Illusions (perceptual)

An optical illusion is characterized by visually perceived images that are deceptive or misleading. Information gathered by the eye is processed by the brain to give a perception that does not tally with a physical measurement of the stimulus source. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that apparently show particular assumptions in the human perceptual system.

Physiological illusions
Physiological illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and so on. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing; and that repetitive stimulation of only a few channels misleads the visual system.

Cognitive illusions
Cognitive illusions are assumed to arise by interaction with in-built assumptions or 'knowledge' of the world, leading to "unconscious inferences", an idea first suggested in the 19th century. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.

Ambiguous illusions are pictures or objects that elicit significant changes in appearance. Perception will 'switch' between the alternates as they are considered in turn as available data does not confirm a single view. The Necker cube is a well known example. Another instance is the Rubin vase.

Distorting illusions offer distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Mueller-Lyer Illusion.

Paradox illusions offer objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in the work of M. C. Escher. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join.

Fiction illusions are the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or hallucinogenic drugs.

Well-known illusions include:
 * Ames room illusion
 * Ames trapezoidal window illusion
 * Benham's top
 * Bezold Effect
 * Cafe wall illusion
 * Chubb illusion
 * Craik-O'Brien-Cornsweet illusion
 * Ebbinghaus illusion
 * Ehrenstein illusion
 * Fraser spiral illusion
 * Grid illusion
 * Hering illusion
 * Hermann grid illusion
 * Hollow-Face illusion
 * Illusory figure
 * Impossible trident illusion
 * Isometric illusion
 * Jastrow illusion
 * Kanizsa triangle
 * Mach bands
 * Missing square puzzle
 * Moon illusion
 * Motion aftereffect
 * Muller-Lyer distortion illusion
 * Necker cube illusion
 * Orbison illusion
 * Penrose triangle aka Impossible triangle illusion
 * Peripheral drift illusion
 * Phi phenomenon
 * Poggendorff illusion
 * Ponzo illusion
 * Pulfrich effect or Pulfrich pendulum illusion
 * Rubin vase
 * Same color
 * Sander illusion
 * Size-weight illusion
 * Stroop effect
 * White's illusion
 * Wundt illusion
 * Zollner illusion

Many famous artists have worked extensively with optical illusions, including M.C. Escher, Salvador Dalí, Giuseppe Arcimboldo, Marcel Duchamp, Oscar Reutersvärd, and Charles Allan Gilbert.Optical illusion is also used in film by the technique of forced perspective.

A Possible Explanation of Many Visual Illusions
The explanation of illusions is widely debated. Recent evidence implies that visual illusions are simply the signature of the empirical statistical way all visual percepts are generated. In this interpretation, these phenomena are simply a consequence of the way vision has evolved to solve the inverse problem (see references and links below).