(A fascinating discovery reported in the latest issue of Science magazine, Aug. 23, 2007).
Out-of-body experiences are associated more with tabloid newspapers, New Age Web sites, and large doses of hallucinogenic drugs than serious scientif ic discussion. Yet they’re often reported by reputable people who suffer from migraine headaches, epilepsy, and other neurological conditions. Intrigued by such accounts, some researchers are trying to figure out how the brain creates an aspect of human consciousness so fundamental that we take it for granted: the perception that the "self" conforms to the borders of the physical body.
Now, two teams of cognitive neuroscientists independently report on pages 1048 and 1096 methods for inducing elements of an out-of-body experience in healthy volunteers. Both groups used head-mounted video displays to give people a different perspective on their own bodies. Each team also drew upon the sense of touch to enhance the illusion. Although details of the experience differed, the people in both experiments reported feelings of dissociation from their bodies. The researchers say their findings will pave the way to new rainimaging studies of body perception and could have practical applications, such as helping virtual-reality programmers design environments that make users feel as if they are really there.
"It’s striking because when you hear about out-of-body experiences, it sounds so deeply weird," says Chris Frith, a cognitive neuroscientist at University College London who did not participate in the new research. "These studies show you can actually manipulate it experimentally." The illusions add to evidence that the brain’s representation of the physical body is malleable and can be modified by information from the senses, Frith says.
For one of the studies, a team led by Bigna Lenggenhager and Olaf Blanke, both of the Swiss Federal Institute of Technology in Lausanne, asked people to stand in front of a camera while wearing video-display goggles. In one experiment, subjects saw the camera’s view of their own back, computer-enhanced to create a three-dimensional "virtual own body." When the subjects’ backs were stroked with a highlighter pen at the same time they saw their virtual back being stroked, they reported that the sensation seemed to be caused by the highlighter on their virtual back, making them feel as if the virtual body was in fact their own body.
Moreover, when the researchers turned off the video display, guided the subjects back a few steps, and then asked them to blindly return to their former position, subjects overshot the spot where they’d actually been standing and walked to a point closer to the apparent location of their virtual body.
Adopting a similar strategy to attempt to induce out-of-body experiences, Henrik Ehrsson of the Karolinska Institute in Stockholm, Sweden, asked men and women to sit in a chair and don a video headset connected to two cameras that provided a stereoscopic view of their backs. As a subject viewed his or her own back from behind, Ehrsson used two plastic rods to simultaneously stroke the subject’s chest and a location behind the subject’s back.
Although people felt the rubbing on their chest, in the headset they could only see Ehrsson’s arm moving behind their back, reinforcing the sense that they were sitting at a location behind their actual body. The experience often elicited surprised giggles, says Ehrsson, who has tried it out himself.
"You really feel that you are sitting in a different place in the room and you’re looking at this thing in front of you that looks like yourself and you know it’s yourself but it doesn’t feel like yourself," he says. "It’s almost like you’re looking at a dummy." Nearly all subjects reported similar impressions on a questionnaire.
Ehrsson also repeated the illusion with electrodes attached to each person’s fingers to measure skin conductance, a physiological measure of emotional arousal. Then he swung a hammer in front of the cameras so that it appeared to hit the region where people perceived themselves to be. The hammer posed no physical danger, but changes in skin conductance indicated that subjects registered a threat (they also reported feeling anxious). By showing that people respond emotionally as if they were located at a position behind their physical body, the findings provide additional evidence that the subjects buy into the illusion, Ehrsson says.
Both experiments show that visual perspective and coordination between the senses of vision and touch are important for the sensation of being within the body, says Peter Brugger, a neuroscientist at University Hospital Zürich in Switzerland. Yet neither study replicated the full-blown out-of-body experiences in which people report "an enormously compelling sensation of separation from the body," he notes. Even so, Brugger says, these illusions may be as close as it is possible to get in the lab.
Previous research has pointed to several brain regions, including the intersection of the temporal and parietal lobes, that may be involved in producing out-of-body experiences in neurological patients, Blanke says. The new illusions can be used to examine which of these brain regions contribute to which aspects of these strange experiences, and that in turn, says Blanke, could lead to a better understanding of how the brain generates a concept of self.
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