Optical illusion

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An optical illusion is characterized by visually perceived images that are deceptive or misleading [1]. 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 [2].

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1 Physiological illusions
2 Cognitive illusions
3 Depth
4 A Possible Explanation of Many Visual Illusions
5 References
6 See also
7 External links

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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.

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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.

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Depth

Overview

Much evidence suggests that both monocular and binocular depth are also determined empirically, placing on the same probabilistic footing two aspects of vision that, historically have been regarded as having different physiological bases.

An Empirical Explanation: The Perception of Line Length and Other Geometrical Illusions

these observations imply that the sense of egocentric distance is another manifestation of the probabilistic strategy that allows the human visual system to contend with the inherent ambiguity of visual stimuli

The phenomenology of distance perception clearly provides some puzzles that need to be explained. As illustrated in Figure 1,[3] it has long been known that the apparent distance of objects bears a peculiar relationship to their physical distance from the observer. When subjects are asked to make judgments with little or no contextual information (e.g., the distance of a luminous but otherwise featureless object in a darkened room), the distances reported differ in several ways from the corresponding physical distances. First, objects in these circumstances are typically perceived to be at a distance of 2-4m, a phenomenon referred to as the "specific distance tendency" (Figure 1A). Second, objects that are relatively near each other in the retinal image appear to be about the same distance from the observer, a phenomenon called the "equidistance tendency" (Figure 1B). Third, when presented at or near eye-level, the distance of objects relatively near the observer tends to be overestimated, whereas the distance of objects that are further away tends to be underestimated (Figure 1C). Finally, the apparent distance of objects on the ground varies with the angle of declination of the line of sight: objects on the ground that are at least several meters away appear closer than they really are and with increasing distance are judged to be progressively more elevated than warranted by their physical position (Figure 1D).

Although a variety of explanations have been proposed in the various studies cited, there has been little or no agreement about the basis of these unusual perceptions of egocentric distance. More often than not, the several tendencies illustrated in Figure 1[4] have simply been accepted as empirical facts that are then used to rationalize other aspects of visual space.

Given the ability of the probabilistic relationship between retinal images and sources to explain a variety of other geometrical percepts, it makes sense to ask whether the probability distributions of the possible sources of visual stimuli also determine apparent distance. Using the same database of natural scene geometry described in the sections on other geometrical illusions, the anomalous perceptions of distance illustrated in Figure 1[5] can all be accounted for by the probability distributions of the physical distances of objects surfaces from human observers (see Yang and Purves, 2003).

The ability to explain these anomalies in the perception of distance based on the statistics the physical distances of object surfaces from the observer in natural scenes offers further evidence that rationalizing perceived geometry in a probabilistic framework is a powerful way of understanding visual space. In addition to successfully explaining the specific anomalies that have been difficult to rationalize in other ways (see Figure 1), these observations imply that the sense of egocentric distance is another manifestation of the probabilistic strategy that allows the human visual system to contend with the inherent ambiguity of visual stimuli.

Well-known illusions include:

Ames room illusion
Ames trapezoidal window illusion
Benham's top
Bezold Effect
Cafe wall illusion
Chubb illusion
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
Simultaneous brightness contrast
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.

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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).

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