A third arm: It's all in your head.
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.
Applications in advanced prostheses
“This might have important bearings on the development of advanced prostheses, where the patient can experience and control an extra arm,” said Guterstam of the practical applications to the study.
Ongoing projects question whether the perceived body can be shrunk to the size of a Barbie doll or if the brain can accept a body of a different sex.
Other seemingly bizarre recent projects have included giving participants the illusion of shaking hands with themselves, having their stomachs slashed with a kitchen knife and seeing themselves from behind. All were designed to trick participants into a false perception of owning another body.
Understanding the sense of self
Olaf Blanke, the director of the Laboratory of Cognitive Neuroscience at the Swiss Federal Institute of Technology, conducts similar out-of-body experiments.
“Ehrsson's work has been very important in understanding how bodily processing is associated with the sense of self," he said. "
The self is not a mysterious entity that we are not able to study in the research laboratory, but is open to investigation and experimental manipulation.”
Gamers: watch this space
For Andrea Serino from the University of Bologna, Italy, the implications of this work are deep, “They have provided evidence that in order to know that this hand is mine, there is no need of any ‘magic’ knowledge, any magic ‘self’," she said, "but just a system integrating visual and tactile information related to the body in the parietal and premotor cortices.”
Serino agreed that this research could be applied in such diverse fields as video-games, virtual reality systems and tele-surgery methods: “It is quite common now to control another body at a distance, but the new thing we can start doing is to induce the feeling that a body we control is our own.”
“In this way, the self could potentially occupy multiple spaces, which is definitely a revolution. We are not that far yet, but we are trying to understand how to go there.”