Mind trick: How the illusion of having 3 hands could help stroke patients

Add to My Stories It gives a whole new meaning to the idea of lending a hand.Scientists have created the illusion of having three hands - and made it so realistic that men and women taking part broke into a sweat when their extra limb was threatened with a knife.The bizarre insight into how the brain can be fooled may speed the development of life-like prosthetic limbs for stroke patients and others who have lost the use of their own arms.Scroll down for video

Experiment: Scientists have created the illusion of having three hands - fooling volunteers' into thinking they had two right limbsTo create the illusion, Swedish scientists set up an experiment in which a person sat at a table and had a skin-toned rubber artificial arm placed next to their right arm.Both their real arm and the artificial one were covered with a cloth up to the shoulder, leaving only the two right hands in clear sight.The researchers then used small brushes to tickle the real hand and the rubber hand at the same time and in exactly the same spot and the men in women taking part were asked how they felt.Perhaps surprisingly, many said they felt as if they had two right hands, the journal PLoS ONE reports.

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A conflict arises in the brain concerning which of the right hands belongs to the participant's body, said researcher 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 thir! d arm.To prove that the rubber arm was experienced as real, the scientists lunged at the hands with a kitchen knife while measuring levels of sweating from the palm.Proving that the brain was truly tricked, the volunteers sweated almost as much when the rubber hand was under threat as when their hand was.But the brain is not too easily fooled. Placing the rubber hand on the opposite side of the table or replacing it with a rubber foot did not pass muster.The research could be used to create more lifelike prosthetic limbs.Researcher Dr Henrik Ehrsson, from the Karolinska Institute's Department of Neuroscience, said: 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.The brains willingness to accept an extra limb could also one day be exploited when fitting fire-fighters with robotic arms for heavy lifting. Or even to make shopping easier.The researcher said: If you had an old lady who didnt have much strength, a wearable robotic arm could be useful.
This is far from the first curious study conducted by Dr Ehrsson.In the past he has used nothing more than goggles, a video camera and a pair of sticks to fool women into believing a man's body was their own, and vice versa.So powerful was the body-swapping illusion, that it was even possible to make someone believe they were shaking your own hand, instead that of the person facing them.But not everything is are possible. For instance, Dr Ehrsson failed in his mission to convince people they had turned into a chair.

Watch Dr Ehrsson threaten an 'extra limb' (video has no audio):