(if you like this sort of thing, you might also consider seeing "Why you can't tickle yourself" http://learning.eng.cam.ac.uk/wolpert/talks/tickle.ram
or the full talk at http://learning.eng.cam.ac.uk/wolpert/talks/wolpert.ram )
Scientists Induce Out-of-Body Sensation
Using virtual reality goggles, a camera and a stick, scientists have induced out-of-body experiences — the sensation of drifting outside of one's own body — - in healthy people, according to experiments being published in the journal Science.
When people gaze at an illusory image of themselves through the goggles and are prodded in just the right way with the stick, they feel as if they have left their bodies.
The research reveals that "the sense of having a body, of being in a bodily self," is actually constructed from multiple sensory streams, said Matthew Botvinick, an assistant professor of neuroscience at Princeton University, an expert on body and mind who was not involved in the experiments.
Usually these sensory streams, which include vision, touch, balance and the sense of where one's body is positioned in space, work together seamlessly, Prof. Botvinick said. But when the information coming from the sensory sources does not match up, when they are thrown out of synchrony, the sense of being embodied as a whole comes apart.
The brain, which abhors ambiguity, then forces a decision that can, as the new experiments show, involve the sense of being in a different body.
The research provides a physical explanation for phenomena usually ascribed to other-worldly influences, said Peter Brugger, a neurologist at University Hospital in Zurich, Switzerland. After severe and sudden injuries, people often report the sensation of floating over their body, looking down, hearing what is said, and then, just as suddenly, find themselves back inside their body. Out-of-body experiences have also been reported to occur during sleep paralysis, the exertion of extreme sports and intense meditation practices.
The new research is a first step in figuring out exactly how the brain creates this sensation, he said.
The out-of-body experiments were conducted by two research groups using slightly different methods intended to expand the so-called rubber hand illusion.
In that illusion, people hide one hand in their lap and look at a rubber hand set on a table in front of them. As a researcher strokes the real hand and the rubber hand simultaneously with a stick, people have the vivid sense that the rubber hand is their own.
When the rubber hand is whacked with a hammer, people wince and sometimes cry out.
The illusion shows that body parts can be separated from the whole body by manipulating a mismatch between touch and vision. That is, when a person's brain sees the fake hand being stroked and feels the same sensation, the sense of being touched is misattributed to the fake.
The new experiments were designed to create a whole body illusion with similar manipulations.
In Switzerland, Dr. Olaf Blanke, a neuroscientist at the École Polytechnique Fédérale in Lausanne, Switzerland, asked people to don virtual reality goggles while standing in an empty room. A camera projected an image of each person taken from the back and displayed 6 feet away. The subjects thus saw an illusory image of themselves standing in the distance.
Then Dr. Blanke stroked each person's back for one minute with a stick while simultaneously projecting the image of the stick onto the illusory image of the person's body.
When the strokes were synchronous, people reported the sensation of being momentarily within the illusory body. When the strokes were not synchronous, the illusion did not occur.
In another variation, Dr. Blanke projected a "rubber body" — a cheap mannequin bought on eBay and dressed in the same clothes as the subject — into the virtual reality goggles. With synchronous strokes of the stick, people's sense of self drifted into the mannequin.
A separate set of experiments were carried out by Dr. Henrik Ehrsson, an assistant professor of neuroscience at the Karolinska Institute in Stockholm, Sweden.
Last year, when Dr. Ehrsson was, as he says, "a bored medical student at University College London", he wondered, he said, "what would happen if you 'took' your eyes and moved them to a different part of a room? Would you see yourself where you eyes were placed? Or from where your body was placed?"
To find out, Dr. Ehrsson asked people to sit on a chair and wear goggles connected to two video cameras placed 6 feet behind them. The left camera projected to the left eye. The right camera projected to the right eye. As a result, people saw their own backs from the perspective of a virtual person sitting behind them.
Using two sticks, Dr. Ehrsson stroked each person's chest for two minutes with one stick while moving a second stick just under the camera lenses — as if it were touching the virtual body.
Again, when the stroking was synchronous people reported the sense of being outside their own bodies — in this case looking at themselves from a distance where their "eyes" were located.
Then Dr. Ehrsson grabbed a hammer. While people were experiencing the illusion, he pretended to smash the virtual body by waving the hammer just below the cameras. Immediately, the subjects registered a threat response as measured by sensors on their skin. They sweated and their pulses raced.
They also reacted emotionally, as if they were watching themselves get hurt, Dr. Ehrsson said.
People who participated in the experiments said that they felt a sense of drifting out of their bodies but not a strong sense of floating or rotating, as is common in full-blown out of body experiences, the researchers said.
The next set of experiments will involve decoupling not just touch and vision but other aspects of sensory embodiment, including the felt sense of the body position in space and balance, they said.