| Motion Aftereffect |
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For example, if one looks at a waterfall for about a minute and then looks at the stationary rocks at the side of the waterfall, these rocks appear to be moving upwards slightly. The illusory upwards movement is the motion aftereffect. This particular motion aftereffect is also known as the Waterfall Illusion . EXPLANATION Neurons coding a particular movement reduce their responses with time of exposure to a constantly moving stimulus; this is Neural Adaptation . Neural adaptation also reduces the responses of these same neurons when responding to a stationary stimulus (see, for example, Barlow & Hill, 1963; Srinivasan & Dvorak, 1979). One theory is that stationariness, for example of rocks beside a waterfall, is coded as the balance between the responses of neurons stimulated by upwards movement and the reponses of neurons stimulated by downwards movement. Neural adaptation of neurons stimulated by downwards movement reduces their responses, tilting the balance in favour of upwards movement. Easy Explanation The motion direction that we perceive is a result of a decision by a majority of our motion sensitive neurons. If some motion detectors continuously respond to motion for a while, they tire or adapt to that motion and become less active. When the motion is stopped, differnt motion detectors, especially those for the opposite direction, become active, and give rise to the illusory motion in that direction. HISTORY Aristotle (approx. 350 B.C.) reported illusory movement after viewing constant movement, but did not specify its direction. The first clear specification of the motion aftereffect was by Jan Evangelista Purkyně (1820) who observed it after looking at a cavalry parade. Robert Addams (1834) reported the waterfall illusion after observing it at the Falls of Foyers in Scotland. According to Verstraten (1996) the term waterfall illusion was coined by Thompson (1888). SEE ALSO REFERENCES
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