[♪ INTRO] Presence hallucinations are the perception of another person existing near to you, when no one is actually there.

And researchers have developed ways to induce them in a lab setting – on purpose. That might sound a little weird, but while hallucinations can be perfectly normal, they can also tell us something about a person’s health under the right conditions.

And determining how susceptible people are to these presence hallucinations could lead directly to earlier diagnosis and treatment for Parkinson’s disease and other conditions. All by making you feel like someone else is there. There are two main methods psychologists have developed to create these kinds of hallucinations.

The first, most recent one, involves Swiss undergraduate participants and sound. Or, more precisely, those participants listening to sounds of themselves walking around… themselves. It goes like this.

Before the experiment takes place, a participant is asked to record their own stimuli. In this case, the researchers asked them to walk around a room on a set path, reciting passages from The Little Prince. These sounds are recorded using a binaural microphone which basically records sounds in 3d just as you would hear them, ent is conducted.

During the experiment, they sit in that same spot with their eyes covered and headphones on, and they can effectively hear themselves walking around the room. They were also played similar audio of other gender-matched participants. After hearing the audio, participants in the Swiss study were given a 14-point questionnaire measuring how closely they felt that there was another presence in the room, and how closely they identified it as themselves.

They were also asked if they had any previous hallucinatory experiences. And sure enough, when listening to their own audio, participants reported feeling that that someone else was in the room with them: themselves. A doppelganger, if you will.

Hearing is one thing, but feeling as though someone else is actually in the room is something else. For example, hearing a TikToker on your phone isn’t the same as feeling they’re in the room with you. Feeling that presence of another person is what makes it a presence hallucination.

The second method for inducing a feeling of presence, first used in a study published in 2014 gets a bit more cyberpunk. It involves the use of a robotic arm. With this method, participants are blindfolded and perform movements with their hand in front of their body.

These movements are tracked and reproduced at the same time against their back, using a robotic arm. In essence, they’re kind of petting and poking their own back, using a robot. This creates a discrepancy.

But, since the brain likes all our sensory inputs to indicate that the same thing is happening to your body, it tries to find the most likely situation that would explain all these weirdly ‘off’ inputs. Even if in some cases, even if that most-likely situation is impossible. Like someone magically appearing in the room to pet you.

And as if that wasn’t strange enough, when the researchers implemented a time delay on the robotic back petting, they were able to make that experience of their body seem separate. Kind of like a ghost in the room. Another presence hallucination.

Plus, the brain’s attempt to resolve these sensory discrepancies effectively throws participants’ experience of their body backwards, into the space around the robotic arm. Participants had of course given their informed consent before the study, but researchers would have been careful to make sure they didn't alert them to what they might experience in the study, in order to avoid biasing the response. Even so, the researchers noted that the experience of this other presence in the room was surprisingly strong.

So much so that several participants opted to stop the experiment. Interestingly, the teams running both of these studies found that those who have hallucinations regularly are more sensitive to the induction of this particular kind of hallucinations than those who do not. And it’s that quirk that takes this whole thing from a weird party trick to a potentially life changing tool for diagnosis.

The same research team that pioneered the robotic arm experiment went on to look at making it into a diagnostic tool. For example, hallucinations are very prevalent in those with Parkinson’s disease. While there are a whole host of symptoms associated with Parkinsons, primarily relating to movement and cognition, there’s a growing suggestion that hallucinations set in before other symptoms.

That means if we can spot them early, we might be able to improve outcomes for these patients, by getting them treatment earlier. And that’s important, because treatments such as levodopa or even exercise are more effective at staving off symptoms earlier in the development of Parkinson’s. However, people are often reluctant to tell their doctors that they’re having hallucinations, whether through fear of what it might mean about their health, because they think an uptick in hallucinations is just part of getting old , or other reasons.

The robotic arm researchers have already observed that some people with Parkinson’s are more sensitive to having hallucinations induced by their method. They’re now looking at developing it into a sort of “stress test” that doctors can use to spot patients who are especially prone to, or experiencing hallucinations, but not reporting them. That wouldn’t be a slam dunk for diagnosing Parkinson’s, but it could help identify patients for further testing.

With a little luck, the diagnostic tool they make may lead to those crucial earlier diagnoses. Thanks for watching this episode of SciShow, and thanks to this month’s President of Science, Matthew Brant! Your generous support makes it possible for us to bring fascinating, surprising science to everyone.

And we love doing that! If you’d like to join our amazing community of patrons, you can get started at patreon.com/scishow. [♪ OUTRO ]