Визуальные иллюзии: природа и классификация

Visually perceived images that differ from objective reality
visual perception

[[File:Gregory categorization of illusions 1991. png|thumb|right|upright=1.5|Gregory's categorization of illusions) 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. and schizophrenia.

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 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.

thumb|Simultaneous contrast illusion. The background is a color gradient and progresses from dark gray to light gray. The horizontal bar appears to progress from light grey to dark grey, but is in fact just one color. Perceptual constancies are sources of illusions. Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed. The luminosity of the object will appear brighter against a black field (that reflects less light) than against a white field, even though the object itself did not change in luminosity. Similarly, the eye will compensate for color contrast depending on the color cast of the surrounding area. In addition to the gestalt principles of perception, water color illusions contribute to the formation of optical illusions. Water color illusions consist of object hole effects and coloration. Object hole effects occur when boundaries are prominent where there is a figure and background with a hole that is 3D volumetric in appearance. Coloration consists of an assimilation of color radiating from a thin colored edge lining a darker chromatic contour. The water color illusion describes how the human mind perceives the wholeness of an object such as top down processing. Thus, contextual factors play into perceiving the brightness of an object.

thumb|"Shepard tables" deconstructed. The two tabletops appear to be different, but they are the same size and shape. Just as it perceives color and brightness constancies, the brain has the ability to understand familiar objects as having a consistent shape or size. For example, a door is perceived as a rectangle regardless of how the image may change on the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The Shepard tables illusion is an example of an illusion based on distortions in shape constancy.