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   Also referred to as dream intrusion, dual-input model, and seepage theory. The term perceptual release theory was introduced in or shortly before 1958 by the American psychiatrist Louis Jolyon West (1924-1999) to denote a hypothetical model for the mediation of hallucinations that proceeds on the assumption that the 'lower' parts of the brain are capable of producing endogenous percepts, and that these can be 'released' (or made to 'seep through') towards the 'higher' (i.e. cortical) centres, thus giving rise to a * release hallucination. The model suggests that sense perceptions and other life experiences leave permanent neural templates or engrams in the brain, which are continually competing with externally generated percepts for the individual's attention. The 19th-century dream researcher Louis-FerdinandAlfred Maury (1817-1892) referred to this continuous stream of endogenous percepts as an * oneiroid, or dreamlike, activity (hence the term dream intrusion). Maury also suggested that during normal wakefulness we are barely aware of this activity because it tends to be inhibited, or kept out of our awareness, by the constant stream of sense perceptions. Hypotheses like West's and Maury's have their roots in the darwinian notion, voiced by the British neurologist John Hughlings Jackson (1835-1911), that the brain's morphological structure reflects the successive stages of evolutionary development, with the 'lower' parts being phylogenetically older and the 'higher' parts being of a more recent evolutionary date. Since hallucinations are traditionally considered more primitive in nature than sense perceptions, their origin has been sought in the lower levels of the brain, i.e. the least organized and least complex regions, but the ones considered 'fittest for survival' under unfavourable conditions. In 1884, Jackson hypothesized that the loss of 'higher' cerebral functions might be brought about by a retrograde evolutionary process called dissolution. The concomitant doctrine, known as * Jackson's law, states that a loss of mental functions due to disease retraces in reverse order the evolutionary development of the brain. This loss of normal functioning was referred to by Jackson as 'negative' symptomatology and the release of cognitive and/or perceptual phenomena from the brain's more 'primitive' centres as 'positive' symptomatology. As a corollary, he argued that positive symptoms (such as hallucinations) might well originate from the normal activity of the brain's 'lower' centres. Throughout the 20th century, Jackson's law proved an important source of inspiration for hallucinations researchers. During the 1920s, for example, it was taken up by the French psychiatrist Jean-Jacques Lhermitte (1877-1959) to furnish his model of *peduncular hallucinosis. During the 1960s, West's perceptual release theory was further refined under the influence of * sensory deprivation experiments and the study of * drug-induced hallucinations. Variants of West's perceptual release theory include the * experiential projector model formulated by the American psychopharmacologists Ronald K. Siegel and Murray E. Jarvik, and the *dual-input model formulated by the American psychiatrist Jacob A. Arlow (1912-2004).

   References

   Lhermitte, J. (1951). Les hallucinations. Clinique etphysiopathologie. Paris: G. Doin.

   Siegel, R.K., Jarvik, M.E. (1975). Drug-induced hallucinations in animals and man.In: Hallucinations. Behavior, experience, and theory. Edited by Siegel, R.K., West, L.J. New York, NY: John Wiley & Sons.

   West, L.J., ed. (1962). Hallucinations.New York, NY: Grune & Stratton.