The illusion is delicate: If the strokes were applied with a different rhythm than the ones displayed in the video or if a subject moved his or her own hand, the feeling would be dispelled. But the findings suggest that our consciousness of a self that is contained in our body is based on the brain processing and correlating inputs from the various senses, the researchers report. "All of these illusions are all about the integration of all available data and the brain coming up with one interpretation," Ehrsson says.
These results further indicate, Blanke says, that self-consciousness begins with the awareness of a self in a particular body. "This research gives us the first building block of how the first-person perspective could naturally emerge," says Thomas Metzinger of the University of Mainz in Germany, a member of Blanke's team. "Can you be conscious without any experience of your body at all?"
"The constant checking that everything is in register is crucial for the sense of a unified corporeal self," or a sense of a self within a particular body, Altschuler adds. "The perception of the corporeal self is in some sense an illusion that is dependent on this registration. If you don't have that, then you become disembodied."
Because the effect is simple and seems to be universal, it may be possible to use it for things like telesurgery (enabling surgeons to operate intuitively from a distance) or for space exploration (allowing controllers to manipulate robots as if they were on distant moons or planets). "We don't really know the constraints," Ehrsson notes. "How much tactile information is necessary? How much delay [between the visual signal and the feeling] is allowed?"
The techniques could also be used to put people into the computer-generated virtual avatars, either for therapy (as in body-image modification for anorexics) or for enhancing game experiences, an application Ehrsson is currently studying.
It remains unclear, researchers say, whether consciousness can be transferred from person to person or even person to object as Blanke's metal box experiment hints. The brain functions underlying the illusion also remain a mystery; preliminary research and clinical reports of epileptics and patients with brain damage point to networks that link the temporal and parietal lobes of the brain. Blanke plans to do neuroimaging studies to try to map the brain circuits involved.
And matching these out-of-body illusions with the actual out-of-body episodes experienced by as many as one in 10 people will also be key, researchers note. Ehrsson says that at least one of his patients reported feeling the same way after the lab test as she did after a spontaneous out-of-body occurrence.
"The advantage of what these guys have done," says neuroscientist Vilayanur Ramachandran of the University of California, San Diego, "is that they have shown a simple way of producing out-of-body experiences in normal people." Getting out of the body may be as simple as tricking the brain.