Motion Aftereffect (Waterfall Illusion)

from Michael’s Visual Phenomena & Optical Illusions


What to do & observe

Fixate on the central cross during the motion and watch the cycle at least three times. Observe the motion aftereffect in the resting figure (the Buddha of Kamakura). [There is a more flashy version on the next page.] The “warping” caused by the motion aftereffect applies to anything you look at.

You may also try to cover one eye, adapt over ≈3 cycles and then test with the other eye (for this, you will need to stop the movie at the right point…). Well, how strong is your “interocular transfer”?

This is often explained in terms of “fatigue” of the class of neurons encoding one motion direction. It is, however, more accurate to interpret this in terms of adaptation or “gain control”. These motion detectors are, for humans, not in the retina but in the brain (Bach & Hoffmann 2000).

We use the motion aftereffect in combination with EEG recordings as a tool to analyse the human motion system (→motion projects). For a more detailed explanation and a neat demo of the “waterfall effect” see George Mather’s MAE page.


Aristotle “De Somnis” (in Parva Naturalia, translated by Beare JI, 1931) chapter 2 (search for ‘rivers’). [Interestingly, Aristotle did not describe the reverse motion, rather “…things really at rest are then seen moving”.]

Lucretius TC (≈-50) De rerum natura, book 4
“Then, when a head-strong horse balks mid-river, and we look down into the rapid course of the current, though our mount does not budge, still it seems a force is sweeping his body sideways, swiftly shoving it upstream, and wherever we cast our glance, every other thing will seem to be borne along and rushing as ourselves, in the same way.” source
In other words: Lucretius looked at the stationary leg of his horse when in the middle of a fast flowing river and noted that it seemed to be moving in the opposite direction to the flow. This we now call “induced motion”.

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
“Having steadfastly looked for a few seconds at a particular part of the cascade, admiring the confluence and decussation of the currents forming the liquid drapery of waters, and then suddenly directed my eyes to the left, to observe the vertical face of the sombre age-worn rocks immediately contiguous to the water-fall, I saw the rocky face as if in motion upwards, and with an apparent velocity equal to that of the descending water, which the moment before had prepared my eyes to behold this singular deception. (p. 373)”

Mather, Verstraten & Anstis (1998) The motion aftereffect: a modern perspective. Cambridge, Mass: MIT Press

George Mather’s MAE page (part of his nice motion site)

Kohn, A, and Movshon, JA (2003) Neuronal adaptation to visual motion in area MT of the macaque. Neuron 39: 681-691 [PDF]

Wade NJ (2018) Pursuing Paradoxes Posed by the Waterfall Illusion. Perception

Bach M, Hoffmann MB (2000) Visual motion detection in man is governed by non-retinal mechanisms. Vision Res 40(18):2379–2385