Editing Color vision (section)

===Color in the primate brain===
[[File:Ventral-dorsal streams.svg|thumb|left|Visual pathways in the human brain. The [[ventral stream]] (purple) is important in color recognition. The [[dorsal stream]] (green) is also shown. They originate from a common source in the [[visual cortex]].]]

Color processing begins at a very early level in the visual system (even within the retina) through initial color opponent mechanisms. Both Helmholtz's trichromatic theory and Hering's opponent-process theory are therefore correct, but trichromacy arises at the level of the receptors, and opponent processes arise at the level of [[retinal ganglion cell]]s and beyond. In Hering's theory, opponent mechanisms refer to the opposing color effect of red–green, blue–yellow, and light-dark. However, in the visual system, it is the activity of the different receptor types that are opposed. Some midget retinal ganglion cells oppose L and M cone activity, which corresponds loosely to red–green opponency, but actually runs along an axis from blue-green to magenta. Small bistratified retinal ganglion cells oppose input from the S cones to input from the L and M cones. This is often thought to correspond to blue–yellow opponency but actually runs along a color axis from yellow-green to violet.

Visual information is then sent to the brain from retinal ganglion cells via the [[optic nerve]] to the [[optic chiasma]]: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasma, the visual tracts are referred to as the [[optic tract]]s, which enter the [[thalamus]] to synapse at the [[lateral geniculate nucleus]] (LGN).

The lateral geniculate nucleus is divided into laminae (zones), of which there are three types: the M-laminae, consisting primarily of M-cells, the P-laminae, consisting primarily of P-cells, and the koniocellular laminae.  M- and P-cells receive relatively balanced input from both L- and M-cones throughout most of the retina, although this seems to not be the case at the fovea, with midget cells synapsing in the P-laminae. The koniocellular laminae receives axons from the small bistratified ganglion cells.<ref name = "Rodieck_1988">{{cite book | vauthors = Rodieck RW | title = The First Steps in Seeing | publisher = Sinauer Associates, Inc. | location = Sunderland, Massachusetts, USA | date = 1998 | isbn = 978-0-87893-757-8 }}</ref><ref name = "Hendry_1970">{{cite journal | vauthors = Hendry SH, Reid RC | title = The koniocellular pathway in primate vision | journal = Annual Review of Neuroscience | volume = 23 | pages = 127–53 | date = 1970-01-01 | pmid = 10845061 | doi = 10.1146/annurev.neuro.23.1.127 }}</ref>

After [[synapse|synapsing]] at the LGN, the visual tract continues on back to the primary [[visual cortex]] (V1) located at the back of the brain within the [[occipital lobe]]. Within V1 there is a distinct band (striation). This is also referred to as "striate cortex", with other cortical visual regions referred to collectively as "extrastriate cortex". It is at this stage that color processing becomes much more complicated.

In V1 the simple three-color segregation begins to break down. Many cells in V1 respond to some parts of the spectrum better than others, but this "color tuning" is often different depending on the adaptation state of the visual system. A given cell that might respond best to long-wavelength light if the light is relatively bright might then become responsive to all wavelengths if the stimulus is relatively dim. Because the color tuning of these cells is not stable, some believe that a different, relatively small, population of neurons in V1 is responsible for color vision. These specialized "color cells" often have receptive fields that can compute local cone ratios. Such "double-opponent" cells were initially described in the goldfish retina by Nigel Daw;<ref>{{cite journal | vauthors = Daw NW | title = Goldfish retina: organization for simultaneous color contrast | journal = Science | volume = 158 | issue = 3803 | pages = 942–4 | date = November 1967 | pmid = 6054169 | doi = 10.1126/science.158.3803.942 | s2cid = 1108881 | bibcode = 1967Sci...158..942D }}</ref><ref>{{cite book| title = Neural Mechanisms of Color Vision: Double-Opponent Cells in the Visual Cortex | vauthors = Conway BR | url = https://books.google.com/books?id=pFodUlHfQmcC&q=goldfish+retina+by+Nigel-Daw&pg=PR7 | publisher = Springer | year = 2002 | isbn = 978-1-4020-7092-1 }}</ref> their existence in primates was suggested by [[David H. Hubel]] and [[Torsten Wiesel]], first demonstrated by C.R. Michael<ref>{{Cite journal|last=Michael|first=C. R.|date=1978-05-01|title=Color vision mechanisms in monkey striate cortex: dual-opponent cells with concentric receptive fields|url=https://journals.physiology.org/doi/abs/10.1152/jn.1978.41.3.572|journal=Journal of Neurophysiology|volume=41|issue=3|pages=572–588|doi=10.1152/jn.1978.41.3.572|pmid=96222|issn=0022-3077}}</ref> and subsequently confirmed by [[Bevil Conway]].<ref>{{cite journal | vauthors = Conway BR | title = Spatial structure of cone inputs to color cells in alert macaque primary visual cortex (V-1) | journal = The Journal of Neuroscience | volume = 21 | issue = 8 | pages = 2768–83 | date = April 2001 | pmid = 11306629 | pmc = 6762533 | doi = 10.1523/JNEUROSCI.21-08-02768.2001 }}</ref> As Margaret Livingstone and David Hubel showed, double opponent cells are clustered within localized regions of V1 called [[Blob (visual system)|blobs]], and are thought to come in two flavors, red–green and blue-yellow.<ref>{{cite book| title = Neurons, and Networks: An Introduction to Behavioral Neuroscience | vauthors = Dowling JE | publisher = Harvard University Press | year = 2001 | isbn = 978-0-674-00462-7 | url = https://books.google.com/books?id=adeUwgfwdKwC&q=Margaret+Livingstone+David+Hubel+double+opponent+blobs&pg=PA376 }}</ref> Red–green cells compare the relative amounts of red–green in one part of a scene with the amount of red–green in an adjacent part of the scene, responding best to local color contrast (red next to green). Modeling studies have shown that double-opponent cells are ideal candidates for the neural machinery of [[color constancy]] explained by [[Edwin H. Land]] in his [[retinex]] theory.<ref>{{cite book | veditors = McCann M | date = 1993 | title = [[Edwin H. Land]]'s Essays | location = Springfield, Va. | publisher = Society for Imaging Science and Technology }}</ref>
[[File:16777216colors.png|thumb|When viewed in full size, this image contains about 16 million pixels, each corresponding to a different color in the full set of RGB colors. The [[human eye]] can distinguish about 10 million different colors.<ref name="Judd_1975">{{cite book | vauthors = Judd DB, Wyszecki G |title=Color in Business, Science and Industry|publisher=[[Wiley-Interscience]]|series=Wiley Series in Pure and Applied Optics|edition=third|location=New York|year= 1975|page=388|isbn=978-0-471-45212-6}}</ref>]]

From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form).  Neurons in V2 then synapse onto cells in the extended V4. This area includes not only  V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO.<ref name="Conway_2007">{{cite journal | vauthors = Conway BR, Moeller S, Tsao DY | title = Specialized color modules in macaque extrastriate cortex | journal = Neuron | volume = 56 | issue = 3 | pages = 560–73 | date = November 2007 | pmid = 17988638 | doi = 10.1016/j.neuron.2007.10.008 | pmc = 8162777 | s2cid = 11724926 | url = https://authors.library.caltech.edu/100800/ }}</ref><ref name="Conway_2009">{{cite journal | vauthors = Conway BR, Tsao DY | title = Color-tuned neurons are spatially clustered according to color preference within alert macaque posterior inferior temporal cortex | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 42 | pages = 18034–9 | date = October 2009 | pmid = 19805195 | pmc = 2764907 | doi = 10.1073/pnas.0810943106 | bibcode = 2009PNAS..10618034C | doi-access = free }}</ref> Area V4 was initially suggested by [[Semir Zeki]] to be exclusively dedicated to color,<ref>{{cite journal | vauthors = Zeki SM | title = Colour coding in rhesus monkey prestriate cortex | journal = Brain Research | volume = 53 | issue = 2 | pages = 422–7 | date = April 1973 | pmid = 4196224 | doi = 10.1016/0006-8993(73)90227-8 }}</ref> and he later showed that V4 can be subdivided into subregions with very high concentrations of color cells separated from each other by zones with lower concentration of such cells though even the latter cells respond better to some wavelengths than to others,<ref name="Zeki_1983">{{cite journal | vauthors = Zeki S | title = The distribution of wavelength and orientation selective cells in different areas of monkey visual cortex | journal = Proceedings of the Royal Society of London. Series B, Biological Sciences | volume = 217 | issue = 1209 | pages = 449–70 | date = March 1983 | pmid = 6134287 | doi = 10.1098/rspb.1983.0020 | bibcode = 1983RSPSB.217..449Z | s2cid = 39700958 }}</ref> a finding  confirmed by subsequent studies.<ref name="Conway_2007" /><ref>{{cite journal | vauthors = Bushnell BN, Harding PJ, Kosai Y, Bair W, Pasupathy A | title = Equiluminance cells in visual cortical area v4 | journal = The Journal of Neuroscience | volume = 31 | issue = 35 | pages = 12398–412 | date = August 2011 | pmid = 21880901 | pmc = 3171995 | doi = 10.1523/JNEUROSCI.1890-11.2011 }}</ref><ref>{{cite journal | vauthors = Tanigawa H, Lu HD, Roe AW | title = Functional organization for color and orientation in macaque V4 | journal = Nature Neuroscience | volume = 13 | issue = 12 | pages = 1542–8 | date = December 2010 | pmid = 21076422 | pmc = 3005205 | doi = 10.1038/nn.2676 }}</ref>  The presence in V4 of orientation-selective cells led to the view that V4 is involved in processing both color and form associated with color<ref name="Zeki_2005">{{cite journal | vauthors = Zeki S | title = The Ferrier Lecture 1995 behind the seen: the functional specialization of the brain in space and time | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 360 | issue = 1458 | pages = 1145–83 | date = June 2005 | pmid = 16147515 | pmc = 1609195 | doi = 10.1098/rstb.2005.1666 }}</ref> but it is worth noting that the orientation selective cells within V4 are more broadly tuned than their counterparts in V1, V2 and V3.<ref name="Zeki_1983" /> Color processing in the extended V4 occurs in millimeter-sized color modules called [[glob (visual system)|globs]].<ref name="Conway_2007"/><ref name="Conway_2009"/> This is the part of the brain in which color is first processed into the full range of [[hue]]s found in [[color space]].<ref>{{Cite journal|last=Zeki|first=S.|date=1980|title=The representation of colours in the cerebral cortex|url=https://www.nature.com/articles/284412a0|journal=Nature|language=en|volume=284|issue=5755|pages=412–418|doi=10.1038/284412a0|pmid=6767195|bibcode=1980Natur.284..412Z |s2cid=4310049|issn=1476-4687}}</ref><ref name="Conway_2007"/><ref name="Conway_2009"/>

Anatomical studies have shown that neurons in extended V4 provide input to the inferior [[temporal lobe]]. "IT" cortex is thought to integrate color information with shape and form, although it has been difficult to define the appropriate criteria for this claim. Despite this murkiness, it has been useful to characterize this pathway (V1 > V2 > V4 > IT) as the [[ventral stream]] or the "what pathway", distinguished from the [[dorsal stream]] ("where pathway") that is thought to analyze motion, among other features.