From: Andy Soos, ENN
Published June 30, 2011 02:10 PM

Motion Affer Effect

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. Why does it happen, though? Is it because we are consciously aware that the background is moving in one direction, causing our brains to shift their frame of reference so that we can ignore this motion? Or is it an automatic, subconscious response? Davis Glasser, a doctoral student in the University of Rochester's Department of Brain and Cognitive Sciences thinks he has found the answer. The results of a study done by Glasser, along with his advisor, Professor Duje Tadin, and colleagues James Tsui and Christopher Pack of the Montreal Neurological Institute, is published in the journal Proceedings of the National Academy of Sciences. In their paper, the scientists show that humans experience the Motion Aftereffect even if the motion that they see in the background is so brief that they can't even tell whether it is heading to the right or the left.


Aristotle (@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 PurkynÄ› (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.

Neural adaptation or sensory adaptation is a change over time in the responsiveness of the sensory system to a constant stimulus. It is usually experienced as a change in the stimulus. For example, if one rests one's hand on a table, one immediately feels the table's surface on one's skin. Within a few seconds, however, one ceases to feel the table's surface. The sensory neurons stimulated by the table's surface respond immediately, but then respond less and less until they may not respond at all; this is neural adaptation.Adaptation is considered to be the cause of perceptual phenomena like afterimages and the motion aftereffect. In the absence of fixational eye movements, visual perception may fade out or disappear due to neural adaptation. 

When an observers’ visual stream adapts to a single direction of real motion, imagined motion can be perceived at various different speeds. If the imagined motion is in the same direction as that experienced during adaptation, imagined speed is slowed; when imagined motion is in the opposite direction, its speed is increased.

The new study shows even when shown a video of a pattern that is moving for only 1/40 of a second (25 milliseconds) — so short that the direction it is moving cannot be consciously distinguished — a subject's brain automatically adjusts. If the subject is then shown a stationary object, it will appear to him as though it is moving in the opposite direction of the background motion.

In recordings from a motion center in the brain called cortical area MT, the researchers found neurons that, following a brief exposure to motion, respond to stationary objects as if they are actually moving. It is these neurons that the researchers think are responsible for the illusory motion of stationary objects that people see during the motion aftereffect.This discovery reveals that the motion aftereffect illusion is not just a compelling visual oddity: It is caused by neural processes that happen essentially every time we see moving objects. The next phase of the group's study will attempt to find out whether this rapid motion adaptation serves a beneficial purpose — in other words, does this rapid adaptation actually improve your ability to estimate the speed and direction of relevant moving objects, such as a baseball flying toward you.

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