expectancy hypothesis of hallucinatory experience
   A generic term for a group of explanatory models that attribute a major part in the mediation of hallucinations and "illusions to a person's expectations and attentional modulation. As noted by the American psychiatrist Mardi Jon Horowitz (b. 1934), this cognitive mechanism would seem particularly relevant to the formation of hallucinations and illusions in which an ambiguous stimulus or 'nidus' (such as a tree in the dark) is 'seen' in the form of an expected, feared, or wished-for shape (such as a person with or without malign intentions). In Horowitz's own words, "A fusion of an ambiguous perceptual nidus, a template of expectancy, and an active memory or fantasy image would provide the 'information' and perceptual quality of an hallucination." Obviously, expectancy alone is not a sufficient condition for the mediation of a hallucination or illusion. Rather, expectancy tends to be conceptualized as a pathoplastic factor which may help to shape or determine the aberrant percept's content. Conscious or subconscious types of expectancy would seem to play an important role in the mediation of a multitude of "sensory deceptions, including " hypnotically induced hallucinations, "reflex hallucinations, "cognitive illusions, "pareidolia, "auditory pareidolia, "change blindness, "inattentional blindness, "inattentional deafness, "auditory deafness, "tactileinsensitivity, "visual inattention, the "verbal transformation effect, the " electronic voice phenomenon (EVP), and " daydreaming, as well as in " apophenia. The " dissociation model of hallucinatory experience as put forward by the German hallucinations researcher Edmund Parish (1861-1916) may be regarded as one of the most rigoureus elaborations of the expectancy hypothesis.
   References
   Aleman, A., Laroi, F. (2008). Hallucinations. The science ofidiosyncratic perception.NewYork, NY: American Psychological Association.
   Haddock, G., Slade, P.D., Bentall, R.P. (1995). Auditory hallucinations and the verbal transformation effect: The role of suggestions. Personality and Individual Differences, 19, 301-306.
   Horowitz, M.J. (1975). Hallucinations: An information-processing approach.In: Hallucinations. Behavior, experience, and theory. Edited by Siegel, R.K., West, L.J. New York, NY: John Wiley & Sons.

Dictionary of Hallucinations. . 2010.

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