Editing Opponent process (section)

==Physiological basis==
===Relation to LMS color space===
[[File:Diagram of the opponent process.png|right|thumb|360px|Diagram of the opponent process {{citation needed|date=June 2022}}]]
Though the [[Young–Helmholtz theory|trichromatic]] and opponent processes theories were initially thought to be at odds, the opponent process theory has been refined so as to claim that the mechanisms responsible for the opponent process receive signals from the three types of cones predicted by the [[trichromatic theory]] and process them at a more complex level.<ref name="Kandel">Kandel E. R., Schwartz J. H. and Jessell T. M., 2000. ''Principles of Neural Science'', 4th ed., McGraw–Hill, New York. pp.&nbsp;577–580.</ref>

Most humans have three different [[cone cell]]s in their retinas that facilitate [[trichromacy|trichromatic color vision]]. Colors are determined by the proportional excitation of these three cone types, i.e. their ''quantum catch''. The levels of excitation of each cone type are the parameters that define [[LMS color space]]. To calculate the opponent process [[CIE 1931 color space#Tristimulus values|tristimulus values]] from the LMS color space, the cone excitations must be compared:{{citation needed|date=September 2022}}
* The luminous opponent channel is equal to the sum of all three cone cells (plus the [[rod cell]]s in some conditions).
* The red–green opponent channel is equal to the difference of the L- and M-cones.
* The blue–yellow opponent channel is equal to the difference of the S-cone and the sum of the L- and M-cones.

===Neurological basis===
{{see also|Lateral geniculate nucleus#Color processing}}
[[File:opponent_process_contrast_sensitivity_functions.svg|thumb|Spatial contrast sensitivity functions for luminance and chromatic contrast.]]
The neurological conversion of color from [[LMS color space]] to the opponent process is believed to take place mostly in the [[lateral geniculate nucleus]] (LGN) of the [[thalamus]], though it may also take place in the [[retina bipolar cell]]s. <!-- Need more info about bipolar cells --> [[Retinal ganglion cell]]s carry the information from the retina to the LGN, which contains three major classes of layers:<ref name=Gho17>[https://doi.org/10.1016/j.pneurobio.2017.06.002 M. Ghodrati, S.-M. Khaligh-Razavi, S. R. Lehky, Towards building a more complex view of the lateral geniculate nucleus: recent advances in understanding its role, Prog. Neurobiol. 156:214–255, 2017.]</ref>
* [[Magnocellular layer]]s (large-cell){{dash}}responsible largely for the luminance channel
* [[Parvocellular layer]]s (small-cell){{dash}}responsible largely for red–green opponency
* [[Koniocellular layer]]s{{dash}}responsible largely for blue–yellow opponency

===Advantage===
Transmitting information in opponent-channel color space could be advantageous over transmitting it in [[LMS color space]] ("raw" signals from each cone type). There is some overlap in the [[wavelength]]s of [[light]] to which the three types of cones (''L'' for ''long-wave'', ''M'' for ''medium-wave'', and ''S'' for ''short-wave'' light) respond, so it is more efficient for the visual system (from a perspective of [[Dynamic range#Human perception|dynamic range]]) to record ''differences'' between the responses of cones, rather than each type of cone's individual response.{{citation needed|date=September 2022}}{{dubious|date=September 2022}}

===Color blindness===
{{main|Color blindness}}
[[Color blindness]] can be classified by the [[cone cell]] that is affected (protan, deutan, tritan) or by the opponent channel that is affected ([[Color blindness#Red–green color blindness|red–green]] or [[Color blindness#Blue–yellow color blindness|blue–yellow]]). In either case, the channel can either be inactive (in the case of [[Color blindness#Dichromacy|dichromacy]]) or have a lower dynamic range (in the case of [[Color blindness#Anomalous trichromacy|anomalous trichromacy]]). For example, individuals with [[deuteranopia]] see little difference between the red and green [[unique hues]].