BRISTOL: Swedish scientists have explored how a brain identifies its own body and how body image can change by successfully creating the illusion of owning three arms or being the size of a Barbie doll in a laboratory setting.
The research not only addresses some of the oldest philosophical and psychological questions about the relationship between body and mind, but also has potential applications in prosthetics and robotics.
“We were interested in probing the limits of what we can experience as part of our own body. We demonstrated that the body image is much more flexible than previously thought, even allowing healthy participants to experience ownership of an extra third arm," said study author Arvid Guterstam from the Swedish medical university Karolinska Institutet.
Fooling the brain in minutes
The experiment involved sitting a participant at a table with a rubber prosthetic arm placed next to their right arm. Touching the subject's right hand and the rubber hand with two small brushes at corresponding location, the scientists stimulated a feeling of owning the prosthetic arm by synchronising the strokes as synchronously as possible.
“Instead of choosing to experience only one hand as your own, we, surprisingly, found that the brain accepts both right hands as part of the body image,” said Guterstam.
The researchers found that in less than a minute they could fool the brain into having a third arm, as Guterstam explained: “When the brain is presented with two equally probable locations of the arm, it accepts both solutions and the subject experience having two right arms.”
Taking ownership of additional limb
To prove that the prosthetic arm was truly perceived to be a third arm, Guterstam 'threatened' either the prosthetic hand or the real hand with a kitchen knife, and measured the degree of palm sweating as a physiological response to this provocation.
According to the results, the subjects had the same stress response when the prosthetic hand was threatened as when the real hand was, but only during the periods when they experienced the third arm illusion.
There was no stress reaction when the prosthetic right arm was replaced with a left arm or a prosthetic foot, the researchers reported in the journal PLoS ONE.
