Wednesday, December 3, 2008

Perceptual illusions of body swapping

Awhile back, I posted on some research about the rubber hand illusion. Now, researchers are looking at not only just a limb of the body, but the entire body as a perceptive illusion.

Researchers at the Karolinska Institute induced these "
out of body" experiences in a group of healthy volunteers through various visual and tactile stimulation experiments. The first several experiments involved a mannequin which was used as the perceptual "other body." Two small cameras were placed on the mannequin's head which were hooked up to small screens positioned in front of the participants' eyes. This gave the illusion that participants' were looking through the eyes of the mannequin. Therefore, if the subjects tilted their heads down, they only saw the mannequin's.body and not their own.

Then, the researchers stroked both the subjects' and mannequin's bodies at the same time. The participants were able to see the stroking via the cameras on the mannequin, however, not their own. Questionnaires taken after this test showed that participants felt the mannequin's body was their own and sensed the touching of the abdomen on the mannequin.

To test the physiological response of owning another's body, researchers measured the skin conductance response, an indicator of stress that the polygraph lie detector test reads, of participants after threatening the mannequin body by "cutting" its abdomen. They observed there was a higher SCR (skin conductance response) when the mannequin was threatened with a knife.

Then, as a way to test whether participants could "body swap," they had experimenters wear the cameras which had been on the mannequin. Subjects were asked to take hold of the experimenter's hand and continuously squeeze. This set-up allowed the subjects to be able to see both parts of their own body as well as the other experimenter. The results indicated the participants felt the experimenter's arm was their own, like they were shaking hands with themselves.

Similar repetitive tests of threatening the experimenter's body (in this case, their hands) with a knife were done and the SCR tested. They found that the SCR results were again significant when the knife was threatening the experimenter's hand.

These series of experiments indicate how people can perceive both another human body as well as an artificial humanoid body as their own whether they are at a standstill or making voluntary movements. The "ownership" idea is especially evident when there deems a physical threat to the other body as shown through the greater SCR responses.

In the words of the author, "
This shows how easy it is to change the brain's perception of the physical self. By manipulating sensory impressions, it's possible to fool the self not only out of its body but into other bodies too

The implications and uses for this research is far ranging. Everything from body image to phantom limb pain/amputee pain to self-identity to robotics. It is a reminder how malleable our minds are and all the multisensory signals our body uses. For those with distorted body image, the hope is that this research can become a possible therapeutic tool as a way to perceive their bodies with more accuracy.

The full study is published on the online journal Public Library of Sciences and can be read here.

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