The motion after-effect is a visual illusion experienced after viewing a moving visual stimulus for a time (seconds to minutes) with stationary eyes, and then fixating a stationary stimulus. The stationary stimulus appears to move in the opposite direction to the original (physically moving) stimulus.
The motion aftereffect is believed to be the result of motion adaptation.
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.
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