"Some years ago, scientists in Switzerland found a way to make people hallucinate. They didn’t use LSD or sensory deprivation chambers."

"Instead, they sat people in a chair and asked them to push a button that, a fraction of a second later, caused a rod to gently press their back. After a few rounds, the volunteers got the creeping sense of someone behind them. Faced with a disconnect between their actions and their sensations, their minds conjured another explanation: a separate presence in the room. In a new study... researchers... found that volunteers were more likely to report hearing a voice when there was a lag between the push of the button and the rod’s touch than when there was no delay. The findings suggest that the neurological roots of hallucinations lie in how the brain processes contradictory signals from the environment, the researchers said."

From "If You’ve Ever Heard a Voice That Wasn’t There, This Could Be Why/With a ghostly finger in a lab, researchers coaxed people to hear phantom voices" (NYT).