Formerly known as illusio, fallacia, and idolum. The term illusion comes from the Latin verb illudere, which means to mock, to delude, to tempt. It is unknown when and by whom the term was introduced, but it has been in use since ancient times. It has also had numerous connotations, most of them revolving around the notion of a false percept or idea. The French alienist Jean-Etienne Dominique Esquirol (1772-1840) is traditionally credited with giving the term illusion its current global meaning, i.e. a percept which is based on an object or stimulus in the external world, but is either misperceived or misinterpreted. Esquirol is also credited with distinguishing illusions from hallucinations, although it has been noted by the American historians of psychiatry Gregory Zilboorg (1890-1960) and George W. Henry (1890-1964) that a similar conceptual distinction was made in the second century AD by the physician Aretaeus of Cappadocia (c. AD 150). In 1832 Esquirol depicted illusions as a type of " sensory deception in which "there is always an actual impression upon the senses of external objects". Some common examples of such illusions are a face seen in the pattern of a carpet, a tree misidentified as a person, and a shadow held to be a cat. Esquirol tentatively attributes the mediation of illusions to the influence of the sense organs. As he argues, "In illusions ... the sensibility of the nervous extremities is altered: it is exalted, enfeebled, or perverted. The senses are active, and the actual impressions solicit the reaction of the brain." Thus Esquirol inserts a caesura between hallucinations and illusions as regards the involvement of the sense organs. And yet he does not regard the sense organs as the necessary locus of their origin. In his view, illusions can be mediated by the sense organs, the afferent nerves, or the brain. But he refuses to regard them as a product of cerebral or mental activity alone, which he believes is true of hallucinations. Like the Greek philosopher Aristotle (384-322 BC) long before him, Esquirol observes that the tendency to create illusions is promoted by affect-laden preoccupations. Theoretically, illusions can occur in any of the sensory modalities. However, most clinical examples involve the visual or auditory modality. Throughout medical history, illusions have been classified in a multitude of ways. The earliest known classification of illusions was the work of the Arab mathematician " Alhazan (c. 965-1040). Alhazan focuses primarily on "physical illusions such as reflections from curved surfaces and atmospheric refraction, but he also draws attention to the part played by knowledge and inference in the mediation of illusions. A division of illusions proposed by Esquirol is based on their having recourse to stimuli originating from within or from outside the body. He calls those illusions which arise as a consequence of stimuli from outside the body - such as a shadow or a tree - "illusions of the senses. Illusions arising in reaction to stimuli from inside the body are designated as " ganglionic illusions. Judging by the examples of ganglionic illusions given by Esquirol -including pain originating from an abdominal adhesion which the affected individual attributed to the popes holding council in her bowels, and a headache attributed to an intracerebral worm - ganglionic illusions in the esquirolian sense would today no doubt be labelled somatic hallucinations or somatic delusions. In addition to these esquirolian subclasses, a third subclass was proposed by the Russian psychiatrist Victor Kandinsky (1849-1889). In referring to this subclass, he uses the terms " deliriumofthe senses and " mistaken identity. Both these terms apply to cases in which a person consistently misidentifies another person. And yet, in spite of conceptual refinements and classificatory additions such as those above, the notion of an illusion has always remained somewhat problematic. As noted in 1894 by the German hallucinations researcher Edmund Parish (1861-1916), "Mere misinterpretations of sense-perceptions should not be regarded as sensory fallacies. In the long run, therefore, no satisfactory theory can be based on Esquirol's distinction, as is sufficiently indicated by the many unsuccessful attempts to reach one. But, generally speaking, nearly all the observers are agreed to consider illusion as a mixture of subjective and objective elements of perception, or as an incomplete sensory delusion, and to restrict the word hallucination entirely to new sensory creations." On the basis of the indebtedness of illusions to both subjective and objective elements of perception, a classification of illusions has been proposed which consists of physical illusions, " physiological illusions, and " cognitive illusions. In this context the term physical illusion refers to an illusion arising primarily as a consequence of the physical properties of an object or stimulus present in the extracorporeal world. Thus physical illusions are naturally occurring phenomena which theoretically can be observed by any person in possession of the necessary perceptual capacities. Some examples of phenomena which fall into this class of illusions are the " rainbow, the " mirage, the " anthelion, mirror images, and the Moiré pattern. In the present context the term physiological illusion is used to denote an illusion arising as a consequence of the perceptual system's inherent characteristics. The occurrence of this type of illusion is as inevitable as the physical illusion, but it does not entail an objectively observable phenomenon. Some examples of illusions which fall into this category are the " afterimage, the " after-effect, and the contrast effect. In the same context the term cognitive illusion is reserved for those illusions most indebted to an active contribution of the brain's (or mind's) unconscious inferences about the nature of the extracor-poreal world. Some examples of phenomena commonly regarded as cognitive illusions are " geometric-optical illusions, so-called impossible figures (as in the drawings by the Dutch graphic artist Maurits Cornelis Escher (1898-1972)), and the " Necker cube. A further division of each of these three classes of illusions has been proposed by the British psychologist Richard Langton Gregory (b. 1923). In an effort to do justice to the various effects elicited by physical, physiological, and cognitive illusions, Gregory suggests further dividing each category into four subcategories: " ambiguous illusions, " distortion illusions, "paradox illusions, and "fiction illusions. The identification and classification of illusions, whether they occur naturally or in the context of neurologic or psychiatric disease, or are designed by experimental psychologists, have yielded a plethora of phenomena and arrangements. Some of the phenomena can be explained with reference to contemporary physical, neuro-physiological, and neuropsychological theories, while others (such as the "Moon illusion and the long-distance "mirage) still lack a satisfying explanation, even though they have been known since ancient times.
   Esquirol, J.-E.D. (1832). Sur les illusions des sens chez les aliénés. Archives Générales de Médecine, 2, 5-23.
   Esquirol, J.-E.D. (1965). Mental maladies. A treatise on insanity. A facsimile of the English edition of 1845. Translated by Hunt, E.K. New York, NY: Hafner Publishing Company.
   Gregory, R.L. (1991). Putting illusions in their place. Perception, 20, 1-4.
   Kandinsky, V. (1885). Kritische und klinische Betrachtungen im Gebiete der Sinnestäuschungen. Erste und zweite Studie. Berlin: Verlag von Friedländer und Sohn.
   Ninio, J. (2001). The science ofillusions.Trans-lated by Philip, F. Ithaca, NY: Cornell University Press.
   Parish, E. (1897). Hallucinations and illusions. A study ofthe fallacies ofperception. London: Walter Scott.
   Zilboorg, G., Henry, G.W. (1941). A history of medical psychology. New York, NY: W.W. Norton & Company.

Dictionary of Hallucinations. . 2010.


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