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http://www.wired.com/2009/10/hallucinations/

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“It appears that, when confronted by lack of sensory patterns in our environment, we have a natural tendency to superimpose our own patterns.”

The findings support the hypothesis that hallucinations happen when the brain misidentifies the source of what it is experiencing, a concept the researchers call “faulty source monitoring.”

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Among the nine participants who scored high on the first survey, five reported having hallucinations of faces during the sensory deprivation, and six reported seeing other objects or shapes that weren’t there. Four also noted an unusually heightened sense of smell, and two sensed an “evil presence” in the room. Almost all reported that they had “experienced something very special or important” during the experiment.

(Corboy: consider the many reports and legends of ascetics feeling tormented
by demons or temptresses. Alexandra David-Neel famously claimed to have visualized a young monk, then found him such a nuisance she had to dematerialize him. Christian ascetics in the deserts of Egypt reported visits from demons. Something happens when human beings are without ordinary social
connection.)

"As expected, volunteers who were less prone to hallucinations experienced fewer perceptual distortions, but they still reported a variety of delusions and hallucinations.

"The researchers were not altogether surprised by such dramatic results from only 15 minutes of sensory deprivation. Although few scientists are studying sensory deprivation today, a small body of research from the 1950s and 1960s supports the idea that a lack of sensory input can lead to symptoms of psychosis.

“Sensory deprivation is a naturalistic analogue to drugs like ketamine and cannabis for acting as a psychosis-inducing context,” Mason wrote, “particularly for those prone to psychosis.”

We still don’t know why some people are more likely to have hallucinations than others, but Fletcher says that some researchers consider the phenomenon particularly important because it suggests that symptoms of mental illness occur on a continuum with normality.