Illusion Author: Elena Pasquinelli [INSTNICOD] Contributors: none Current version (on 2005-07-05) The notion of illusion is widely used in the domain of mediated interactions and constitutes one of the most addressed phenomena in current studies about perception. Some perceptual illusions were known to the ancient Greeks (for instance, the so-called Aristotle’s illusion), but it is in the XIX century that the first scientific description of illusions were given. In 1832 L. A. Necker illustrated how a rhomboid reverses in depth, sometimes one face appearing the nearer, sometimes the other (perceptual reversal or alternation); W. Wundt described the Horizontal-Vertical illusion: a vertical line looks longer than the horizontal line of equal length that it encounters (distortion illusion); interest in illusions grew higher suite to the publication of some figures showing distortions which could affect the use of optical instruments, thus producing errors: the Poggendorff figure of 1860 (a straight line crossing a rectangle appears displaced), the Hering illusion of 1861 and the Wundt of 1896 (straight parallel lines look bowed outwards or inwards), the Mueller-Lyer arrow figure of 1889 (the outward-going arrow heads produce expansion of the shaft, and the inward-going heads contraction). Distortion phenomena were then explained with reference to the stimulus pattern, (for example, in the case of the Mueller-Lyer figure that the acute angles tend to be overestimated and the obtuse angles to be underestimated). The number of phenomena that are described as illusions has greatly grown during the last two centuries [Gregory, 1968], and the term “illusions” is used in relationship to: •

ambiguities (as the Necker cube, the visual effects provoked by mist or retinal rivalry)

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distortions (as the Size-weight illusion or other classic geometric illusions, such as the Horizontal-Vertical illusion, but also mirages)

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paradoxes (as the impossible triangle of L. S. Penrose and R. Penrose of 1958, which cannot be seen as a sensible three-dimensional figure, the so-called impossible figures and impossible objects in general).

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fictions (as the rainbow, the faces one can ‘see’ in the fire, galleons in the clouds and so on, the after-images and figures such as the Kanisza triangle).

However, the notion of illusion is not theoretically neutral, but has come to be associated with the indirect, inferential approach to perception: illusions are defined as systematic perceptual errors or systematic discrepancies between what is in the world and what we end up perceiving of it, with the intermediate of inferential, cognitive processes [Gregory, 1997]. Between direct views of perception the ecological approach to perception denies that perception can be untrue, and then discards the concept of illusory perception as error [Turvey et al., 1981]. Sensorimotor and enactive accounts of perception strongly criticize the idea that perception is bounded to the representation or mirroring of the external reality, and then the idea of adequacy-inadequacy between perception and the unperceived world. However, the notion of illusion is not completely discarded [O’Regan & Noe, 2001] Illusions seem nevertheless to have a great heuristic value for theories of perception (and as a matter of fact, illusory phenomena are largely exploited by ecological theories of perception [Turvey, 1996]) because they permit to distinguish a class of phenomena which presents some structural differential features: the awareness that there is something wrong in the actual perceptual experience, associated with a reaction of surprise when this awareness is achieved. This awareness does not depend on a comparison of internal perceptual experiences with external states of the world, but rather on the detection of some violation of coherence (between experiences in different sensory modalities, between past and present experiences, between knowledge or information and present perception); the reaction of surprise alerts the subject to the possibility of error and hence represents an epistemic value and a potentially adaptive behavior the systematic character of the experience, both at the intersubective and at the intrasubjective level. In reference to these features, a characterization of the notion of illusion can hence be proposed that is neutral in relation to theoretical assumptions about perception.

References: Gregory, R. L. (1968). Perceptual illusions and Brain models. Proceedings of the Royal Society, B 171, 179-296. Gregory, R. L. (1997). Knowledge in perception and illusion. Philosophical O'Regan, K., & Noë, A. (2001). A sensorimotor account of vision and visual consciousness. Behavioral and Brain Sciences, 24(5), 939-1011.Transactions of the Royal Society of London, B 352, 1121-1128. Turvey, M. (1981). Ecological Laws of Perceiving and Acting: In Reply to Fodor and Pylyshyn. Cognition, 9, 237-304. Turvey, M. T. (1996). Dynamic touch. American Psychologist, 51(11), 1134-1152.

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