page_title: Optical illusion
section_title: Cognitive illusions
hierarchical_section_title: Optical illusion / Cognitive illusions
caption_attribution_description: English: "The Organ Player" - Pareidolia phenomenon in Neptune's Grotto (Alghero, Sardinia)
context_page_description: An optical illusion
  is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type.
context_section_description: 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 the German physicist and physician Hermann Helmholtz. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.
  Ambiguous illusions are pictures or objects that elicit a perceptual "switch" between the alternative interpretations. The Necker cube is a well-known example; other instances are the Rubin vase and the "squircle", based on Kokichi Sugihara's ambiguous cylinder illusion.
  Distorting or geometrical-optical illusions are characterized by distortions of size, length, position or curvature. A striking example is the Café wall illusion. Other examples are the famous Müller-Lyer illusion and Ponzo illusion.
  Paradox illusions (or impossible object illusions) are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircase 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.
  Fictions are when a figure is perceived even though it is not in the stimulus.
caption: "The Organ Player" – Pareidolia phenomenon in Neptune's Grotto stalactite cave (Alghero, Sardinia)
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