You don't need psychedelic drugs to start seeing colors and objects that aren't really there. Just 15 minutes of near-total sensory deprivation can bring on hallucinations in many otherwise sane individuals.

Psychologists stuck 19 healthy volunteers into a sensory-deprivation room, completely devoid of light and sound, for 15 minutes. Without the normal barrage of sensory information flooding their brains, many people reported experiencing visual hallucinations, paranoia and a depressed mood.

"This is a pretty robust finding," wrote psychiatrist Paul Fletcher of the University of Cambridge, who studies psychosis but was not involved in the study. "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."

"This is the idea that hallucinations come about because we misidentify the source of our own thoughts," psychologist Oliver Mason of the University College London wrote in an e-mail to Wired.com. "So basically something that actually is initiated within us gets misidentified as from the outside." Mason and colleagues published their study in October in the Journal of Nervous and Mental Disease.

To choose people for their study, the researchers asked more than 200 volunteers to complete a questionnaire called the "Revised Hallucinations Scale," which measures the predisposition of healthy people to see things that aren't really there. The scientists picked participants who scored in either the upper or lower 20th percentile, so they could compare how short-term sensory deprivation affects a range of individuals.

Study participants sat in a padded chair in the middle of an anechoic chamber, a room designed to dampen all sound and block out light. The researchers describe the setup as a "room within a room," with thick outer walls and an inner chamber formed by metallic acoustic panels and a floating floor. In between the outer and inner walls are large fiberglass wedges. "This results in a very low-noise environment in which the sound pressure due to outside levels is below the threshold of hearing," the researchers wrote.

Though participants had a panic button, none of them used it. After spending 15 minutes deprived of sight and sound, each person completed a test called the "Psychotomimetic States Inventory," which measures psychosis-like experiences and was originally developed to study recreational drug users.

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.

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.

"Perhaps this reflects different ways of dealing with sense data, which under certain circumstances might be advantageous," Fletcher wrote.

Next, the researchers hope to study how sensory deprivation affects schizophrenic patients and people who use recreational drugs that increase the risk of psychosis.

"There are claims that schizophrenic patients paradoxically find that their psychotic symptoms such as hearing voices are improved by sensory deprivation," Mason wrote, "though the evidence for this is very long in the tooth indeed. What happens to people who already hear voices when in the chamber?"

Via MindHacks.

Image: daveknapik/Flickr

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