Editing Visual system (section)

==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).