Editing Motion perception (section)

==Motion in depth==
{{Main|Monocular depth cues|Structure from motion|Stereoscopic motion}}
As in other aspects of vision, the observer's visual input is generally insufficient to determine the true nature of stimulus sources, in this case their velocity in the real world. In [[monocular vision]] for example, the visual input will be a 2D projection of a 3D scene. The motion cues present in the 2D projection will by default be insufficient to reconstruct the motion present in the 3D scene. Put differently, many 3D scenes will be compatible with a single 2D projection. The problem of motion estimation generalizes to [[binocular vision]] when we consider occlusion or motion perception at relatively large distances, where [[binocular disparity]] is a poor cue to depth. This fundamental difficulty is referred to as the [[inverse problem]].<ref>{{cite journal | vauthors = Lages M, Heron S | title = On the inverse problem of binocular 3D motion perception | journal = PLOS Computational Biology | volume = 6 | issue = 11 | pages = e1000999 | date = November 2010 | pmid = 21124957 | pmc = 2987932 | doi = 10.1371/journal.pcbi.1000999 | bibcode = 2010PLSCB...6E0999L | doi-access = free }}</ref>

Nonetheless, some humans do perceive motion in depth. There are indications that the brain uses various cues, in particular temporal changes in disparity as well as monocular velocity ratios, for producing a sensation of motion in depth.<ref name="BlakeWilson2011">{{cite journal | vauthors = Blake R, Wilson H | title = Binocular vision | journal = Vision Research | volume = 51 | issue = 7 | pages = 754–70 | date = April 2011 | pmid = 20951722 | pmc = 3050089 | doi = 10.1016/j.visres.2010.10.009 }}</ref> Two different binocular cues of the perception motion in depth are hypothesized: Inter-ocular velocity difference (IOVD) and changing disparity (CD) over time. Motion in depth based on inter-ocular velocity differences can be tested using dedicated binocularly uncorrelated random-dot kinematograms.<ref>{{cite journal |vauthors=Shioiri S, Saisho H, Yaguchi H |title=Motion in depth based on inter-ocular velocity differences |journal=Vision Research |volume=40 |issue=19 |pages=2565–72 |date=2000 |pmid=10958908 |doi=10.1016/s0042-6989(00)00130-9|s2cid=15342293 }}</ref> Study results indicate that the processing of these two binocular cues – IOVD and CD – may use fundamentally different low-level stimulus features, which may be processed jointly that later stages.<ref name="pmid33362456">{{cite journal |vauthors=Himmelberg MM, Segala FG, Maloney RT, Harris JM, Wade AR |title=Decoding Neural Responses to Motion-in-Depth Using EEG |journal=Frontiers in Neuroscience |volume=14 |issue= |pages=581706 |date=2020 |pmid=33362456 |pmc=7758252 |doi=10.3389/fnins.2020.581706 |doi-access=free }}</ref><ref>{{cite journal |vauthors=Brooks KR |title=Interocular velocity difference contributes to stereomotion speed perception |journal=Journal of Vision |volume=2 |issue=3 |pages=218–31 |date=2002 |pmid=12678584 |doi=10.1167/2.3.2 |url=|doi-access=free }}</ref> Additionally, as monocular cue, also the changing size of retinal images contributes to motion in depth detection.