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Cognitive illusions
Cognitive illusions are assumed to arise
by interaction with assumptions about the world, leading to "unconscious
inferences", an idea first suggested in the 19th century by Hermann Helmholtz.
Cognitive illusions are commonly divided into ambiguous illusions, distorting
illusions, paradox illusions, or fiction illusions.
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Ambiguous illusions are pictures or
objects that elicit a perceptual 'switch' between the alternative
interpretations. The Necker cube is a well known example; another instance is
the Rubin vase.
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Distorting illusions are
characterized by distortions of size, length, or curvature. A striking example
is the Café wall illusion. Another example is the famous Müller-Lyer illusion.
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Paradox illusions are generated by
objects that are paradoxical or impossible, such as the Penrose triangle or
impossible staircases seen, for example, in M. C. Escher's Ascending and
Descending and Waterfall. The triangle is an illusion dependent on a
cognitive misunderstanding that adjacent edges must join.
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Fictional illusions are defined as
the perception of objects that are genuinely not there to all but a single
observer, such as those induced by schizophrenia or a hallucinogen. These are
more properly called hallucinations.
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