The motion aftereffect (MAE) is a visual illusion experienced after viewing a moving visual stimulus for a time (tens of milliseconds 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.
Another example can be seen when one looks at the center of a rotating spiral for several seconds. The spiral can exhibit outward or inward motion. When one then looks at any stationary pattern, it appears to be moving in the opposite direction. This form of the motion aftereffect is known as the spiral aftereffect.
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 spontaneous, baseline activity of these same neurons when responding to a stationary stimulus (see, for example, Barlow & Hill, 1963; Srinivasan & Dvorak, 1979; Glasser, Tsui, Pack, & Tadin, 2011). One theory is that perception of stationary objects, for example rocks beside a waterfall, is coded as the balance among the baseline responses of neurons coding all possible directions of motion. Neural adaptation of neurons stimulated by downward movement reduces their baseline activity, tilting the balance in favor of upward movement. The MAE is often used in psychophysical experiments to investigate cortical specialization. This includes Levinson and Sekuler's (1976) Direction aftereffect experiment.
An alternate explanation for MAE is based on increase in excitability of neurons having a preference for a direction that is opposite to the adapting direction (Bajaj 2013). Adapting direction selective neurons hyperpolarize due to long duration intracellular sodium and calcium ion accumulation. This causes extracellular imbalances and an increase in brain tissue excitability; which spreads via ionic diffusion in extracellular space and glial assisted mechanisms. This causes the opposite direction neurons to spike when a stationary stimulus is presented, as these neurons have no hyperpolarizing intracellular imbalances but get surrounded by depolarizing extracellular imbalances. So after continuously looking at motion in a particular direction when one looks at a stationary stimulus, it appears to move in the opposite direction, as neurons selective to opposite direction have got depolarized and exhibit spiking activity. This explanation is supported by experimental data on increase in neural excitability with motion adaptation.
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 (1880).
See also 
- Addams, R. (1834). An account of a peculiar optical phenomenon 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.
- Bajaj K. (2013). Visual Motion Aftereffect in an Excitable Brain Tissue - Explaining the waterfall illusion, ASIN: B00C2C1VYI.
- Barlow, H.B., & Hill, R.M. (1963). Evidence for a physiological explanation of the waterfall illusion. Nature, 200, 1345-1347.
- Glasser, D.M., Tsui, J.M., Pack, C.C., & Tadin, D. (2011). Perceptual and neural consequences of rapid motion adaptation. PNAS Plus, 108(45), E1080–E1088. doi:10.1073/pnas.1101141108
- Purkinje, J.E. (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.
- Mather, G., Verstraten, F., & Anstis, S. (1998). The motion aftereffect: A modern perspective. Cambridge, Mass: MIT Press