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.”

Wired: Just 15 Minutes of Sensory Deprivation Triggers Hallucinations