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   Sensory deprivation experiments are also known as isolation experiments. Both terms are used to denote an experimental study in which a test person is subjected to a drastically reduced and depatterned sensory input. Biomedical case reports of the hallucinatory effects of sensory deprivation date back to the 19th century. In 1861, for example, the German psychiatrist Christian Friedrich Wilhelm Roller (1802-1978) published an account of the effects of solitary confinement on mental health. Similar reports stem from solitary sailors, mountaineers, and polar explorers. The phenomenon itself has been known since ancient times, as witness many of the 'spiritual journeys' reported by shamans and mystics. The systematic study of sensory deprivation was initiated in or shortly before 1951 by the Canadian psychobiologist Donald Olding Hebb (1904-1985). Hebb had received a government assignment to evaluate the effects of Chinese and Russian brainwashing techniques based on sensory deprivation. An unexpected finding in his initial study was the spontaneous report of hallucinations by 14 test persons. Out of this group, 11 individuals reported only * simple and * geometric visual hallucinations, whereas the other 3 also reported * scenic hallucinations. As Hebb wrote in 1954, "It appears that the activity has a rather regular course of development from simple to complex. The first symptom is that the visual field, when the eyes are closed, changes from a dark to a light contour; next there are reports of dots of light, lines, or simple geometrical patterns. All 14 subjects reported such imagery (in runs lasting from two to six days), which was a new experience to them. The next step, reported by 11 subjects, is seeing something like wallpaper patterns. Then came isolated objects, without background, reported by 7 out of 14, and finally integrated scenes usually containing dreamlike distortions, reported by three of the 11." These findings prompted a series of similar experiments by Hebb and his colleagues Wood-burn Heron, Harold Bexton, Benjamin Doane, and Thomas Scott, and soon thereafter by many others in countries around the world. The means of sensory deprivation evolved from the use of cardboard arm cuffs, cotton gloves, and translucent or opaque goggles in sound-deadened isolation cubicles, to the use of sophisticated waterimmersion techniques. Basically, however, each experiment involved the exposure ofhealthy individuals to prolonged periods of reduced and depatterned sensory input. As these studies confirmed, sensory deprivation may evoke *visual illusions and hallucinations, *metamorphopsias, and in some cases also * tactile, * somaesthetic, *olfactory, and * auditory illusions in the majority of healthy individuals within several hours to days. Next, the circumstances most favourable to the genesis of hallucinations were studied. Some studies indicated that hallucinations are most likely to occur when sensory input is diminished rather than absent. As Doane et al. concluded, "Unpatterned sensory stimulation increases the probability of hallucinatory activity". Hallucinations were also found to increase in frequency and variation when motility was restricted and when test persons were lying on their back rather than sitting in a chair. It is still unclear whether hallucinatory activity is influenced directly by such means of restraint or whether this influence should be attributed to the stress associated with restraint.

   In some studies suggestions and expectations served to increase the frequency and variation of visual hallucinations and to induce additional *auditory and * kinaesthetic hallucinations -whereas in others, the suggestion that soft music would be played failed to induce any * musical hallucinations. Pathophysiologically, hallucinations occurring in the context of sensory deprivation tend to be considered * release phenomena. As a technique, sensory deprivation may be regarded as diametrically opposed to * sleep deprivation (which results in a continuous bombardment of sensory stimuli). The expression 'pharmacologically induced sensory deprivation' is sometimes used to denote the effects ofa group of hallucinogens known as * dissociatives.

   References

   Bexton, W.H., Heron, W., Scott, T.H. (1954). Effects of decreased variation in the sensory environment. Canadian Journal of Psychology, 8, 70-76.

   Hebb, D.O. (1954). The problem of consciousness and introspection. In: Brain mechanisms and consciousness. Edited by Delafresnaye, J.F. Springfield, IL: Charles C. Thomas. Roller, Chr. (1861). Über Seelenstörungen in Einzelhaft. Allgemeine Zeitschrift für Psychiatrie und psychisch-gerichtliche Medizin, 20, 195-213.

   Vernon, J. (1963). Inside the black room. Studies of sensory deprivation. London: Souvenir Press.

   Zubek, J.P., ed. (1969). Sensory deprivation: Fifteen years of research. New York, NY: Appleton-Century-Crofts.