Editing Superior colliculus (section)

===Eye movements===
{{Main|Eye movement}}
In primates, [[eye movement (sensory)|eye movements]] can be divided into several types: [[Fixation (visual)|fixation]], in which the eyes are directed toward a motionless object, with eye movements only to compensate for movements of the head; [[smooth pursuit]], in which the eyes move steadily to track a moving object; [[saccade]]s, in which the eyes move very rapidly from one location to another; and [[vergence]], in which the eyes move simultaneously in opposite directions to obtain or maintain single binocular vision. The superior colliculus is involved in all of these, but its role in saccades has been studied most intensively.<ref>{{cite journal |last1=Basso |first1=MA |last2=May |first2=PJ |title=Circuits for Action and Cognition: A View from the Superior Colliculus |journal=Annual Review of Vision Science |date=2017 |volume=3 |pages=197–226 |doi=10.1146/annurev-vision-102016-061234 |pmid=28617660|pmc=5752317 }}</ref><ref>{{cite journal |last1=Furlan |first1=M |last2=Smith |first2=AT |last3=Walker |first3=R |title=Activity in the human superior colliculus relating to endogenous saccade preparation and execution |journal=Journal of Neurophysiology |date=2015 |volume=114 |issue=2 |pages=1048–1058 |doi=10.1152/jn.00825.2014 |pmid=26041830|pmc=4725108 }}</ref><ref>{{cite journal |last1=Moschovakis |first1=A K |title=The superior colliculus and eye movement control |journal=Current Opinion in Neurobiology |date=1996 |volume=6 |issue=6 |pages=811–816 |doi=10.1016/s0959-4388(96)80032-8 |pmid=9000018}}</ref>

Each of the two colliculi — one on each side of the brain — contains a two-dimensional map representing half of the visual field. The [[Fovea centralis|fovea]] — the region of maximum sensitivity — is represented at the front edge of the map, and the periphery at the back edge. Eye movements are evoked by activity in the deep layers of the SC. During fixation, neurons near the front edge — the foveal zone — are tonically active. During smooth pursuit, neurons a small distance from the front edge are activated, leading to small eye movements. For saccades, neurons are activated in a region that represents the point to which the saccade will be directed. Just prior to a saccade, activity rapidly builds up at the target location and decreases in other parts of the SC. The coding is rather broad, so that for any given saccade the activity profile forms a "hill" that encompasses a substantial fraction of the collicular map: The location of the peak of this "hill" represents the saccade target.<ref>{{cite journal |last1=Soetedjo |first1=R |last2=Kaneko |first2=CRS |last3=Fuchs |first3=AF |title=Evidence against a moving hill in the superior colliculus during saccadic eye movements in the monkey |journal=Journal of Neurophysiology |date=2002 |volume=87 |issue=6 |pages=2778–2789 |doi=10.1152/jn.2002.87.6.2778 |pmid=12037180}}</ref>

The SC encodes the target of a gaze shift, but it does not seem to specify the precise movements needed to get there.<ref>[[#refSparks2003|Sparks & Gandhi, 2003]]</ref> The decomposition of a gaze shift into head and eye movements and the precise trajectory of the eye during a saccade depend on integration of collicular and non-collicular signals by downstream motor areas, in ways that are not yet well understood. Regardless of how the movement is evoked or performed, the SC encodes it in "retinotopic" coordinates: that is, the location of the SC 'hill" corresponds to a fixed location on the retina. This seems to contradict the observation that stimulation of a single point on the SC can result in different gaze shift directions, depending on initial eye orientation. However, it has been shown that this is because the retinal location of a stimulus is a non-linear function of target location, eye orientation, and the spherical geometry of the eye.<ref>[[#refKlier2001|Klier et al., 2001]]</ref>

There has been some controversy about whether the SC merely commands eye movements, and leaves the execution to other structures, or whether it actively participates in the performance of a saccade. In 1991, Munoz et al., on the basis of data they collected, argued that, during a saccade, the "hill" of activity in the SC moves gradually, to reflect the changing offset of the eye from the target location while the saccade is progressing.<ref>[[#refMunoz|Munoz et al., 1991]]</ref> At present, the predominant view is that, although the "hill" does shift slightly during a saccade, it does not shift in the steady and proportionate way that the "moving hill" hypothesis predicts.<ref>[[#refSoetedjo|Soetedjo et al., 2002]]</ref> However, moving hills may play another role in the superior colliculus; more recent experiments have demonstrated a continuously moving hill of visual memory activity when the eyes move slowly while a separate saccade target is retained.<ref>[[#refDash|Dash et al., 2015]]</ref>

The output from the motor sector of the SC goes to a set of midbrain and brainstem nuclei, which transform the "place" code used by the SC into the "rate" code used by oculomotor neurons. Eye movements are generated by six muscles, arranged in three orthogonally-aligned pairs. Thus, at the level of the final common path, eye movements are encoded in essentially a Cartesian coordinate system.

Although the SC receives a strong input directly from the retina, in primates it is largely under the control of the cerebral cortex, which contains several areas that are involved in determining eye movements.<ref>[[#refPierrot|Pierrot-Deseilligny et al., 2003]]</ref> The [[frontal eye fields]], a portion of the motor cortex, are involved in triggering intentional saccades, and an adjoining area, the supplementary eye fields, are involved in organizing groups of saccades into sequences. The parietal eye fields, farther back in the brain, are involved mainly in reflexive saccades, made in response to changes in the view.  Recent evidence<ref>{{Cite journal|last1=Yan|first1=Yin|last2=Zhaoping|first2=Li|last3=Li|first3=Wu|date=2018-10-09|title=Bottom-up saliency and top-down learning in the primary visual cortex of monkeys|journal=Proceedings of the National Academy of Sciences|volume=115|issue=41|pages=10499–10504|doi=10.1073/pnas.1803854115|pmid=30254154|pmc=6187116|bibcode=2018PNAS..11510499Y |doi-access=free}}</ref><ref>{{Cite journal|date=2012-01-12|title=Neural Activities in V1 Create a Bottom-Up Saliency Map|journal=Neuron|language=en|volume=73|issue=1|pages=183–192|doi=10.1016/j.neuron.2011.10.035|issn=0896-6273|doi-access=free|last1=Zhang|first1=Xilin|last2=Zhaoping|first2=Li|last3=Zhou|first3=Tiangang|last4=Fang|first4=Fang|pmid=22243756}}</ref> suggests that the [[primary visual cortex]] (V1) guides reflexive eye movements, according to [[V1 Saliency Hypothesis]],  using a bottom-up saliency map of the visual field generated in V1 from external visual inputs.<ref>{{Cite journal|date=2002-01-01|title=A saliency map in primary visual cortex|url=https://www.sciencedirect.com/science/article/abs/pii/S1364661300018179|journal=Trends in Cognitive Sciences|language=en|volume=6|issue=1|pages=9–16|doi=10.1016/S1364-6613(00)01817-9|issn=1364-6613|last1=Li|first1=Zhaoping|pmid=11849610|s2cid=13411369|url-access=subscription}}</ref>

The SC only receives [[visual perception|visual]] inputs in its superficial layers, whereas the deeper layers of the colliculus receive also auditory and somatosensory inputs and are connected to many sensorimotor areas of the brain. The colliculus as a whole is thought to help orient the head and eyes toward something seen and heard.<ref name=Kustov>[[#refKustov|Kustov & Robinson, 1996]]</ref><ref>[[#refKlier|Klier et al., 2003]]</ref><ref>[[#refKrauzlis|Krauzlis et al., 2004]]</ref><ref>[[#refSparks|Sparks, 1999]]</ref>

The superior colliculus also receives auditory information from the inferior colliculus. This auditory information is integrated with the visual information already present to produce the [[ventriloquism effect]].