When Henrik Ehrsson tells me that his latest study is “weird”, I pay attention. This is a man, after all, who once convinced me I was the size of a doll, persuaded me that I had three arms, and ripped me out of my own body before stabbing me in the chest. Guy knows weird.

Ehrsson’s team at the Karolinska Institute in Stockholm specialises in studying our sense of self, by creating simple yet spectacular illusions that subvert our everyday experiences. For example, it seems almost trite to suggest that all of us experience our lives from within our own bodies. But with just a few rods, a virtual reality headset, and a camera, Ehrsson can give people an out-of-body experience or convince them that they’ve swapped bodies with a mannequin or another person.

These illusions tell us that our sense of self isn’t the fixed, stable, hard-wired sensation that it seems. Instead, our brain uses the information from our senses to continuously construct the feeling that we own our own bodies. Feed the senses with the wrong information, and you can make the brain believe all manner of impossible things.

Loretxu Bergouignan joined Ehrsson’s team in 2009. She had been studying memory, and she wanted to know if that brittle sense of self is important for encoding our experience. After all, we take in all the events of our lives from inside our own bodies. As Bergouignan writes, “There is always an “I” that experiences the original event, and an I that re-experiences the event during the act of remembering.” If she put someone through an out-of-body illusion, could they still make new memories? Is that first-person perspective of the world important for storing information about it?

It’s the type of experiment that fits perfectly in Ehrsson’s group, who specialise in answering questions that seem almost too weird to ask in the first place. Still, he recalls, “I thought it was a very high-risk project. But sometimes you need to try riskier projects.”

Bergouignan recruited 32 local students for a “memory experiment” and gave them materials to study beforehand. When the volunteers arrived, Bergouignan fitted them with earphones and a virtual reality headset hooked up to a camera.

Half the time, the camera sat just above their heads and gave them the view that they would normally see (A below). The rest of the time, the volunteers sat facing the camera, so they saw themselves through their headsets (B). The researchers then pushed one rod towards the camera, while synchronously tapping the volunteers on the chest with a second rod. They could see themselves being prodded from afar, but also felt the same prods on their chests. That was enough to induce an out-of-body illusion. (Having experienced this before myself, I can attest to how convincing it is!)

Once the volunteers were under, an eccentric professor—really, an actor—entered and asked them questions about the material they had learned. He was scripted to be eccentric and memorable. He punctuated his questions with random monologues, bizarre provocative statements, and personal asides. “It was like performance theatre where the actors interact with the audience in a real-world environment,” says Ehrsson.

A week later, the volunteers returned to the lab, and Bergouignan asked them about their experiences with the professor. This was the real memory test, and the results were clear. The volunteers who experienced the out-of-body illusions were uniformly worse at recalling the details of the day than those who interacted with the professor from their usual in-body perspectives.

The team repeated the experiment with a slight variation. This time, the camera was at a 30 degree angle, so the volunteers floated out of their bodies but could still see the professor’s face (C). Again, they remembered the events more poorly than their in-body peers.

ExperimentThese are fascinating results. Remember that all the volunteers go through the same events. They’re all in the same place. The professor always sticks to the same semi-structured script. And yet, the angle from which they experienced those events strongly affected their ability to remember them.

The out-of-body illusion wasn’t more distracting; under its influence, the volunteers were just as good at simple mental tasks as they normally were. And it wasn’t just bizarre and off-putting either; after all,  we’re *better* at remembering bizarre events than everyday ones. Instead, the illusion seemed to hamper memories by taking volunteers out of their normal perspectives.

Bergouignan supported this view by placing some of her volunteers in a brain-scanner. She was especially interested in their hippocampus—a seahorse-shaped region near the floor of the brain that acts as a funnel between our experiences and our memories. It binds information from our senses and emotions into cohesive forms that can be stored, and then helps to reactivate that stored information when we want to remember something.

When the in-body volunteers sat in the scanner and recalled their time with the weird professor, their hippocampus behaved in the normal way. It became more active when they first tried to remember the events, and then less active with each subsequent attempt.

But the out-of-body hippocampi did exactly the opposite. “The first time they tried to remember, there was nothing in the hippocampus. It was silent,” says Ehrsson. But the more they tried to remember, the more active the hippocampus became.

This suggests that the volunteers aren’t just remembering a little less when they’re out of their bodies. “Their hippocampus is impaired in a more profound way,” says Ehrsson. He suspects that without the first-person perspective, the hippocampus can’t encode experiences in its usual coordinated way, and volunteers end up with fragmented memories that they struggle to recall. And as they struggle, Ehrsson speculates that they could be creating false memories out of the fragments.

This is a good example of embodied cognition, where basic aspects of our bodies like sensory information can influence “higher” mental skills like our memories. “When we walk around minding our own business, we always have this sense of being located inside our bodies,” says Ehrsson. “You need to have that experience of the world to encode and recall your own memories. This has never been shown before perhaps because it’s so difficult to manipulate.”

That’s a stretch, says Howard Eichenbaum, who studies memories at Boston University. “It is well known that material learned in a particular context is better remembered in the same context than a different one… and the hippocampus plays a key role in using context to guide memory retrieval,” he says. For example, deep-sea divers are worse at remembering what happened to them underwater after they resurface. “The out-of-body effects here may be a special case of context dependent memory.”

Ehrsson acknowledges this, and wants to see if it’s possible to reverse the memory defects by putting volunteers back into the out-of-body illusion.

This could have possible applications. There are many disorders like schizophrenia or borderline disorder where people say that they feel detached from themselves. Many people with post-traumatic stress disorder (PTSD) claim that they remember experiencing traumatic events from outside of their own bodies, and struggle to remember those events clearly. Perhaps those two things are linked. “It’s not that they don’t want to remember what happened for emotional reasons,” says Ehrsson, “but it could be that the memories are damaged because they weren’t located inside themselves,” he says.

“Many such patients report that the subjective element of experience is altered, but this is difficult to verify externally,” says Brian Levine from the Baycrest in Toronto. But Ehrsson’s team have provided scientists with a way of probing these experiences, by manipulating something as inherently subjective as “self-ness”.

“They did not confirm any of their predictions in patients, though,” Levine adds. “There is a big difference between dissociations due to psychological trauma, and the dissociations induced in this experiment. So more work is need to make the connection to clinical syndromes.”

Reference: Bergouignan, Nyberg & Ehrsson. 2013. Out-of-body–induced hippocampal amnesia. PNAS http://dx.doi.org/org/10.1073/pnas.1318801111

Meg Marshall