Optical illusion

In visual perception, an optical illusion (also called a visual 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 immersed in water; an example for a physiological paradox is the motion aftereffect (where, despite movement, position remains unchanged). An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known. Pathological visual illusions arise from pathological changes in the physiological visual perception mechanisms causing the aforementioned types of illusions; they are discussed e.g. under visual hallucinations. Optical illusions, as well as multi-sensory illusions involving visual perception, can also be used in the monitoring and rehabilitation of some psychological disorders, including phantom limb syndrome and schizophrenia.

Physical visual illusions A familiar phenomenon and example for a physical visual illusion is when mountains appear to be much nearer in clear weather with low humidity (Foehn) than they are. This is because haze is a cue for depth perception, signalling the distance of far-away objects (Aerial perspective). The classical example of a physical illusion is when a stick that is half immersed in water appears bent. This phenomenon was discussed by Ptolemy (c. 150) and was often a prototypical example for an illusion.

Physiological visual illusions Physiological illusions, such as the afterimages following bright lights, or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation or interaction with contextual or competing stimuli of a specific type—brightness, color, position, tile, size, movement, etc. The theory is that a stimulus follows its individual dedicated neural path in the early stages of visual processing and that intense or repetitive activity in that or interaction with active adjoining channels causes a physiological imbalance that alters perception. The Hermann grid illusion and Mach bands are two illusions that are often explained using a biological approach. Lateral inhibition, where in receptive fields of the retina receptor signals from light and dark areas compete with one another, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands. Once a receptor is active, it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion, the gray spots that appear at the intersections at peripheral locations are often explained to occur because of lateral inhibition by the surround in larger receptive fields. However, lateral inhibition as an explanation of the Hermann grid illusion has been disproved. More recent empirical approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled.

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 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, like with the Kanizsa triangle, using illusory contours. Specific examples of typical cognitive illusions include:

The Ponzo illusion, where two parallel lines of the same length appear to be different sizes due to their placement within converging lines that create a false sense of depth. The Poggendorff illusion, where two straight lines partially obsecured by an intervening shape, typically a rectangle, appears misaligned when, in fact, they are collinear. The Müller-Lyer illusion, where two lines of the same length appear to be different lengths due to the outward or inward pointing arrow-like fins attached to their ends.

Explanation of cognitive illusions

Perceptual organization

To make sense of the world it is necessary to organize incoming sensations into information which is meaningful. Gestalt psychologists believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole. Gestalt organization can be used to explain many illusions including the rabbit–duck illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in the figure–ground illusion the figure and ground are reversible.

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