Illusions

An illusion is a distortion of a sensory perception, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people. Illusions can occur with each of the human senses, but visual illusions are the most well known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words. Some illusions are based on general assumptions the brain makes during perception. These assumptions are made using organizational principles, like Gestalt, an individual's ability of depth perception and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.


Illusions, such as optical illusions, are erroneous perceptions. They are the result of complex interactions of visual cues and are not in all cases well understood. Often they seem to occur when there are few cues upon which to base a perceptual judgment, and one or more are supplying misinformation.

For example, looking with one eye through a small hole at a balloon in a box eliminates most of the cues for depth perception. In this situation, if the balloon is slowly inflated it gives the impression of actually remaining the same size but "moving toward" the viewer. This is because the size of the retinal image is ordinarily an important cue for distance. We judge relatively large things to be closer to us than relatively smaller objects. Young children who have learned this will sketch trees and people as tiny objects to indicate they are "far away" in the drawing they are producing.

Illusions may be learned and influenced by one's cultural context. Many popular textbook illusions, for example, feature straight-lined figures like arrows and lines. Research suggests that in primitive non-Western cultures, where buildings and natural surroundings offer few examples of straight lines, natives have developed no sets, or perceptual expectations, about straight-lined figures, and thus are not "fooled" by such ililusions.


Illusion In Art And Magic

• Stage magic is a popular form of entertainment based on illusion. Magicians use tricks to give their audiences the impression that seemingly impossible events have occurred.
  

• In fantasy works, actual magic may work by affecting the senses or producing an image, rather than producing a real change; this magic is frequently called illusion to distinguish it from more substantive forms of magic.

• Mimes are known for a repertoire of illusions that are created by physical means. The mime artist creates an illusion of acting upon or being acted upon an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls, "climbing stairs", "leaning", "descending ladders", "pulling and pushing", et cetera. Amongst mimes, these illusions are sometimes referred to as pantomime

In psychiatry and philosophy the term illusion refers to a specific form of sensory distortion. Unlike a hallucination, which is a sensory experience in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation so it is perceived in a distorted manner. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or other auditory source) would be an illusion.

Perhaps less common than visual illusions (or maybe more subtle) touch illusions also exist. These "illusory" tactile objects can be used to create "virtual objects".