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{{BioPsy}}
 
{{BioPsy}}
The '''visual system''' is the part of the [[nervous system]] which allows organisms to [[visual perception|see]].
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[[Image:Gray722.png||thumb|200px|The visual system includes the eyes, the connecting pathways through to the visual cortex and other parts of the brain. The illustration shows the mammalian system.]]
It interprets the information from [[visible light]] to build a representation of the world surrounding the [[body]]. The visual system has the unenviable task of reconstructing a three dimensional world from a two dimensional projection of that world. Note that different [[species]] are be able to see different part of the [[light spectrum]]; for example, some can see into the [[ultraviolet]], while others can see into the [[infrared]].
 
   
This article mostly describes the visual system of [[mammal]]s, although other "higher" animals have similar visual systems. In this case, the
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The '''visual system''' is the part of the [[central nervous system]] which enables organisms to [[visual perception|process visual detail]], as well as enabling several non-image forming photoresponse functions. It interprets information from [[visible light]] to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the reception of light and the formation of monocular representations; the construction of a binocular perception from a pair of two dimensional projections; the identification and categorization of visual objects; assessing distances to and between objects; and guiding body movements in relation to visual objects. The psychological manifestation of visual information is known as [[visual perception]], a lack of which is called [[blindness]]. Non-image forming visual functions, independent of visual perception, include the [[pupillary light reflex]] (PLR) and circadian [[Entrainment (chronobiology)|photoentrainment]].
visual system consists of:
 
   
* The [[eye]], especially the [[retina]]
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==Introduction==
* The [[optic nerve]]
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[[File:Schematic diagram of the human eye en.svg|thumb|300px|The human eye<br><small> ''The image projected onto the retina is inverted due to the optics of the eye.''</small>]]This article mostly describes the visual system of [[mammal]]s, although other "higher" animals have similar visual systems. In this case, the visual system consists of:
* The [[optic chiasm]]
 
* The [[optic tract]]
 
* The [[lateral geniculate nucleus]]
 
* The [[optic radiations]]
 
* The [[visual cortex]]
 
   
<table align=right cellspacing=5 width=1><tr><td>[[Image:eye-diagram.png|thumb|300px|Optical layout of the eye]]<br> <small>''Light is inverted by the lens and projected onto the retina; blue-attuned cone cells will be most strongly stimulated by blue light, while yellow/red-attuned cone cells will not be.''</small></td></tr></table>
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* The [[Visual system#Eye|eye]], especially the [[Visual system#Retina|retina]]
==Eye==
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* The [[Visual system#Optic nerve|optic nerve]]
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* The [[Visual system#Optic chiasm|optic chiasma]]
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* The [[Visual system#Optic tract|optic tract]]
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* The [[Visual system#Lateral geniculate body|lateral geniculate body]]
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* The [[Visual system#Optic radiation|optic radiation]]
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* The [[Visual system#Visual cortex|visual cortex]]
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* The [[Visual system#Visual association cortex|visual association cortex]].
   
The [[eye]] is a complex biological device. The functioning of a [[Charge-coupled device|CCD camera]] makes an apt metaphor for the workings of the [[eye]], which takes [[visible light]] and converts it into a stream of information that can be transmitted via [[nerve]]s.
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Different [[species]] are able to see different parts of the [[light spectrum]]; for example, [[bee]]s can see into the [[ultraviolet]],<ref>{{harvnb|Bellingham|Wilkie|Morris|Bowmaker|1997|pp=775–781}}</ref> while [[pit viper]]s can accurately target prey with their [[pit organ]]s, which are sensitive to infrared radiation.<ref>{{harvnb|Safer|Grace|2004|pp=55–61}}.</ref>
   
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 lens inverts the [[light]] and projects an image onto the retina.
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==History==
[[Image:Cajal Retina.jpg|thumb|left|[[S. Ramón y Cajal]], ''Structure of the [[Mammal]]ian Retina, 1900'']]
 
===Retina===
 
   
The retina consists of a large number of [[photoreceptor]] cells which contain a particular [[protein]] [[molecule]] called an [[opsin]]. In humans, there are two types of opsins, rods and cones. Either opsin absorbs a [[photon]] (a particle of light) and transmits a signal to the [[cell (biology)|cell]] through a [[signal transduction pathway]], resulting in hyperpolarization of the photoreceptor. (For more information, see [[photoreceptor]]).
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In the second half of the 19th century, many motifs of the [[nervous system]] were identified such as the [[neuron doctrine]] and brain localisation, which related to the [[neuron]] being the basic unit of the nervous system and [[Functional specialization (brain)|functional localisation in the brain]], respectively. These would become tenets of the fledgling [[neuroscience]] and would support further understanding of the visual system.
   
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. Cones are found primarily in the center (or [[fovea]]) of the retina. 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 are used primarily to distinguish [[color]] and other features of the visual world at normal levels of light.
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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">{{harvnb|Gross|1994|pp=455–69}}</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 [[Edouard Hitzig]] of the motor cortex (1871).<ref name="Gross" /><ref name="Schiller">{{harvnb|Schiller|1986|pp=1351–86}}</ref> Based on selective damage to parts of the brain and the functional effects this would produce ([[lesion study|lesion studies]]), [[David Ferrier]] proposed that visual function was localised to the [[parietal lobe]] of the brain in 1876.<ref name="Schiller" /> 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" />
   
In the retina, the photoreceptors synapse directly onto [[bipolar cell]]s, which in turn synapse onto [[ganglion cell]]s of the outermost layer, who will 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 photoreceptors 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 and [[amacrine]] cells, 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]] or sensitive to color and indifferent to [[motion]].
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==Biology of the visual system==
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===Eye===
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{{Main|Eye}}
   
The final result of all this processing is five different populations of ganglion cells that send information to the brain: M cells, with large center-surround receptive fields that are sensitive to [[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; another population that is intrinsically [[photosensitive]]; and a final population that is used for eye movements.
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The eye is a complex biological device. 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]].
   
==Optic nerve==
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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.
   
[[Image: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]]. [[V1]] in red at bottom of image.
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====Retina====
([[1543]] image from [[Andreas Vesalius]]' ''Fabrica'')]]
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[[Image:Cajal Retina.jpg|thumb|left|[[S. Ramón y Cajal]], ''Structure of the [[Mammal]]ian Retina, 1900'']]
The information about the image received by the [[eye]] is transmitted to the brain via the optic nerve. In humans, the optic nerve is the only sensory system that is connected directly to the brain and does not connect through the [[Medulla oblongata|medulla]], due to the necessity of processing the complex visual information quickly.
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{{Main|Retina}}
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The retina consists of a large number of [[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 of the 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]] (LGN) but to the [[Pretectal area|pretectal olivary nucleus]] (PON).<ref>{{Citation | last = Güler | first = A.D. | coauthors = et al | date = | year = 2008 | month = May | 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 | publisher = | issn = | pmid = 18432195 | doi = 10.1038/nature06829| bibcode = | oclc =| id = | url = http://www.ncbi.nlm.nih.gov/pubmed/18432195?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum | format = Abstract | accessdate = 2010-06-03 | laysummary = | laysource = | laydate = | quote = | pmc = 2871301 | postscript = . }}</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 hyperpolarization of the photoreceptor. (For more information, see [[Photoreceptor cell]]).
   
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. This parallel processing is important for reconstructing the visual world; each type of information will go through a different route to [[perception]]. Another population of [[photosensitive ganglion cells]] sends information to the [[pretectum]] for regulating [[circadian rhythms]], and a final population sends information to both the [[pretectum]] and the [[superior colliculus]] for controlling eye movements ([[saccades]]).
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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. Cones are found primarily in the center (or [[fovea]]) of the retina.{{Citation needed|date=May 2010}} 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 are used primarily to distinguish [[color]] and other features of the visual world at normal levels of light.{{Citation needed|date=May 2010}}
   
==Optic chiasm==
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In the retina, the photoreceptors synapse directly onto [[bipolar cell of the retina|bipolar cell]]s, which in turn synapse onto [[ganglion cell]]s of the outermost layer, which will 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 photoreceptors 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.{{Citation needed|date=May 2010}}
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'''Mechanism of generating visual signals''': The retina adapts to its change in light through the use of the rods. In the dark, the retinal has a bent shape called cis-retinal. When light is present, the retinal changes 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. In the dark, the rhodopsin absorbs no light therefore releasing glutamate cells which inhibit the bipolar cell. This inhibits the release of neurotransmitters to the ganglion cell. In the light, glutamate secretion ceases which no longer inhibits 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, 2010.</ref><ref>http://webvision.med.utah.edu/GCPHYS1.HTM</ref>
   
The optic nerves from both eyes meet and cross at the [[optic chiasm]], at the base of the [[frontal lobe]] of the brain. At this point the information from both eyes is combined and split according to the [[field of view]]. The corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively (the brain is cross-wired), to be processed. That is, though we might expect the right brain to be responsible for the image from the left eye, and the left brain for the image from the right eye, in fact, the right brain deals with the left half of the ''field of view'', and similarly for the left brain. (Note that the right eye actually perceives part of the left field of view, and vice versa).
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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:
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#M cells, with large center-surround receptive fields that are sensitive to [[Depth perception|depth]], indifferent to color, and rapidly adapt to a stimulus;
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#P cells, with smaller center-surround receptive fields that are sensitive to color and [[shape]];
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#K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
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#[[Photosensitive ganglion cell|another population that is intrinsically photosensitive]]; and
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#a final population that is used for eye movements. {{Citation needed|date=May 2010}}
   
==Optic tract==
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A 2006 [[University of Pennsylvania]] study calculated the approximate [[Bandwidth (computing)|bandwidth]] of human retinas to be about 8960 kilobits per second, whereas [[guinea pig]] retinas transfer at about 875 kilobits.<ref>[http://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight Calculating the speed of sight – being-human – 28 July 2006 – New Scientist<!-- Bot generated title -->]</ref>
   
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.
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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="autogenerated1">{{harvnb|Zaidi|Hull|Peirson|Wulff|2007|pp=2122–8}}</ref><ref>[http://www.medicalnewstoday.com/articles/91836.php Normal Responses To Non-visual Effects Of Light Retained By Blind Humans Lacking Rods And Cones<!-- Bot generated title -->]</ref><ref>[http://www.eurekalert.org/pub_releases/2007-12/cp-bhl121307.php Blind humans lacking rods and cones retain normal responses to nonvisual effects of light<!-- Bot generated title -->]</ref> The peak spectral sensitivity was 481&nbsp;nm. This shows that there are two pathways for sight in the retina one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photoreceptive ganglion cells which act as rudimentary visual brightness detectors.
[[Image:Lateral geniculate nucleus.png|left|thumb|200px|Six layers in the LGN]]And light always travels in a straight line to the eye and can be a candle light can be seen 50km away by the human eye.
 
   
==Lateral geniculate nucleus==
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====Photochemistry====
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 some other [[primate]]s such as [[macaque]]s. Layers 1, 4, and 6 correspond to information from one [[eye]]; layers 2, 3, and 5 correspond to [[information]] from the other [[eye]]. Layer one (1) contains M cells, which correspond to the M (magnocellular) cells of the optic nerve of the opposite eye, and are concerned with depth or motion. Layers four and six (4 & 6) 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 (2, 3, & 5) of the LGN connect to the M cells and P (parvocellular) cells of the optic nerve for the same side of the brain as its respective LGN. The six layers of the LGN are the area of a [[credit card]], but about three times the thickness of a credit card, rolled up into two ellipsoids about the size and shape of two small birds eggs. In between the six layers are smaller cells that recieve 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 ([[caudal end]]) in the [[occipital lobe]].
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{{Main|Visual cycle}}
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In the visual system, '''retinal''', technically called ''retinene<sub>1</sub>'' or "retinaldehyde", is a light-sensitive [[retinene]] 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. {{Citation needed|date=May 2010}}
   
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===Fibers to thalamus===
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====Optic nerve====
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{{Main|Optic nerve}}
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[[Image: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_.28V1.29|V1]] in red at bottom of image.
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(1543 image from [[Andreas Vesalius]]' ''Fabrica'')]]
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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 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'>
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{{harvnb|Nolte|2002|pp=410–447}} </ref> as well as other motor responses.
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A final population of [[photosensitive ganglion cell]]s, containing [[melanopsin]], sends information via the [[retinohypothalamic tract]] (RHT) to the [[pretectum]] (pupillary reflex), to several structures involved in the control of [[circadian rhythms]] and [[sleep]] such as the [[suprachiasmatic nucleus]] (SCN, the biological clock), and to the [[ventrolateral preoptic nucleus]] ([[VLPO]], a region involved in sleep regulation).<ref>
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{{harvnb|Lucas|pp=245–7}}
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</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="autogenerated1" />
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====Optic chiasm====
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{{Main|Optic chiasm}}
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The optic nerves from both eyes meet and cross at the optic chiasm,<ref>{{harvnb|al-Haytham|1021|p=98}} <!--[http://www.stanford.edu/~kendric/DPC3/medieval_eye_files/medieval_eye.pdf Another link to al-Haytham's sketch of optic chiasm]--></ref><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 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.
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====Optic tract====
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{{Main|Optic tract}}
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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.
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[[Image:Lateral geniculate nucleus.png|left|thumb|200px|Six layers in the LGN]]
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===Lateral geniculate nucleus===
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{{Main|lateral geniculate nucleus}}
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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 catarhinians, including cercopithecidae and apes. Layers 1, 4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal visual field; layers 2, 3, and 5 correspond to [[information]] from the ipsilateral (uncrossed) fibers of the temporal visual field. Layer one (1) contains M cells which correspond to the M (magnocellular) cells of the optic nerve of the opposite eye and are concerned with depth or motion. Layers four and six (4 & 6) 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 (2, 3, & 5) of the LGN connect to the M cells and P (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 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 ([[caudal 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 and subcortical layers and reciprocal innervation from the visual cortex.{{Citation needed|date=May 2010}}
   
 
[[Image:Gray722.png|left|thumb|200px|[[Gray's Anatomy|Gray's]] FIG. 722– Scheme showing central connections of the [[optic nerve]]s and optic tracts.]]
 
[[Image:Gray722.png|left|thumb|200px|[[Gray's Anatomy|Gray's]] FIG. 722– Scheme showing central connections of the [[optic nerve]]s and optic tracts.]]
==Optic radiations==
 
The [[optic radiations]] carry information from the midbrain [[lateral geniculate nucleus]] to layer 4 of the [[visual cortex]]. The P layer neurons of the LGN relay to V1 layer 4C &beta;. The M layer neurons relay to V1 layer 4C &alpha;. The K layer neurons in the LGN relay to large neurons called blobs in layers 2 and 3 of V1.
 
   
There is a direct correspondence from an angular position in the [[field of view]] of the [[eye]], all the way through the optic tract to a nerve position in V1.
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===Optic radiation===
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{{Main|Optic radiation}}
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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. {{Citation needed|date=May 2010}}
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There is a direct correspondence from an angular position in the [[field of view]] of the eye, all the way through the optic tract to a nerve position in V1.
 
At this juncture in V1, the image path ceases to be straightforward; there is more cross-connection within the visual cortex.
 
At this juncture in V1, the image path ceases to be straightforward; there is more cross-connection within the visual cortex.
   
==Visual cortex==
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===Visual cortex===
[[Image:Ba 17 18 19.png|thumb|200px|[[Visual cortex]]: V1, V2, V3, V4, V5 (also called MT)]]
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{{Main|Visual cortex}}
The [[visual cortex]] is the most massive system in the human brain and is responsible for higher-level processing of the visual image. It lies at the rear of the brain (highlighted in the image), above the [[cerebellum]]. The interconnections between layers of the [[cortex]], the thalamus, the cerebellum, the [[hippocampus]] and the remainder of the areas of the brain are under active investigation. Currently, much of what is known stems from patients with damage to known areas of the brain, with a corresponding study of the cognitive functions which have been spared.
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[[Image:Brodmann areas 17 18 19.png|thumb|200px|[[Visual cortex]]: V1, V2, V3, V4, V5 (also called MT)]]
[[Image:Hippocampus.png|thumb|128px|[[Hippocampus]]' location in the brain (red).]]
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The visual cortex is the largest system in the human brain 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_.28V1.29|primary visual cortex]], (also called V1 and striate cortex). 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 name=KandelEricBook>{{Citation |title= Principles of Neural Science |edition= 4 |last1= Kandel |first1= Eric R. |last2= Jessell |first2= Thomas M. |last3= Sanes |first3=Joshua R. |chapter= Chapter 27: Central Visual Pathways |pages=533–540 |date=2000 |publisher= McGraw-Hill}}</ref>
   
[http://cercor.oxfordjournals.org/cgi/content/abstract/10/12/1211"Lesions Affecting the Parahippocampal Cortex Yield Spatial Memory Deficits in Humans", ''Cerebral Cortex'', Vol. '''10''', No. 12, 1211-1216, December 2000]
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===Visual association cortex===
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{{Main|Two Streams hypothesis}}
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As visual information passes forward through the visual hierarchy, the complexity of the neural representations increase. Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular retinotopic location, neurons in the lateral occipital complex respond selectively to complete object (e.g., a figure drawing), and neurons in visual association cortex may respond selectively to human faces, or to a particular object.
   
Zeineh et al., "Dynamics of the Hippocampus During Encoding and Retrieval of Face-Name Pairs", ''Science'' 2003 '''299''': 577-580.
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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>{{Citation |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. |author=Mishkin M, Ungerleider LG. |pmid=7126325 |doi=10.1016/0166-4328(82)90081-X |postscript=.}}</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 as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli.
   
See also:[[Hippocampus#Role in spatial memory and navigation]], and the [[Fusiform gyrus]] in the [[temporal lobe]] of the cortex.
+
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>{{Citation|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|postscript=.}}</ref>
   
 
==See also==
 
==See also==
*[[Edinger-Westphal nucleus]]
+
{{Portal|Neuroscience}}
  +
  +
*[[Human echolocation|Echolocation]]
  +
*[[Computer vision]]
  +
*[[Helmholtz–Kohlrausch effect]] - how [[color balance]] affects vision
 
*[[Memory-prediction framework]]
 
*[[Memory-prediction framework]]
 
*[[Visual perception]]
 
*[[Visual perception]]
  +
*[[Visual modularity]]
   
 
==References==
 
==References==
*[[David H. Hubel]] ([[1989]]), ''Eye, Brain and Vision''. New York: Scientific American Library.
+
{{Reflist|2}}
*[[David Marr]] ([[1982]]), ''Vision: A Computational Investigation into the Human Representation and Processing of Visual Information''. San Francisco: W. H. Freeman.
+
*R.W. Rodiek (1988). "The Primate Retina". ''Comparative Primate Biology'' Vol. '''4''' of '''''Neurosciences'''''. (H.D. Steklis and J. Erwin, editors.) pp. 203-278. New York: A.R. Liss.
+
==Further reading==
*[http://webvision.med.utah.edu/VisualCortex.html Matthew Schmolesky, The Primary Visual Cortex]
+
{{refbegin}}
*Martin J. Tovée ([[1996]]), ''An introduction to the visual system''. Cambridge University Press, ISBN 0521483395 (References, pp.180-198. Index, pp.199-202. 202 pages.)
+
*{{Citation|last=al-Haytham|author-link=Ibn al-Haytham|year=1021|title=[[Book of Optics]]|url=http://books.google.com/?id=3VfY8PgmhDMC&pg=RA1-PA97&lpg=RA1-PA97&dq=al-haytham+visual+system|accessdate=2008-07-05|isbn=9780292781498}}: the Google books link shows Alhazen's sketch of [[optic nerve]] 522 years before [[Vesalius]]' engraving.
*[[Andreas Vesalius]] ([[1543]]) ''[[De Humanis Corporis Fabrica]]'' (On the Workings of the Human Body)
+
*{{Citation|first=J|last=Bellingham|first2=SE|last2=Wilkie|first3=AG|last3=Morris|last4=Bowmaker|first4=JK, Hunt,DM|year=1997|title=Characterisation of the ultraviolet-sensitive opsin gene in the honey bee, Apis mellifera|journal=European Journal of Biochemistry |volume=243|pages=775–781|doi=10.1111/j.1432-1033.1997.00775.x|pmid=9057845|last5=Hunt|first5=DM|issue=3}}.
*[[Torsten Wiesel]] and [[David H. Hubel]] ([[1963]]), "The effects of visual deprivation on the morphology and physiology of cell's lateral geniculate body". ''Journal of Neurophysiology'' '''26''', 978-993.
+
*{{Citation|doi=10.1093/cercor/4.5.455|volume=4|issue=5|pages=455–469|last=Gross|first=Charles G.|title=How Inferior Temporal Cortex Became a Visual Area|journal=Cerebral Cortex|accessdate=2008-06-09|date=1994-09-01|url=http://cercor.oxfordjournals.org/cgi/content/abstract/4/5/455|pmid=7833649}}
  +
*{{Citation|author-link=David H. Hubel|first=David H.|last=Hubel|year=1989|title=Eye, Brain and Vision|location=New York|publisher=Scientific American Library}}.
  +
*{{Citation
  +
| last =Lucas
  +
| first =R. J., S. Hattar, M. Takao, D. M. Berson, R. G. Foster, and K. W. Yau
  +
| authorlink =
  +
| coauthors =S. Hattar, M. Takao, D. M. Berson, R. G. Foster, and K. W. Yau
  +
| title =Diminished Pupillary Light Reflex at High Irradiances in Melanopsin-Knockout Mice
  +
| journal =Science
  +
| volume =299
  +
| issue =5604
  +
| pages =245–247
  +
| publisher =
  +
| date =2003
  +
| doi =10.1126/science.1077293
  +
| pmid =12522249
  +
}}.
  +
*{{Citation|author-link=David Marr (neuroscientist)|first=David|last=Marr|year=1982|title=Vision: A Computational Investigation into the Human Representation and Processing of Visual Information|location=San Francisco|publisher=W. H. Freeman}}.
  +
*{{Citation
  +
| last =Nolte
  +
| first =John
  +
| authorlink =
  +
| coauthors =
  +
| title =The Human Brain: An Introduction to Its Functional Anatomy. 5th Ed
  +
| publisher =Mosby
  +
| year=2002
  +
| location =St. Louis
  +
| pages =410–447
  +
| url =
  +
| doi =
  +
| id = }}.
  +
*{{Citation|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.
  +
*{{Citation|first=AB|last=Safer|first2=MS|last2=Grace|title=Infrared imaging in vipers: differential responses of crotaline and viperine snakes to paired thermal targets|journal=Behav Brain Res|volume=154|pages=55–61|date=2004-09-23|issue=1|pmid=15302110|doi=10.1016/j.bbr.2004.01.020}}.
  +
*{{Citation|doi=10.1016/0042-6989(86)90162-8|issn=0042-6989|volume=26|issue=9|pages=1351–86|last=Schiller|first=P H|title=The central visual system|journal=Vision research|year=1986|url=http://linkinghub.elsevier.com/retrieve/pii/0042-6989(86)90162-8|pmid=3303663}}
  +
*{{Citation|url=http://webvision.med.utah.edu/VisualCortex.html|first=Matthew|last=Schmolesky|title=The Primary Visual Cortex|accessdate=2005-01-01}}.
  +
*{{Citation|first=Martin J.|last=Tovée|year=1996|title=An introduction to the visual system|publisher=Cambridge University Press|isbn=0-521-48339-5}} (References, pp.&nbsp;180–198. Index, pp.&nbsp;199–202. 202 pages.)
  +
*{{Citation|author-link=Andreas Vesalius|first=Andreas|last=Vesalius|year=1543|title=De Humani Corporis Fabrica (On the Workings of the Human Body)}}
  +
*{{Citation|author-link=Torsten Wiesel|author2-link=David H. Hubel|first=Torsten|last=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|pp=978–993}}.
  +
*{{Citation|last=Zaidi|first=FH |coauthors=''et al.''|title=Short-wavelength light sensitivity of circadian, pupillary, and visual awareness in humans lacking an outer retina|journal=Curr Biol. |date=2007-12-18|volume=17|pages=2122–8|doi=10.1016/j.cub.2007.11.034|issue=24|pmid=18082405|pmc=2151130}}
  +
{{refend}}
  +
  +
==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.
  +
*[http://www.physorg.com/news115919015.html Hagfish research has found the “missing link” in the evolution of the eye. See: ''Nature Reviews Neuroscience. '']
   
 
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Gray722

The visual system includes the eyes, the connecting pathways through to the visual cortex and other parts of the brain. The illustration shows the mammalian system.

The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the reception of light and the formation of monocular representations; the construction of a binocular perception from a pair of two dimensional projections; the identification and categorization of visual objects; assessing distances to and between objects; and guiding body movements in relation to visual objects. The psychological manifestation of visual information is known as visual perception, a lack of which is called blindness. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex (PLR) and circadian photoentrainment.

IntroductionEdit

Schematic diagram of the human eye en

The human eye
The image projected onto the retina is inverted due to the optics of the eye.

This article mostly describes the visual system of mammals, although other "higher" animals have similar visual systems. In this case, the visual system consists of:

Different species are able to see different parts of the light spectrum; for example, bees can see into the ultraviolet,[1] while pit vipers can accurately target prey with their pit organs, which are sensitive to infrared radiation.[2]

HistoryEdit

In the second half of the 19th century, many motifs of the nervous system were identified such as the neuron doctrine and brain localisation, 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), movement (motor cortex), and vision (visual cortex), was first proposed by Franz Joseph Gall in 1810.[3] 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 Edouard Hitzig of the motor cortex (1871).[3][4] Based on selective damage to parts of the brain and the functional effects this would produce (lesion studies), David Ferrier proposed that visual function was localised to the parietal lobe of the brain in 1876.[4] In 1881, Hermann Munk more accurately located vision in the occipital lobe, where the primary visual cortex is now known to be.[4]

Biology of the visual systemEdit

EyeEdit

Main article: Eye

The eye is a complex biological device. 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 CCD camera.

Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil (controlled by the iris) and is further refracted by the lens. The cornea and lens act together as a compound lens to project an inverted image onto the retina.

RetinaEdit

Cajal Retina

S. Ramón y Cajal, Structure of the Mammalian Retina, 1900

Main article: Retina

The retina consists of a large number of photoreceptor cells which contain particular protein molecules called opsins. In humans, two types of opsins are involved in conscious vision: rod opsins and cone opsins. (A third type, melanopsin in some of the 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 (LGN) but to the pretectal olivary nucleus (PON).[5]) An opsin absorbs a photon (a particle of light) and transmits a signal to the cell through a signal transduction pathway, resulting in hyperpolarization of the photoreceptor. (For more information, see Photoreceptor cell).

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. Cones are found primarily in the center (or fovea) of the retina.[citation needed] 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 are used primarily to distinguish color and other features of the visual world at normal levels of light.[citation needed]

In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which will then conduct action potentials to the brain. A significant amount of visual processing arises from the patterns of communication between neurons in the retina. About 130 million photoreceptors 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 and amacrine cells, 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 or sensitive to color and indifferent to motion.[citation needed] Mechanism of generating visual signals: The retina adapts to its change in light through the use of the rods. In the dark, the retinal has a bent shape called cis-retinal. When light is present, the retinal changes 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. In the dark, the rhodopsin absorbs no light therefore releasing glutamate cells which inhibit the bipolar cell. This inhibits the release of neurotransmitters to the ganglion cell. In the light, glutamate secretion ceases which no longer inhibits the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected.[6][7]

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:

  1. M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus;
  2. P cells, with smaller center-surround receptive fields that are sensitive to color and shape;
  3. K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
  4. another population that is intrinsically photosensitive; and
  5. a final population that is used for eye movements. [citation needed]

A 2006 University of Pennsylvania study calculated the approximate bandwidth of human retinas to be about 8960 kilobits per second, whereas guinea pig retinas transfer at about 875 kilobits.[8]

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.[9][10][11] The peak spectral sensitivity was 481 nm. This shows that there are two pathways for sight in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photoreceptive ganglion cells which act as rudimentary visual brightness detectors.

PhotochemistryEdit

Main article: Visual cycle

In the visual system, retinal, technically called retinene1 or "retinaldehyde", is a light-sensitive retinene 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. [citation needed]

Fibers to thalamusEdit

Optic nerveEdit

Main article: Optic nerve
1543,Vesalius'Fabrica,VisualSystem,V1

Information flow from the eyes (top), crossing at the optic chiasma, joining left and right eye information in the optic tract, and layering left and right visual stimuli in the lateral geniculate nucleus. 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 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)[12] as well as other motor responses.

A final population of photosensitive ganglion cells, containing melanopsin, sends information via the retinohypothalamic tract (RHT) to the pretectum (pupillary reflex), to several structures involved in the control of circadian rhythms and sleep such as the suprachiasmatic nucleus (SCN, the biological clock), and to the ventrolateral preoptic nucleus (VLPO, a region involved in sleep regulation).[13] 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.[9]

Optic chiasmEdit

Main article: Optic chiasm

The optic nerves from both eyes meet and cross at the optic chiasm,[14][15] 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 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.[12] A small region in the center of the field of view is processed redundantly by both halves of the brain.

Optic tractEdit

Main article: 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.

Lateral geniculate nucleus

Six layers in the LGN

Lateral geniculate nucleusEdit

Main article: 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 humans and other primates starting from catarhinians, including cercopithecidae and apes. Layers 1, 4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal visual field; layers 2, 3, and 5 correspond to information from the ipsilateral (uncrossed) fibers of the temporal visual field. Layer one (1) contains M cells which correspond to the M (magnocellular) cells of the optic nerve of the opposite eye and are concerned with depth or motion. Layers four and six (4 & 6) 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 (2, 3, & 5) of the LGN connect to the M cells and P (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 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 (caudal 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 and subcortical layers and reciprocal innervation from the visual cortex.[citation needed]
Gray722

Gray's FIG. 722– Scheme showing central connections of the optic nerves and optic tracts.

Optic radiationEdit

Main article: 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. [citation needed]

There is a direct correspondence from an angular position in the field of view of the eye, all the way through the optic tract to a nerve position in V1. At this juncture in V1, the image path ceases to be straightforward; there is more cross-connection within the visual cortex.

Visual cortexEdit

Main article: Visual cortex
Brodmann areas 17 18 19

Visual cortex: V1, V2, V3, V4, V5 (also called MT)

The visual cortex is the largest system in the human brain 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 primary visual cortex, (also called V1 and striate cortex). 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.[16]

Visual association cortexEdit

Main article: Two Streams hypothesis

As visual information passes forward through the visual hierarchy, the complexity of the neural representations increase. Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular retinotopic location, neurons in the lateral occipital complex respond selectively to complete object (e.g., a figure drawing), and neurons in 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,[17] 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 as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli.

However, there is still much debate about the degree of specialization within these two pathways, since they are in fact heavily interconnected.[18]

See alsoEdit

.

ReferencesEdit

  1. Bellingham et al. 1997, pp. 775–781
  2. Safer & Grace 2004, pp. 55–61.
  3. 3.0 3.1 Gross 1994, pp. 455–69
  4. 4.0 4.1 4.2 Schiller 1986, pp. 1351–86
  5. Güler, A.D.; et al (May 2008), "Melanopsin cells are the principal conduits for rod-cone input to non-image-forming vision" (Abstract), Nature 453 (7191): 102–5, doi:10.1038/nature06829, PMID 18432195, PMC: 2871301, http://www.ncbi.nlm.nih.gov/pubmed/18432195?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum, retrieved on 2010-06-03. 
  6. Saladin, Kenneth D. Anatomy & Physiology: The Unity of Form and Function. 5th ed. New York: McGraw-Hill, 2010.
  7. http://webvision.med.utah.edu/GCPHYS1.HTM
  8. Calculating the speed of sight – being-human – 28 July 2006 – New Scientist
  9. 9.0 9.1 Zaidi et al. 2007, pp. 2122–8
  10. Normal Responses To Non-visual Effects Of Light Retained By Blind Humans Lacking Rods And Cones
  11. Blind humans lacking rods and cones retain normal responses to nonvisual effects of light
  12. 12.0 12.1 Nolte 2002, pp. 410–447
  13. Lucas, pp. 245–7
  14. al-Haytham 1021, p. 98
  15. Vesalius 1543
  16. Kandel, Eric R.; Jessell, Thomas M.; Sanes, Joshua R. (2000), "Chapter 27: Central Visual Pathways", Principles of Neural Science (4 ed.), McGraw-Hill, pp. 533–540 
  17. Mishkin M, Ungerleider LG. (1982), "Contribution of striate inputs to the visuospatial functions of parieto-preoccipital cortex in monkeys.", Behav. Brain Res. 6 (1): 57–77, doi:10.1016/0166-4328(82)90081-X, PMID 7126325. 
  18. Farivar R. (2009), "Dorsal-ventral integration in object recognition.", Brain Res. Rev. 61 (2): 144–53, doi:10.1016/j.brainresrev.2009.05.006, PMID 19481571. 

Further readingEdit


External linksEdit

Sensory system - Visual system - edit
Eye | Optic nerve | Optic chiasm | Optic tract | Lateral geniculate nucleus | Optic radiation | Visual cortex
Nervous system - Sensory system - edit
Special sensesVisual system | Auditory system | Olfactory system | Gustatory system
Somatosensory systemNociception | Thermoreception | Vestibular system |
Mechanoreception (Pressure, Vibration & Proprioception) | Equilibrioception 



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