Knife Wielding Scientists Give Test Subjects Third Arm… Sort Of

Last Updated on June 2, 2020 by

It’s an illusion that may not be optical, but is certainly spectacular.  And it may teach us quite a bit about how the brain works.  The scientists were actually able to make test subjects – without the assistance of drugs or any other methods of consciousness altering – think they actually had a third arm and were even able to feel the brush strokes of the third hand even though the hands were not actually attached to their bodies.

The mysterious experiment led scientists at Karolinska Institute in Stockholm in a completely different direction from where they were intending to go originally.  They had hypothesized confidently that the test subjects would take one of the hands as their own and then not have to be reassured that they only had two hands.  But as the experiment went on, what they discovered was quite different.

The test subjects began to accept both hands as their own.  The experiment involved the test subjects being placed in a room where their hands were placed on the table in front of them.  A sheet was draped around their bodies and the scientists placed a realistic looking right hand next to the test subject’s.  Being encouraged to stay very still, the Stockholm scientists then stroked their hands in perfect unison with two brushes in the same place on both the test subject’s hand and on the prosthetic model.  As time went on, eventually a disconnect in the patients’ brain arose and they reported experiencing having a third hand.

But how did the scientists measure the subjective belief of the test subjects?  After convincing the patients they had a third arm, the scientists wanted to test this theory and threatened both the real and prosthetic arms with a knife.  They measured the reactions of the patients based on the perspiration of their palms from the nerve wracking experience and noted that in both cases the patients reacted as though they were nervous around the knife wielding scientists.  They were not, however, as nervous when other inanimate objects (such as a prosthetic left arm or foot) were threatened with a knife.

And so they say the incredibly strange find may have far reaching benefits to those who suffer from strokes and need to retrain their limbs.  But if we are training our brains to suddenly be able to integrate external objects as parts of our own body, it seems like the real application is clear.  As technology improves and cybernetics is being gradually introduced into the human body, this may be just the type of conditioning that allows some people to cross the barrier between external stimuli and nerve endings in the brain.  By using the same principle, perhaps we can one day find a way to make that connection in the consciousness and the subconscious that lets someone “feel” a prosthetic arm as if it was their own.  Of course there would have to be a sensory system on the prosthetic arm as well, but it seems they may have found a way to make sure less information gets lost in translation.