Editing Motion perception (section)

==First-order motion perception==
[[Image:Phi phenomenom no watermark.gif|right|thumb|225px|Example of [[Beta movement]], often confused with [[phi phenomenon]], in which a succession of still images gives the illusion of a moving ball<ref name="PhiIsNotBeta">Steinman, Pizlo & Pizlo (2000) [http://www.psych.purdue.edu/Magniphi/PhiIsNotBeta/phi1.html Phi is not Beta] slideshow based on ARVO presentation.</ref>]]

When two or more stimuli are alternatively switched on and off, they can produce two distinct motion perceptions. The first, known as [[beta movement]], is demonstrated in the yellow-ball figure and forms the basis for electronic [[news ticker]] displays. However, at faster alternation rates, and when the distance between the stimuli is optimal, an illusory "object"—matching the background color—appears to move between the stimuli, alternately occluding them. This phenomenon is called the [[phi phenomenon]] and is often described as an example of "pure" motion detection, uncontaminated by form cues, unlike beta movement.<ref name="PhiIsNotBeta"/> Nevertheless, this description is somewhat paradoxical since creating such motion without figural percepts is impossible.

The phi phenomenon has been referred to as "first-order" motion perception. Werner E. Reichardt and Bernard Hassenstein have modelled it in terms of relatively simple "motion sensors" in the visual system, that have evolved to detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighbouring point on the retina after a short delay. Sensors that are proposed to work this way have been referred to as either ''Hassenstein-Reichardt detectors'' after the scientists [[Bernhard Hassenstein]] and [[Werner E. Reichardt|Werner Reichardt]], who first modelled them,<ref name="Reichardt1961">{{cite book | vauthors = Reichardt W |year= 1961 |chapter=Autocorrelation, a principle for the evaluation of sensory information by the central nervous system |editor=W.A. Rosenblith |title=Sensory Communication |pages=303–317 |publisher=MIT Press}}</ref> motion-energy sensors,<ref name="AdelsonBergen">{{cite journal | vauthors = Adelson EH, Bergen JR | title = Spatiotemporal energy models for the perception of motion | journal = Journal of the Optical Society of America A | volume = 2 | issue = 2 | pages = 284–99 | date = February 1985 | pmid = 3973762 | doi = 10.1364/JOSAA.2.000284 | citeseerx = 10.1.1.148.4141 | bibcode = 1985JOSAA...2..284A }}</ref> or Elaborated Reichardt Detectors.<ref name="VanSantenSperling">{{cite journal | vauthors = van Santen JP, Sperling G | title = Elaborated Reichardt detectors | journal = Journal of the Optical Society of America A | volume = 2 | issue = 2 | pages = 300–21 | date = February 1985 | pmid = 3973763 | doi = 10.1364/JOSAA.2.000300 | bibcode = 1985JOSAA...2..300S | s2cid = 5699316 }}</ref> These sensors are described as detecting motion by spatio-temporal [[correlation]] and are considered by some to be plausible models for how the visual system may detect motion. (Although, again, the notion of a "pure motion" detector suffers from the problem that there is no "pure motion" stimulus, i.e. a stimulus lacking perceived figure/ground properties). There is still considerable debate regarding the accuracy of the model and exact nature of this proposed process. It is not clear how the model distinguishes between movements of the eyes and movements of objects in the visual field, both of which produce changes in luminance on points on the retina.