26 February 2011
Third-arm-illusion experiment conducted by Arvid Guterstam at the Karolinska Institutet in Sweden
The participant is seated at the table having a realistic prosthetic arm placed next to their right arm. The subject then sees her two real arms and the extra prosthetic arm, made out of rubber.
To produce the feeling of owning the rubber arm, the scientist touches the subject’s right hand and the rubber hand with two small brushes at corresponding location – synchronising the strokes as perfectly as possible.
"What happens then is that a conflict arises in the brain concerning which of the right hands belongs to the participant’s body," said Arvid Guterstam.
"What one could expect is that only one of the hands is experienced as one’s own, presumably the real arm. But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of the body image, and the subjects experience having an extra third arm."
To prove the theory, the scientist ‘threatened’ either the prosthetic hand or the real hand with a kitchen knife, and measuring the degree of sweating of the palm as a physiological response to this provocation.
Results showed that 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. The study has implications for prosthetics research.
"It may be possible in the future to offer a stroke patient, who has become paralysed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralysed arm remains within the patient’s body image," said Henrik Ehrsson.
"It is also conceivable that people with demanding work situations could benefit of an extra arm, such as firemen during rescue operations, or paramedics in the field".
The study is published in the online scientific journal PLoS ONE.
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