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It's easy to make your brain think it's in someone else's body

By messing with the brain's sense of location, a team of researchers in Sweden figure out how make people believe they're wearing each other's bodies.

The subjects in the MRI machine felt as though they were the person lying outside the machine, looking back at their own bodies. Arvid Guterstam

Some scientists just can't stop threatening people with knives and rubber mallets. But it's not out of sadism (we hope). Fear can be a vivid indicator of who, or rather where, you think you are.

A team of researchers at Sweden's Karolinska Institutet has been investigating how our brains orient us in space and help us navigate through the world, with fascinating implications for questions of identity and empathy. First they gave participants the illusion of inhabiting an invisible body, so that people felt an actual physical sensation when an empty space was touched and responded with fear when that empty space was threatened. Now, in research published this week in the journal Current Biology, the researchers have gone on to body-swapping.

To create the illusion of inhabiting someone else's body, the researchers had test subjects lie down in a functional MRI machine wearing a head-mounted display that showed the perspective of someone lying on a table elsewhere in the room, from which that person could see the test subject. It would be like looking at a close-up TV screen of someone sitting in a chair in your living room watching you lie on the couch. The MRI machine monitored the subjects' brain activity.

To conjure up the sensation that the person in the machine was in the other person's body, the scientists then touched the two bodies at the same time in the exact same places.

"In a matter of seconds, the brain merges the sensation of touch and visual input from the new perspective, resulting in the illusion of owning the stranger's body and being located in that body's position in the room, outside the participant's physical body," graduate student Arvid Guterstam, lead author of the paper, said in a statement this week.

In other words, when both bodies were touched, the person in the MRI machine felt that he was in the body that he could see being touched, which was the one lying outside the machine.

To further test the illusion, the researchers did the same thing they did in the invisibility experiment -- they threatened the body outside of the machine with a knife.

The knife threat led to increased skin sweating and levels of neural activity in fear centers of the brain during periods when the illusion was experienced compared to when it was broken, "suggesting that the brain interpreted the stranger's body as one's own," say the researchers in the comments on a YouTube video that shows a simulation of the experiment.

When they then pretended to attack the person in the machine with a rubber sledgehammer, the results showed a calmer reaction, which led the researchers to conclude that the subject felt as though the attack was happening to someone else in the room, even though it was, in fact, happening to him.

When the bodies outside the machine were moved to a different position in the room, scientists could watch the out-of-body sensations change by monitoring the brain in a functional MRI machine. Malin Björnsdotter/Arvid Guterstam

Why does all this happen? While the process by which our brains act like super-advanced GPS systems is still not fully understood, in 2014 a different group of researchers was awarded the Nobel Prize for figuring out the role certain neurons known as "place cells" play in guiding us around (PDF).

Guterstam and his team were able to give their test subjects the illusion of being located in several different spots in the room, and used the MRI to watch as different areas of the brain lit up corresponding to the location where the other body was placed. The team found out that certain sections of the brain -- including the hippocampus where place cells dwell -- were responsible for the sense of self-location, while other sections controlled the sense of body ownership. The interplay between the areas was found to be handled by a part of the brain known as the posterior cingulate cortex.

"This finding is particularly interesting because it indicates that place cells are not only involved in navigation and memory encoding, but are also important for generating the conscious experience of one's body in space," said principal investigator Henrik Ehrsson, professor at the Karolinska Institutet's Department of Neuroscience.

They're also important because, um, they just might lead to the coolest party game ever, don't you think?