waterfall illusion
   Also known as waterfall effect and waterfall phenomenon. All three terms refer to a variant of the * motion aftereffect characterized by an optical *illusion of upward motion in stationary objects. The term waterfall illusion was coined in or shortly before 1880 by the British physicist Sil-vanus Phillips Thompson (1851-1916). Theintro-duction of the term waterfall effect has been attributed to the British psychologist Richard Langton Gregory (b. 1923) and the introduction of the term waterfall phenomenon to the American optometrists Horace B. Barlow (b. 1921) and Richard M. Hill. The Greek philosopher Aristotle (384-322 BC) has been credited with being the first author to describe the phenomenon, referring to it as * river illusion. The first modern description of the waterfall illusion can be found in a paper published in 1834 by the chemist and natural philosopher Robert Addams. Addams reported the phenomenon after having observed it at the Falls of Foyers, on the borders of Loch Ness, Scotland. The waterfall illusion can be induced by looking for some time at a descending mass of water and then shifting one's gaze to the stationary objects in the environment. These stationary objects then appear to be moving in the opposite direction. The waterfall illusion is commonly classified as a * physiological illusion. Physiologically, the waterfall illusion and other motion aftereffects have been associated with a process called neural adaptation, i.e. a selective response of neurons in the visual association cor-textomovementinthe visual field.Amove-ment aftereffect that has been classified by some as a special variant of the waterfall illusion is known as * Archimedes's spiral. The waterfall illusion should not be confused with the *hygric hallucination.
   References
   Addams, R. (1834). An account of a peculiar optical phaenomenon seen after having looked at a moving body, &c. London and Edinburgh Philosophical Magazine and Journal of Science, 5, 373-374.
   Barlow, H.B., Hill, R.M. (1963). Evidence for a physiological explanation of the waterfall phenomenon and figural aftereffects. Nature, 200, 1434-1435. Gregory, R.L. (1981). Eye andbrain. Third revised edition. New York, NY: McGraw-Hill. Mather, G., Verstraten, F., Anstis, S. (1998). The motion aftereffect: A modern perspective. Cambridge, MA: MIT Press.
   Thompson, P. (1880). Optical illusions ofmotion. Brain, 3, 289-298.

Dictionary of Hallucinations. . 2010.

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