Illusions

(1) an inadequate understanding of a perceived object that goes beyond the bounds of ordinary errors of perception; illusions represent a phenomenon that is largely unconscious and not subject to voluntary correction; (2) false, unrealizable hopes and dreams.

An examination of a few examples of illusory perception may be found in authors of antiquity (Aristotle, Lucretius, and Vi-truvius). During the Renaissance and in modern times, interest in illusion was cultivated by studies in optics and elaboration of the theory of perspective. However, it was only with the work of S. Oppel (1854, Germany) that the study of illusions acquired a systematic character. Illusion was first interpreted as an independent phenomenon by Gestalt psychology (M. Wertheimer, 1912, Germany). The most significant investigations of illusion in contemporary psychology have been conducted by the schools of the Soviet psychologist D. N. Uznadze and the Swiss psychologist J. Piaget.

So-called opticogeometric illusions (optical illusions)—that is, illusions in perceiving weight, time, various types of seeming and induced movement, and effects of a succession of fixed arrangements—are generally known; illusions that arise under complex special conditions have also been studied. Each illusion produces in experiments its own curve of errors of perception, which are determined by quantitative indexes.

The majority of illusions are either weakened (primary illusions) or strengthened (secondary illusions) with age, thus revealing a close connection with the total course of perceptual development, specifically with the formation of its constancy. Illusions have long been used in art, making possible special effects of expressiveness. It is necessary to take account of the laws of illusory perception in any type of direct observation and evaluation and also in architecture and the design of manufactured articles.

Illusions in medicine. Illusory deceptions of the senses can occur in healthy persons, but are characteristic mainly of persons with acute or chronic mental disturbances. They often arise as a result of fatigue, exhaustion, fear, or an anxious-depressed state. A suspicious attitude toward the environment can cause illusions—for example, a fearful traveler in the forest is capable of mistaking a bush for a hidden malefactor; to a delirious patient, spots on the wall may appear to be trees or animals; an intoxicated alcoholic may hear reproaches and abuse addressed to himself in the speech of the street crowd.

Illusions may be visual, auditory, tactile, gustatory, or olfactory. They differ from hallucinations by the presence of a real object of perception. Besides illusions as psychopathological phenomena, there are “physical illusions”—phenomena that are observed by everyone, regardless of mental peculiarities.