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The primary visual cortex is divided into six functionally distinct layers, labeled 1 to 6. Layer 4, which receives most visual input from the [[lateral geniculate nucleus]] (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives mostly [[Magnocellular cell|magnocellular]] input from the LGN, while layer 4Cβ receives input from [[Parvocellular cell|parvocellular]] pathways.<ref>{{cite journal | vauthors = Hubel DH, Wiesel TN | title = Laminar and columnar distribution of geniculo-cortical fibers in the macaque monkey | journal = The Journal of Comparative Neurology | volume = 146 | issue = 4 | pages = 421–450 | date = December 1972 | pmid = 4117368 | doi = 10.1002/cne.901460402 | s2cid = 6478458 }}</ref><ref>{{cite book |last1=Churchland |first1=Patricia Smith |last2=Sejnowski |first2=Terrence Joseph |author1-link=Patricia Churchland |author2-link=Terry Sejnowski |title=[[The Computational Brain]] |publisher=[[MIT Press]] |___location=Cambridge, Massachusetts |year=1992 |isbn=978-0-262-53120-7 |page=149}}</ref>
The average number of neurons in the adult human primary visual cortex in each hemisphere has been estimated at 140 million.<ref name="Leuba-Kraftsik-1994">{{cite journal | vauthors = Leuba G, Kraftsik R | title = Changes in volume, surface estimate, three-dimensional shape and total number of neurons of the human primary visual cortex from midgestation until old age | journal = Anatomy and Embryology | volume = 190 | issue = 4 | pages = 351–366 | date = October 1994 | pmid = 7840422 | doi = 10.1007/BF00187293 | s2cid = 28320951 }}</ref> The volume of each V1 area in an adult human is about 5400mm<math>{}^3</math> on average. A study of 25 hemispheres from 15 normal individuals with average age 59 years at autopsy found a very high variation, from 4272 to 7027mm<math>{}^3</math> for the right hemisphere (mean 5692mm<math>{}^3</math>), and from 3185 to 7568mm<math>{}^3</math> for the left hemisphere (mean 5119mm<math>{}^3</math>), with 0.81 correlation between left and right hemispheres.<ref>{{cite journal |last1=Andrews |first1=Timothy J. |last2=Halpern |first2=Scott D. |last3=Purves |first3=Dale |title=Correlated Size Variations in Human Visual Cortex, Lateral Geniculate Nucleus, and Optic Tract |journal=Journal of Neuroscience |date=1997 |volume=17 |issue=8 |pages=2859–2868 |doi=10.1523/JNEUROSCI.17-08-02859.1997 |doi-access=free|pmc=6573115 }}</ref> The same study found average V1 area 2400mm<math>{}^2</math> per hemisphere, but with very high variability. (Right hemisphere mean 2477mm<math>{}^2</math>, range 1441–3221mm<math>{}^2</math>. Left hemisphere mean 2315mm<math>{}^2</math>, range 1438–3365mm<math>{}^2</math>.)
=== Function ===
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In addition to its role in spatial processing, the retinotopic map in V1 is intricately connected with other visual areas, forming a network that contributes to the integration of various visual features and the construction of a coherent visual percept. This dynamic mapping mechanism is fundamental to our ability to navigate and interpret the visual world effectively.<ref name= kepler1604 >Johannes Kepler (1604) Paralipomena to Witelo whereby The Optical Part of Astronomy is Treated (Ad Vitellionem Paralipomena, quibus astronomiae pars optica traditvr, 1604), as cited by A.Mark Smith (2015) From Sight to Light. Kepler modeled the eye as a water-filled glass sphere, and discovered that each point of the scene taken in by the eye projects onto a point on the back of the eye (the retina).</ref> The correspondence between a given ___location in V1 and in the subjective visual field is very precise: even the [[Blind spot (vision)|blind spots]] of the retina are mapped into V1. In terms of evolution, this correspondence is very basic and found in most animals that possess a V1. In humans and other animals with a [[Fovea centralis|fovea]] ([[Cone cell|cones]] in the retina), a large portion of V1 is mapped to the small, central portion of visual field, a phenomenon known as [[cortical magnification]]. Perhaps for the purpose of accurate spatial encoding, neurons in V1 have the smallest [[receptive field]] size (that is, the highest resolution) of any visual cortex microscopic regions.
The tuning properties of V1 neurons (what the neurons respond to) differ greatly over time. Early in time (40 ms and further) individual V1 neurons have strong tuning to a small set of stimuli. That is, the neuronal responses can discriminate small changes in visual [[Orientation (mental)|orientations]], [[spatial frequencies]] and [[color]]s (as in the optical system of a [[camera obscura]], but projected onto [[retina]]l cells of the eye, which are clustered in density and fineness).<ref name= kepler1604 /> Each V1 neuron propagates a signal from a retinal cell, in continuation. Furthermore, individual V1 neurons in humans and other animals with [[binocular vision]] have ocular dominance, namely tuning to one of the two eyes. In V1, and primary sensory cortex in general, neurons with similar tuning properties tend to cluster together as [[cortical column]]s. [[David Hubel]] and [[Torsten Wiesel]] proposed the classic ice-cube organization model of cortical columns for two tuning properties: [[ocular dominance columns|ocular dominance]] and orientation. However, this model cannot accommodate the color, spatial frequency and many other features to which neurons are tuned {{Citation needed|date=November 2011}}. The exact organization of all these cortical columns within V1 remains a hot topic of current research.
The receptive fields of V1 neurons<ref>{{cite journal |vauthors=DeAngelis GC, Ohzawa I, Freeman RD |date=October 1995 |title=Receptive-field dynamics in the central visual pathways |journal=Trends in Neurosciences |volume=18 |issue=10 |pages=451–458 |doi=10.1016/0166-2236(95)94496-r |pmid=8545912 |s2cid=12827601}}</ref><ref>{{Cite book |title=The Visual Neurosciences, 2-vol. Set |vauthors=DeAngelis GC, Anzai A |date=2003-11-21 |publisher=The MIT Press |isbn=978-0-262-27012-0 |veditors=Chalupa LM, Werner JS |volume=1 |___location=Cambridge |pages=704–719 |language=en |chapter=A Modern View of the Classical Receptive Field: Linear and Nonlinear Spatiotemporal Processing by V1 Neurons |doi=10.7551/mitpress/7131.003.0052 |chapter-url=https://direct.mit.edu/books/book/5395/chapter/3948206/A-Modern-View-ofthe-Classical-Receptive-Field}}</ref> resemble Gabor functions, so the operation of the visual cortex has been compared to the [[Gabor transform]].{{Citation needed|date=May 2023}}
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Furthermore, the reciprocal feedback connections from V2 to V1 play a significant role in modulating the activity of V1 neurons. This feedback loop is thought to be involved in processes such as attention, perceptual grouping, and figure-ground segregation. The dynamic interplay between V1 and V2 highlights the intricate nature of information processing within the visual system.
Moreover, V2's connections with subsequent visual areas, including V3, V4, and V5, contribute to the formation of a distributed network for visual processing. These connections enable the integration of different visual features, such as motion and form, across multiple stages of the visual hierarchy.<ref>Taylor, Katherine. and Jeanette Rodriguez.
In terms of anatomy, V2 is split into four quadrants, a [[Dorsum (biology)|dorsal]] and [[ventral]] representation in the left and the right [[cerebral hemisphere|hemispheres]]. Together, these four regions provide a complete map of the visual world. V2 has many properties in common with V1: Cells are tuned to simple properties such as orientation, spatial frequency, and color. The responses of many V2 neurons are also modulated by more complex properties, such as the orientation of [[illusory contours]],<ref name="illusory contours">{{cite journal | vauthors = von der Heydt R, Peterhans E, Baumgartner G | title = Illusory contours and cortical neuron responses | journal = Science | volume = 224 | issue = 4654 | pages = 1260–1262 | date = June 1984 | pmid = 6539501 | doi = 10.1126/science.6539501 | bibcode = 1984Sci...224.1260V }}</ref><ref name="A. Anzai, X. Peng 2007"/> [[binocular disparity]],<ref name="stereoscopic edges">{{cite journal | vauthors = von der Heydt R, Zhou H, Friedman HS | title = Representation of stereoscopic edges in monkey visual cortex | journal = Vision Research | volume = 40 | issue = 15 | pages = 1955–1967 | date = 2000 | pmid = 10828464 | doi = 10.1016/s0042-6989(00)00044-4 | s2cid = 10269181 | doi-access = free }}</ref> and whether the stimulus is part of the figure or the ground.<ref>{{cite journal | vauthors = Qiu FT, von der Heydt R | title = Figure and ground in the visual cortex: v2 combines stereoscopic cues with gestalt rules | journal = Neuron | volume = 47 | issue = 1 | pages = 155–166 | date = July 2005 | pmid = 15996555 | pmc = 1564069 | doi = 10.1016/j.neuron.2005.05.028 }}</ref><ref>{{cite journal | vauthors = Maruko I, Zhang B, Tao X, Tong J, Smith EL, Chino YM | title = Postnatal development of disparity sensitivity in visual area 2 (v2) of macaque monkeys | journal = Journal of Neurophysiology | volume = 100 | issue = 5 | pages = 2486–2495 | date = November 2008 | pmid = 18753321 | pmc = 2585398 | doi = 10.1152/jn.90397.2008 }}</ref> Recent research has shown that V2 cells show a small amount of attentional modulation (more than V1, less than V4), are tuned for moderately complex patterns, and may be driven by multiple orientations at different subregions within a single receptive field.
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