions that are active when an individual is awake and at rest. The visual system's default mode can be monitored during [[resting state fMRI]]:
Fox, et al. (2005) found that "[http://www.pnas.org/content/102/27/9673.full the human brain is intrinsically organized into dynamic, anticorrelated functional networks"],<ref>{{cite journal | last1 = Fox | first1 = Michael D. | display-authors = etal | year = 2005| title =  From The Cover: The human brain is intrinsically organized into dynamic, anticorrelated functional networks| journal = PNAS | volume = 102 | issue = 27| pages = 9673–9678 | doi = 10.1073/pnas.0504136102 | pmid = 15976020 | pmc = 1157105 | bibcode = 2005PNAS..102.9673F | doi-access = free }}</ref> in which the visual system switches from resting state to attention.

In the [[parietal lobe]], the [[lateral intraparietal cortex|lateral]] and ventral intraparietal cortex are involved in visual attention and saccadic eye movements. These regions are in the [[intraparietal sulcus]] (marked in red in the adjacent image).

==Development==

===Infancy===
{{See also|Infant vision}}
Newborn infants have limited [[color perception]].<ref name="Lane2012">{{cite book|last=Lane|first=Kenneth A.|title=Visual Attention in Children: Theories and Activities|url=https://books.google.com/books?id=FdVDkAOK_WUC&pg=PA7|access-date=4 December 2014|year=2012|publisher=SLACK|isbn=978-1-55642-956-9|page=7}}</ref> One study found that 74% of newborns can distinguish red, 36% green, 25% yellow, and 14% blue. After one month, performance "improved somewhat."<ref name="AdamsCourage1994">{{cite journal|last1=Adams|first1=Russell J.|last2=Courage|first2=Mary L.|last3=Mercer|first3=Michele E.|title=Systematic measurement of human neonatal color vision|journal=Vision Research|volume=34|issue=13|year=1994|pages=1691–1701|issn=0042-6989|doi=10.1016/0042-6989(94)90127-9|pmid=7941376|s2cid=27842977}}</ref> Infant's eyes do not have the ability to [[accommodation (eye)|accommodate]]. Pediatricians are able to perform non-verbal testing to assess [[visual acuity]] of a newborn, detect [[nearsightedness]] and [[astigmatism]], and evaluate the eye teaming and alignment. Visual acuity improves from about 20/400 at birth to approximately 20/25 at 6 months of age. This happens because the nerve cells in the [[retina]] and brain that control vision are not fully developed.

===Childhood and adolescence===

[[Depth perception]], focus, tracking and other aspects of vision continue to develop throughout early and middle childhood. From recent studies in the [[United States]] and [[Australia]] there is some evidence that the amount of time school aged children spend outdoors, in natural light, may have some impact on whether they develop [[myopia]]. The condition tends to get somewhat worse through childhood and adolescence, but stabilizes in adulthood. More prominent myopia (nearsightedness) and astigmatism are thought to be inherited. Children with this condition may need to wear glasses.

===Adulthood===

Vision is often one of the first senses affected by aging. A number of changes occur with aging:
* Over time, the [[Lens (anatomy)|lens]] becomes yellowed and may eventually become brown, a condition known as brunescence or [[Brunescent cataract|brunescent]] [[cataract]]. Although many factors contribute to yellowing, lifetime exposure to [[ultraviolet light]] and [[ageing|aging]] are two main causes.
* The lens becomes less flexible, diminishing the ability to accommodate ([[presbyopia]]).
* While a healthy adult pupil typically has a size range of 2–8&nbsp;mm, with age the range gets smaller, trending towards a moderately small diameter.
* On average [[Tears|tear production]] declines with age. However, there are a number of age-related conditions that can cause excessive tearing.

==Other functions==

===Balance===
Along with [[proprioception]] and [[vestibular function]], the visual system plays an important role in the ability of an individual to control balance and maintain an upright posture.  When these three conditions are isolated and balance is tested, it has been found that vision is the most significant contributor to balance, playing a bigger role than either of the two other intrinsic mechanisms.<ref>{{cite journal |vauthors=Hansson EE, Beckman A, Håkansson A |title=Effect of vision, proprioception, and the position of the vestibular organ on postural sway |journal=Acta Otolaryngol. |volume=130 |issue=12 |pages=1358–63 |date=December 2010 |pmid=20632903 |doi=10.3109/00016489.2010.498024 |s2cid=36949084 |url= http://lup.lub.lu.se/search/ws/files/5263076/1671500.pdf}}</ref> The clarity with which an individual can see his environment, as well as the size of the visual field, the susceptibility of the individual to light and glare, and poor depth perception play important roles in providing a feedback loop to the brain on the body's movement through the environment.  Anything that affects any of these variables can have a negative effect on balance and maintaining posture.<ref>{{cite journal |vauthors=Wade MG, Jones G |title=The role of vision and spatial orientation in the maintenance of posture |journal=Phys Ther |volume=77 |issue=6 |pages=619–28 |date=June 1997 |pmid=9184687 |doi= 10.1093/ptj/77.6.619|doi-access=free }}</ref> This effect has been seen in research involving elderly subjects when compared to young controls,<ref>{{cite journal |vauthors=Teasdale N, Stelmach GE, Breunig A |title=Postural sway characteristics of the elderly under normal and altered visual and support surface conditions |journal=J Gerontol |volume=46 |issue=6 |pages=B238–44 |date=November 1991 |pmid=1940075 |doi=10.1093/geronj/46.6.B238 }}</ref> in [[glaucoma]] patients compared to age matched controls,<ref name="Shabana, 2005">{{cite journal |vauthors=Shabana N, Cornilleau-Pérès V, Droulez J, Goh JC, Lee GS, Chew PT |title=Postural stability in primary open angle glaucoma |journal=Clin. Experiment. Ophthalmol. |volume=33 |issue=3 |pages=264–73 |date=June 2005 |pmid=15932530 |doi=10.1111/j.1442-9071.2005.01003.x |s2cid=26286705 }}</ref> [[cataract]] patients pre and post surgery,<ref name="Schwartz, 2005">{{cite journal |vauthors=Schwartz S, Segal O, Barkana Y, Schwesig R, Avni I, Morad Y |title=The effect of cataract surgery on postural control |journal=Invest. Ophthalmol. Vis. Sci. |volume=46 |issue=3 |pages=920–4 |date=March 2005 |pmid=15728548 |doi=10.1167/iovs.04-0543 |doi-access=free }}</ref> and even something as simple as wearing safety goggles.<ref>{{cite journal |vauthors=Wade LR, Weimar WH, Davis J |title=Effect of personal protective eyewear on postural stability |journal=Ergonomics |volume=47 |issue=15 |pages=1614–23 |date=December 2004 |pmid=15545235 |doi=10.1080/00140130410001724246 |s2cid=22219417 }}</ref> [[Monocular vision]] (one eyed vision) has also been shown to negatively impact balance, which was seen in the previously referenced cataract and glaucoma studies,<ref name="Shabana, 2005"/><ref name="Schwartz, 2005"/> as well as in healthy children and adults.<ref>{{cite journal |vauthors=Barela JA, Sanches M, Lopes AG, Razuk M, Moraes R |title=Use of monocular and binocular visual cues for postural control in children |journal=J Vis |volume=11 |issue=12 |page= 10|year=2011 |pmid=22004694 |doi=10.1167/11.12.10 |doi-access=free }}</ref>

According to Pollock et al. (2010) [[stroke]] is the main cause of specific visual impairment, most frequently visual field loss ([[Homonymous hemianopsia|homonymous hemianopia]], a visual field defect). Nevertheless, evidence for the efficacy of cost-effective interventions aimed at these visual field defects is still inconsistent.<ref>{{cite journal | doi = 10.1111/j.1747-4949.2010.00516.x | volume=5 | title=Vision | year=2010 | journal=International Journal of Stroke | issue=3_suppl | page=67}}</ref>

==Clinical significance==

[[File:Hemianopsia en.jpg|thumb|350px|right|'''[[Visual pathway lesions]]''' <br />
From top to bottom: <br /> 1. Complete loss of vision, right eye <br /> 2. [[Bitemporal hemianopia]] <br /> 3. [[Homonymous hemianopsia]] <br /> 4. [[Quadrantanopia]] <br /> 5&6. Quadrantanopia with [[macular sparing]]]]

Proper function of the visual system is required for sensing, processing, and understanding the surrounding environment. Difficulty in sensing, processing and understanding light input has the potential to adversely impact an individual's ability to communicate, learn and effectively complete routine tasks on a daily basis.

In children, early diagnosis and treatment of impaired visual system function is an important factor in ensuring that key social, academic and speech/language developmental milestones are met.

[[Cataract]] is clouding of the lens, which in turn affects vision. Although it may be accompanied by yellowing, clouding and yellowing can occur separately. This is typically a result of ageing, disease, or drug use.

[[Presbyopia]] is a visual condition that causes [[Far-sightedness|farsightedness]]. The eye's lens becomes too inflexible to [[Accommodation (eye)|accommodate]] to normal reading distance, focus tending to remain fixed at long distance.

[[Glaucoma]] is a type of blindness that begins at the edge of the visual field and progresses inward. It may result in [[tunnel vision]]. This typically involves the outer layers of the optic nerve, sometimes as a result of buildup of fluid and excessive pressure in the eye.<ref name="Publications">{{cite book|author=Harvard Health Publications|title=The Aging Eye: Preventing and treating eye disease|url=https://books.google.com/books?id=6cwilvu0CXQC&pg=PA20|access-date=15 December 2014|publisher=Harvard Health Publications|isbn=978-1-935555-16-2|page=20|year=2010}}</ref>

[[Scotoma]] is a type of blindness that produces a small [[Blind spot (vision)|blind spot]] in the visual field typically caused by injury in the primary visual cortex.

[[Homonymous hemianopia]] is a type of blindness that destroys one entire side of the visual field typically caused by injury in the primary visual cortex.

[[Quadrantanopia]] is a type of blindness that destroys only a part of the visual field typically caused by partial injury in the primary visual cortex. This is very similar to homonymous hemianopia, but to a lesser degree.

[[Prosopagnosia]], or face blindness, is a brain disorder that produces an inability to recognize faces. This disorder often arises after damage to the [[fusiform face area]].

[[Visual agnosia]], or visual-form agnosia, is a brain disorder that produces an inability to recognize objects. This disorder often arises after damage to the [[ventral stream]].

==Other animals==
{{See also|Eye|Vision in birds|Parietal eye|Vision in fish|Arthropod visual system|Cephalopod eye}}
Different [[species]] are able to see different parts of the [[light spectrum]]; for example, [[bee]]s can see into the [[ultraviolet]],<ref>{{cite journal |vauthors=Bellingham J, Wilkie SE, Morris AG, Bowmaker JK, Hunt DM |title=Characterisation of the ultraviolet-sensitive opsin gene in the honey bee, Apis mellifera |journal=Eur. J. Biochem. |volume=243 |issue=3 |pages=775–81 |date=February 1997 |pmid=9057845 |doi=10.1111/j.1432-1033.1997.00775.x |doi-access=free }}</ref> while [[pit viper]]s can accurately target prey with their [[pit organ]]s, which are sensitive to infrared radiation.<ref>{{cite journal |vauthors=Safer AB, Grace MS |title=Infrared imaging in vipers: differential responses of crotaline and viperine snakes to paired thermal targets |journal=Behav. Brain Res. |volume=154 |issue=1 |pages=55–61 |date=September 2004 |pmid=15302110 |doi=10.1016/j.bbr.2004.01.020 |s2cid=39736880 }}</ref> The [[mantis shrimp]] possesses arguably the most complex visual system of any species. The eye of the mantis shrimp holds 16 color receptive cones, whereas humans only have three. The variety of cones enables them to perceive an enhanced array of colors as a mechanism for mate selection, avoidance of predators, and detection of prey.<ref>{{Cite web |url=https://www.aqua.org/explore/animals/mantis-shrimp |title=(2018) "Peacock Mantis Shrimp" ''National Aquarium'' |access-date=2018-03-06 |archive-date=2018-05-04 |archive-url=https://web.archive.org/web/20180504093928/http://aqua.org/explore/animals/mantis-shrimp |url-status=dead }}</ref> Swordfish also possess an impressive visual system. The eye of a [[swordfish]] can generate [[heat]] to better cope with detecting their [[prey]] at depths of 2000 feet.<ref>David Fleshler(10-15-2012) ''[http://www.sun-sentinel.com/news/local/breakingnews/fl-giant-eyeball-mystery-20121015,0,2019024.story South Florida Sun-Sentinel] {{Webarchive|url=https://archive.today/20130203120659/http://www.sun-sentinel.com/news/local/breakingnews/fl-giant-eyeball-mystery-20121015,0,2019024.story |date=2013-02-03 }}'',
*[https://www.newscientist.com/article/dn6861-swordfish-heat-their-eyes-for-the-hunt.html Swordfish heat their eyes]</ref> Certain [[one-celled]] [[Microorganism|microorganisms]], the [[warnowiid]] [[dinoflagellate]]s have eye-like [[ocelloid]]s, with analogous structures for the lens and retina of the multi-cellular eye.<ref>[http://www.sci-news.com/biology/science-warnowiid-dinoflagellate-plankton-eyes-02973.html Single-Celled Planktonic Organisms Have Animal-Like Eyes, Scientists Say]
*[http://www.biomedcentral.com/1471-2148/9/116 "Molecular phylogeny of ocelloid-bearing dinoflagellates (Warnowiaceae) as inferred from SSU and LSU rDNA sequences"]</ref> The armored shell of the [[chiton]] ''[[Acanthopleura granulata]]'' is also covered with hundreds of [[aragonite]] crystalline eyes, named [[Simple eye in invertebrates|ocelli]], which can form [[image]]s.<ref>{{cite journal | doi = 10.1126/science.aad1246 | pmid=26586760 | volume=350 | title=Multifunctionality of chiton biomineralized armor with an integrated visual system | year=2015 | journal=Science | pages=952–6  | last1 = Li | first1 = L | last2 = Connors | first2 = MJ | last3 = Kolle | first3 = M | last4 = England | first4 = GT | last5 = Speiser | first5 = DI | last6 = Xiao | first6 = X | last7 = Aizenberg | first7 = J | last8 = Ortiz | first8 = C| issue=6263 | doi-access = free | hdl = 1721.1/100035 | hdl-access = free }}</ref>

Many [[fan worm]]s, such as ''[[Acromegalomma interruptum]]'' which live in tubes on the sea floor of the [[Great Barrier Reef]], have evolved compound eyes on their tentacles, which they use to detect encroaching movement. If movement is detected, the fan worms will rapidly withdraw their tentacles. Bok, et al., have discovered opsins and [[G protein]]s in the fan worm's eyes, which were previously only seen in simple [[Cilium|ciliary]] photoreceptors in the brains of some [[Invertebrate|invertebrates]], as opposed to the [[rhabdomeric]] receptors in the eyes of most invertebrates.<ref name="Bok2017">{{cite journal |last1=Bok |first1=Michael J. |last2=Porter |first2=Megan L. |last3=Nilsson |first3=Dan-Eric |title=Phototransduction in fan worm radiolar eyes |journal=Current Biology |date=July 2017 |volume=27 |issue=14 |pages=R698–R699 |doi=10.1016/j.cub.2017.05.093|pmid=28743013 |hdl=1983/3793ef99-753c-4c60-8d91-92815395387a |doi-access=free |bibcode=2017CBio...27.R698B |hdl-access=free }}  cited by [https://phys.org/news/2017-08-evolution-fan-worm-eyes.html Evolution of fan worm eyes (August 1, 2017) Phys.org]</ref>

Only [[higher primate]] [[Old World]] (African) [[Monkey|monkeys]] and apes ([[macaque]]s, [[ape]]s, [[orangutan]]s) have the same kind of three-cone [[Photoreceptor cell|photoreceptor]] color vision humans have, while lower primate [[New World]] (South American) monkeys ([[spider monkey]]s, [[squirrel monkey]]s, [[Capuchin monkey|cebus monkeys]]) have a two-cone photoreceptor kind of color vision.<ref>{{Cite book|title=Vision and art: the biology of seeing|last=Margaret.|first=Livingstone|date=2008|publisher=Abrams|others=Hubel, David H.|isbn=978-0-8109-9554-3|location=New York|oclc=192082768}}</ref>

Biologists have determined that humans have extremely good vision compared to the overwhelming majority of animals, particularly in daylight, surpassed only by a few large species of [[Bird of prey|predatory birds]].<ref>{{Cite web |last=Renner |first=Ben |date=January 9, 2019 |title=Which species, including humans, has the sharpest vision? Study debunks old beliefs |url=https://studyfinds.org/which-species-greatest-vision-study-debunks-old-beleifs/ |access-date=February 25, 2024 |website=Study Finds |language=en-US}}</ref><ref>{{Citation |last=Kirk |first=E. Christopher |title=The Evolution of High Visual Acuity in the Anthropoidea |date=2004 |work=Anthropoid Origins: New Visions |pages=539–602 |editor-last=Ross |editor-first=Callum F. |url=https://link.springer.com/chapter/10.1007/978-1-4419-8873-7_20 |access-date=2024-11-02 |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4419-8873-7_20 |isbn=978-1-4419-8873-7 |last2=Kay |first2=Richard F. |editor2-last=Kay |editor2-first=Richard F.|url-access=subscription }}</ref> Other animals such as [[dog]]s are thought to rely more on senses other than vision, which in turn may be better developed than in humans.<ref>{{Cite web |last1=Gibeault |first1=Stephanie |date=March 22, 2018 |title=Do Dogs Have Self-Awareness? |url=https://www.akc.org/expert-advice/lifestyle/do-dogs-have-self-awareness/ |access-date=February 25, 2024 |website=American Kennel Club |language=en}}</ref><ref>{{Cite web |last= |date=September 14, 2023 |title=Animal senses: How they differ from humans |url=https://animalpha.com/animal-senses-how-they-differ-from-humans/ |access-date=February 25, 2024 |website=Animalpha |language=en}}</ref>

==History==
In the second half of the 19th century, many motifs of the nervous system were identified such as the neuron doctrine and brain localization, which related to the [[neuron]] being the basic unit of the nervous system and functional localisation in the brain, respectively. These would become tenets of the fledgling [[neuroscience]] and would support further understanding of the visual system.

The notion that the [[cerebral cortex]] is divided into functionally distinct cortices now known to be responsible for capacities such as [[touch]] ([[somatosensory cortex]]), [[Motion (physics)|movement]] ([[motor cortex]]), and vision ([[visual cortex]]), was first proposed by [[Franz Joseph Gall]] in 1810.<ref name="Gross, 1994">{{cite journal |author=Gross CG |title=How inferior temporal cortex became a visual area |journal=Cereb. Cortex |volume=4 |issue=5 |pages=455–69 |year=1994 |pmid=7833649 |doi=10.1093/cercor/4.5.455 }}</ref> Evidence for functionally distinct areas of the brain (and, specifically, of the cerebral cortex) mounted throughout the 19th century with discoveries by [[Paul Broca]] of the [[language center]] (1861), and [[Gustav Fritsch]] and [[Eduard Hitzig]] of the motor cortex (1871).<ref name="Gross, 1994"/><ref name="Schiller, 1986">{{cite journal |author=Schiller PH |title=The central visual system |journal=Vision Res. |volume=26 |issue=9 |pages=1351–86 |year=1986 |pmid=3303663 |doi=10.1016/0042-6989(86)90162-8 |s2cid=5247746 |issn=0042-6989 }}</ref> Based on selective damage to parts of the brain and the functional effects of the resulting [[lesion]]s, [[David Ferrier]] proposed that visual function was localized to the [[parietal lobe]] of the brain in 1876.<ref name="Schiller, 1986"/> In 1881, [[Hermann Munk]] more accurately located vision in the [[occipital lobe]], where the [[primary visual cortex]] is now known to be.<ref name="Schiller, 1986"/>

In 2014, a textbook "Understanding vision: theory, models, and data" <ref name=":1" /> illustrates how to link neurobiological data and visual behavior/psychological data through theoretical principles and computational models.

==See also==

{{columns-list|colwidth=22em| 
*[[Achromatopsia]]
*[[Akinetopsia]]
*[[Apperceptive agnosia]]
*[[Associative visual agnosia]]
*[[Asthenopia]]
*[[Astigmatism (eye)|Astigmatism]]
*[[Color blindness]]
*[[Human echolocation|Echolocation]]
*[[Computer vision]]
*[[Helmholtz–Kohlrausch effect]] – how [[color balance]] affects vision
*[[Magnocellular cell]]
*[[Memory-prediction framework]]
*[[Prosopagnosia]]
*[[Scotopic sensitivity syndrome]]
*[[Recovery from blindness]]
*[[Visual agnosia]]
*[[Visual modularity]]
*[[Visual perception]]
*[[Visual processing]]
}}

==References==
{{Reflist}}

==Further reading==
* {{cite journal |vauthors=Davison JA, Patel AS, Cunha JP, Schwiegerling J, Muftuoglu O |title=Recent studies provide an updated clinical perspective on blue light-filtering IOLs |journal=Graefes Arch. Clin. Exp. Ophthalmol. |volume=249 |issue=7 |pages=957–68 |date=July 2011 |pmid=21584764 |pmc=3124647 |doi=10.1007/s00417-011-1697-6 }}
* {{cite journal |vauthors=Hatori M, Panda S |title=The emerging roles of melanopsin in behavioral adaptation to light |journal=Trends Mol Med |volume=16 |issue=10 |pages=435–46 |date=October 2010 |pmid=20810319 |pmc=2952704 |doi=10.1016/j.molmed.2010.07.005 }}
* Heiting, G., (2011). Your infant's vision Development. Retrieved February 27, 2012 from http://www.allaboutvision.com/parents/infants.htm
* {{cite book|author=Hubel, David H.|title=Eye, brain, and vision|publisher=[[Scientific American Library]]|location=New York|year=1995|isbn=978-0-7167-6009-2|oclc=32806252}}
* {{cite book |vauthors=Kolb B, Whishaw I |title=Introduction to Brain and Behaviour Fourth Edition |publisher=Worth Publishers |location=New York |year=2012 |isbn=978-1-4292-4228-8 |oclc=918592547 }}
* {{cite book|author1=Marr, David|author2=Ullman, Shimon|author3=Poggio, Tomaso|title=Vision: A Computational Investigation into the Human Representation and Processing of Visual Information|publisher=[[The MIT Press]]|location=Cambridge, Mass|year=2010|isbn=978-0-262-51462-0|oclc=472791457}}
* {{Cite journal|first=R.W.|last=Rodiek|year=1988|title=The Primate Retina|journal=Comparative Primate Biology|volume=4|series=Neurosciences|location=New York|publisher=A.R. Liss}}. (H.D. Steklis and J. Erwin, editors.) pp.&nbsp;203–278.
* {{cite journal |first=Matthew|last=Schmolesky|title=The Primary Visual Cortex |website=NIH National Library of Medicine |year=1995 |url=https://www.ncbi.nlm.nih.gov/books/NBK11524/ |pmid=21413385}}
* The Aging Eye; See into Your future. (2009). Retrieved February 27, 2012 from https://web.archive.org/web/20111117045917/http://www.realage.com/check-your-health/eye-health/aging-eye
* {{cite book|last=Tovée|first=Martin J.|title=An introduction to the visual system|publisher=[[Cambridge University Press]]|location=Cambridge, UK|year=2008|isbn=978-0-521-88319-1|oclc=185026571}}
* {{Cite book |author-link=Andreas Vesalius|first=Andreas|last=Vesalius|year=1543|title=De Humani Corporis Fabrica |trans-title=On the Workings of the Human Body}}
* {{Cite journal|author-link=Torsten Wiesel|author2-link=David H. Hubel|first1=Torsten|last1=Wiesel|first2=David H.|last2=Hubel|year=1963|title=The effects of visual deprivation on the morphology and physiology of cell's lateral geniculate body|journal=[[Journal of Neurophysiology]]|volume=26|issue=6|pages=978–993|pmid=14084170|doi=10.1152/jn.1963.26.6.978|s2cid=16117515}}.

==External links==
*[http://webvision.med.utah.edu/ "Webvision: The Organization of the Retina and Visual System"] – John Moran Eye Center at University of Utah
*[http://www.visionscience.com/ VisionScience.com] – An online resource for researchers in vision science.
*[http://www.journalofvision.org/ Journal of Vision] – An online, open access journal of vision science.
*[https://journals.sagepub.com/articles/IPE/ i-Perception] – An online, open access journal of perception science.
*[http://www.physorg.com/news115919015.html Hagfish research has found the "missing link" in the evolution of the eye. See: ''Nature Reviews Neuroscience. '']
*{{cite book|title=Neuroscience Online, the Open-Access Neuroscience Electronic Textbook|publisher=The University of Texas Health Science Center at Houston (UTHealth)|chapter-url=http://neuroscience.uth.tmc.edu/s2/chapter14.html|author=Valentin Dragoi|access-date=27 April 2014|chapter=Chapter 14: Visual Processing: Eye and Retina|archive-date=1 November 2017|archive-url=https://web.archive.org/web/20171101011143/http://neuroscience.uth.tmc.edu/s2/chapter14.html|url-status=dead}}

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