If you've ever heard a voice that wasn't there, maybe that's why

With a ghostly finger in a laboratory, researchers persuaded people to hear ghost voices.

A few years ago, Swiss scientists found a way to make people hallucinate. They did not use LSD or sensory deprivation chambers. Instead, they sat people in a chair and asked them to press a button that, a split second later, caused a rod to gently press against their backs. After a few laps, the volunteers had the scary feeling of having someone behind them. Faced with a disconnect between their actions and sensations, their minds conjured up another explanation: a distinct presence in the room.

In a new study published in the journal Psychological Medicine, researchers from the same lab used the ghostly finger configuration to probe another type of hallucination: hearing voices. They found that volunteers were more likely to report hearing a voice when there was a delay between pressing the button and touching the stem than when there was no delay.

< p class="css-at9mc1 evys1bk0">The findings suggest that the neurological roots of hallucinations lie in how the brain processes conflicting signals from the environment, the researchers said.

Hearing voices is more common than you think, said Pavo Orepic, a postdoctoral researcher at the University of Geneva and author of the new paper. In surveys, scientists have found that many people without a psychiatric diagnosis (perhaps 5 to 10 percent of the general population) report hearing a disembodied voice at some point in their lives.

"There is actually a continuum of these experiences," Dr. Orepic said. "So we all hallucinate — at certain times, like if you're tired you'll hallucinate more, for example — and some people are more prone to doing it."

In the new study, as in previous work, Dr. Orepic and colleagues asked volunteers to sit in a chair and press the button that made their backs touch the rod. In some sessions, there was no delay between the push and the touch, while others had a half-second delay — enough time to give volunteers the feeling that someone was there. proximity.

During all trials, volunteers listened to recordings of pink noise, a softer version of white noise. Some recordings contained pieces of their own voice, while others contained fragments of someone else's voice or no voice at all. In each trial, volunteers were asked if they had heard someone talking.

The study found that when people were already experiencing the particular sensation of a ghostly presence, they were more likely to say they heard a voice when there was none. Additionally, they were more likely to hear a nonexistent voice if, earlier in the experiment, they had heard bursts of noise containing someone else's voice.

This suggests that the brain was connecting the hallucinated presence and the voice, Dr. Orepic said.

Intriguingly, the volunteers without a lag between pressure of the button and the stem have sometimes reported hearing a non-existent voice as well. , and they were more likely to do so if they had recently heard clips of their own voice. If the volunteers subconsciously decided they were responsible for the sensation of the finger on their back, they might have been primed to hear their own voice, the researchers said.

Together, the researchers' findings support the idea that hallucinations may result from difficulty recognizing one's own actions, as well as being primed to expect a particular outcome, Dr. Orepic said . Over time, people who encountered a ghostly presence during the test were increasingly likely to hear voices, implying that the brain was somehow relying on the past experience to create the impression that someone was speaking.

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