Ever feel like you aren't quite yourself some days? That things are a little out of place? The way your brain understands the world and your position in it might be a lot more complex than you think. A study from Sweden's Karolinska Institutet created an "out-of-body experience" to look at how a test subject can be fooled into accepting — at a fundamental, instinctive level — what they see rather than what they know to be true.
To make sense of its environment and to understand its place in the world, the brain takes in a vast amount of information via the senses — continuously processing that information to create its own internal map. Previous studies with rats have pinpointed specific areas of the brain that contain "place cells" that allow the brain to calculate position and retrace familiar routes. That study — which won the Nobel Prize for Physiology or Medicine demonstrated "a cellular basis for higher cognitive function" and showed that the rat brain creates and stores a mental map of the world. It remains unknown whether the human brain has a corresponding system for determining its location.
An article, published in the journal Current Biology, describes how Swedish neuroscientists created an out-of-body experience for participants who were placed inside a brain scanner to record the results. By means of this illusion, they were able to analyze the way in which the brain perceives the body's location and how it constructs an internal model of the external world.
The new study, of 15 participants, at first, might sound a little odd. Each of the subjects was placed individually inside a brain scanner wearing a head-mounted display showing the room they were in. Instead of seeing themselves, the researchers projected onto the display the viewpoint of another person lying down somewhere else in the room. As a result, the participant could see what appeared to be their own legs in front of them with the legs of another person emerging from a brain scanner in a far corner of the room. To reinforce the illusion, the scientists then touched the participant's real body in the scanner with an object while, in the projection, the stranger's body was seen to be touched in the same manner. This was done in full view of the participant.
The scientists then began to make threatening motions toward the stranger's legs with a knife while measuring the participant's stress response. Despite being fully aware of the experiment's setup and procedure and knowing objectively where they really were, the knife threat made the participants sweat, and scientists measured increased levels of neural activity in the fear centers of their brains. This, says the team, suggests that the brain interpreted the stranger's body as its own. The findings were further supported when the participants saw their actual legs threatened with a large rubber sledgehammer: since they perceived the legs as someone else's their recorded stress responses were lower.
As peculiarly unscientific as knife-wielding and rubber sledgehammer attacks seem, the team says this part of the experiment has some very important implications. Arvid Guterstam, lead author of the study, says: "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."
Next, the scientists used the out-of-body illusion to "teleport" the participants between various places in the scanner room giving them viewpoints from different locations each time. By analyzing brain activity, the researchers say, it was shown that perceived self-location can be decoded from activity patterns in specific areas in the temporal and parietal lobes. The scientists say there is a "systematic relationship" between the information gathered from patterns of brain activity and the apparent truth of what each participant saw. The brain was instinctively convinced by the illusion that the screen created, that the body was physically located somewhere else in the room. One of the places that the scientists showed was responsible for creating this mental model of the world was the hippocampus. This part of the brain is specifically linked to memory and spatial awareness and is the location of the "place cells" of the rats described by the Nobel Prize team.
"The sense of being a body located somewhere in space is essential for our interactions with the outside world and constitutes a fundamental aspect of human self-consciousness," says Arvid Guterstam. "Our results are important because they represent the first characterization of the brain areas that are involved in shaping the perceptual experience of the bodily self in space." Henrik Ehrsson, professor of neuroscience at the Karolinska Institutet says: "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."