Motion aftereffect

The motion aftereffect (MAE) -is a visual illusion perceived after watching a moving visual stimulus for about a minute and then looking at stationary stimulus. The stationary stimulus appears to move slightly for about 15 seconds, opposite to the direction of 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.

Another example can be seen when one is spun in one direction, as in an office chair that can spin on a central post. After spinning for about 30 seconds in one direction and stopping, the surrounding room seems to spin in the opposite direction to how the person was spinning.

The brain's ability to process actual sensory input as opposed to perceived reality can be overpowered by these illusions, and if additional environmental factors are removed and the illusions are strengthened, the body loses normal control. In the waterfall illusion, the person may fall backwards, on the spinning chair, they may find themselves unwittingly start to turn in the opposite direction.

Example movie which produces distortion illusion after you watch it and look away.

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 perception of stationary objects, for example 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, different 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

• Afterimage
• Motion perception

References

• Addams, R. (1834). An account of a peculiar optical phænomenon seen after having looked at a moving body. London and Edinburgh Philosophical Magazine and Journal of Science, 5, 373–374
• Aristotle (approx. 350 B.C.) Parva Naturalia.
• Barlow, H. B., & Hill, R. M. (1963). Evidence for a physiological explanation of the waterfall illusion. Nature, 200, 1345-1347.
• Purkinje JE (1820) Beiträge zur näheren Kenntniss des Schwindels aus heautognostischen Daten. Medicinische Jahrbücher des kaiserlich-königlichen österreichischen Staates, 6, 79–125.
• Srinivasan, M. V., & Dvorak, D. R. (1979). The waterfall illusion in an insect visual system. Vision Research, 19, 1435-1437.
• Thompson, P. (1880). Optical illusions of motion. Brain, 3, 289-298.
• Verstraten, F. A. J. (1996). On the ancient history of the direction of the motion aftereffect. Perception, 25, 1177-1188.

Bibliography

• Mather, G., Verstraten, F., & Anstis, S. (1998). The motion aftereffect: A modern perspective. Cambridge, Mass: MIT Press