Editing Visual system (section)

===== Mechanism of generating visual signals =====
The retina adapts to change in light through the use of the rods. In the dark, the [[chromophore]] [[retinal]] has a bent shape called cis-retinal (referring to a ''cis'' conformation in one of the double bonds). When light interacts with the retinal, it changes conformation 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. At baseline in the dark, the rhodopsin absorbs no light and releases [[glutamate]], which inhibits the bipolar cell. This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. When there is light present, glutamate secretion ceases, thus no longer inhibiting 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 Education|McGraw-Hill]], 2010.</ref><ref>{{Cite web |url=http://webvision.med.utah.edu/GCPHYS1.HTM |title=Webvision: Ganglion cell Physiology |access-date=2018-12-08 |archive-url=https://web.archive.org/web/20110123202041/http://webvision.med.utah.edu/GCPHYS1.HTM |archive-date=2011-01-23 }}</ref>

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:<ref name=Tov2008 />
#M cells, with large center-surround receptive fields that are sensitive to [[Depth perception|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;
#[[Photosensitive ganglion cell|another population that is intrinsically photosensitive]]; and
#a final population that is used for eye movements.<ref name=Tov2008 />

A 2006 [[University of Pennsylvania]] study calculated the approximate [[Bandwidth (computing)|bandwidth]] of human retinas to be about 8,960 [[Kilobit|kilobits]] per second, whereas [[guinea pig]] retinas transfer at about 875 kilobits.<ref>{{cite web|url=https://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight|title=Calculating the speed of sight}}</ref>

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="Zaidi, 2007">{{cite journal |display-authors=etal |vauthors=Zaidi FH, Hull JT, Peirson SN |date=December 2007 |title=Short-wavelength light sensitivity of circadian, pupillary, and visual awareness in humans lacking an outer retina |journal=[[Curr. Biol.]] |volume=17 |issue=24 |pages=2122–8 |doi=10.1016/j.cub.2007.11.034 |pmc=2151130 |pmid=18082405|bibcode=2007CBio...17.2122Z }}</ref> The peak [[spectral sensitivity]] was 481&nbsp;nm. This shows that there are two pathways for vision in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photo-receptive ganglion cells which act as rudimentary visual brightness detectors.