visual illusion
   Also known as optical illusion. Both terms are commonly used to denote a visual percept that has its basis in a stimulus derivative of the extra-corporeal environment (also referred to as a point de repère) which is either misperceived or misinterpreted. The French term point de repère translates loosely as 'guiding mark' or 'target'. It refers to any object or stimulus that may act as the source material for the development of an illusion. Thus a cloud may form the point de repère for an illusory face or animal, and a treetrunk for the illusory perception of a human figure. The class of visual illusions constitutes an exceptionally rich plethora of individual phenomena, some of which can be observed in nature (such as the * rainbow, the * mirage, and the colours of the peacock's feather) and some of which form the basic ingredients of everyday visual perception (such as the * afterimage and the * aftereffect). When visual illusions arise out of a random pattern such as clouds or a stain on the wall, they are referred to as * pareidoliae. Because visual illusions have the capacity to shed light on the workings of the perceptual system, experimental psychologists have designed a vast group of synthetic phenomena that also fall into this category. The classification of visual illusions is bewilderingly diverse. In some classifications, the term illusion is even applied to phenomena lacking a source in the external environment, such as *scotomata (associated with migraine), and the filling-in of the * blind spot. In 1881 the British psychologist James Sully (1842-1923) divided *illusions into *active illusions and *passive illusions, as a means of designating the relative contribution of perceptual and cognitive processes in their mediation. On the basis of their indebtedness to both subjective and objective elements of perception, the group of visual illusions has been divided into * 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 in the external world. Thus physical illusions are conceptualized as naturally occurring phenomena that can theoretically be observed by any person with proper vision. Some examples of physical illusions are the rainbow, the mirage, the * anthelion, mirror images, and the Moiré pattern. 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 is the physical illusion, but it is not an objectively observable phenomenon. Some examples of illusions placed in this category are the afterimage, the aftereffect, and the contrast effect. 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 physical world. Some examples of phenomena commonly designated 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)), *Jastrow's duck-rabbit, and the * Necker cube. A further subdivision 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 proposes that each category be further divided into four subcategories, referred to as * ambiguous illusions, * distortion illusions, *paradox illusions, and * fiction illusions. In 1959 the term visual illusion was used in a somewhat different sense by the Canadian neuroscientists Wilder Graves Penfield (1891-1976) and Sean Francis Mullan (b. 1925). These authors used the term to denote a qualitative change in visual perception in which things seem clearer or blurred, nearer or farther, larger or smaller, fatter or thinner. Judging by the examples given by Penfield and Mullan, their conception ofthe notion ofvisual illusion seems to come close to the notion of *metamorphopsia. As used by these authors, visual illusions are classified as *psychical illusions, which are in turn classified as *psychical states (i.e. as *aurae occurring in the wake of an epileptic seizure or during a cortical probing experiment). In the specific context of Penfield and Mullan's work, the term visual illusion is used in opposition to the terms * auditory illusion, *illusion of recognition, *illusional emotion, and a nameless remaining group containing relatively rare phenomena such as illusions of increased awareness, illusions of alteration in the speed of movements, and visuo-vestibular disturbances.
   References
   Gregory, R.L., Gombrich, E.H., eds. (1973). Illusion in nature and art. London: Gerald Duckworth & Company: Gerald Duckworth & Company.
   Gregory, R.L. (1991). Putting illusions in their place. Perception, 20, 1-4.
   Mullan, S., Penfield, W. (1959). Illusion of comparative interpretation and emotion. Archives of Neurology and Psychiatry, 81, 269-284.
   Ninio, J. (2001). The science of illusions.Translated by Philip, F. Ithaca, NY: Cornell University Press.

Dictionary of Hallucinations. . 2010.

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