ILLUSION

a misrepresentation of a real sensory stimulusthat is, an interpretation that contradicts objective reality as defined by general agreement. For example, a child who perceives tree branches at night as if they are hobglobins may be said to be having an illusion. An illusion is distinguished from a hallucination, an experience that seems to originate without an external source of stimulation. Neither experience is necessarily a sign of psychiatric disturbance, and both are regularly and consistently reported by virtually everyone. false perceptual experience of actual stimuli. An illusion is not a simple error in perception; rather, it has features whose nature it is to evoke an incorrect perception. Illusions must be distinguished from hallucinations: the former, unlike the latter, result from some external perceptual stimulus. Illusions can be classified according to their sources. Some illusions are produced when the stimulus presented to the senses has been altered by some environmental condition; these are called stimulus-distortion illusions. A standard example is the stick that appears bent when part of it is immersed in water, an effect attributable to the refraction of light rays. A similar illusion is a mirage, in which, for example, a shimmering pool of water is perceived over a highway or desert heated by the sun. A familiar auditory stimulus-distortion illusion is the perception that the pitch of a siren changes as the siren moves. This is called the Doppler effect and is caused by the relative compression and expansion of the sound waves as the siren moves toward and then away from the listener. When the mechanisms by which stimuli are received, processed, and interpreted are misled, so-called perceiver-distortion illusions can result. For example, paradoxical cold results when a spot on the skin that normally responds only to cold responds to something very warm, relaying an illusory sensation of cold. In responding to stimuli, human sense organs adapt themselves in certain ways, and this phenomenon can give rise to illusions. For example, the retina will adjust itself to a light of a given brightness, thus temporarily narrowing its range of sensitivity. If a second light of exactly the same intensity is seen immediately after the first, it will seem less bright because the eye is less sensitive. Many illusions are generated by tricking the normal processes by which the brain interprets sense data. An illusion of this sort essentially brought about the development of the motion-picture industry. ( See movement perception.) In a process called closure, the brain tends to perceive as whole slightly incomplete figures. For example, the brain compensates if part of the image of a figure falls on the blind spot of the retina, so that the viewer does not perceive a gap. In other cases, a drawing of a figure can include competing visual cues, causing the brain to misjudge the figure's visual properties. Such drawings include the many familiar optical illusions in which straight lines appear to be curved or shapes equal in size appear to be unequal. Certain illusions stem from the ambiguity of visual cues. For example, in a well-known drawing that can be seen either as a vase or as two profiles facing each other, it is uncertain what is the figure and what is the ground against which the figure is set. Thus, the brain can choose, using the profiles as the ground to perceive the vase or the vase as the ground to perceive the profiles. Some illusions involve two senses simultaneously. In the phenomenon called synesthesia, objects of one sense are experienced as objects of another. The most common instances of synesthesia involve sounds and colours, whereby a person might experience a colour when hearing a tone, or vice versa. Further, an experience presented via one sense might be so vivid that it provokes the illusion of a complementary experience appropriate to another sense, as in the impression of smelling the food in a painting of a sumptuous banquet. Additional reading Works on the psychology of illusion include R.L. Gregory, The Intelligent Eye (1970), a fascinating account of how we see and are sometimes deceived by appearances; and S. Tolansky, Optical Illusions (1964), with most of the known visual illusions illustrated, one of the few modern books on this subject. Louis Jolyon West The Editors of the Encyclopdia Britannica