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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>
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}}</ref>
=== Function ===
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