People who have lost limbs often feel imaginary pain in amputated and no longer existing legs and arms due to the fact that the brain tries to "rebuild" the work of the center of movements after they disappear and makes mistakes in doing so, neurophysiologists say in an article published in the journal Nature Communications.
“Despite the fact that the hand is no longer there, people with phantom pains feel that it is still in place - it seems to them that their hand is on fire and reacts very painfully to touch, and no painkillers can help get rid of such symptoms. We wanted to understand if it can be dealt with using engineering technologies, rather than medical intervention,”said Benjamin Seymour of the University of Cambridge (UK).
Memory scars
Seymour and his colleagues found that the cause of these pains is an incorrect "reorganization" of the brain, leading to the appearance of incorrect connections between the neurons that controlled the movements of an amputated arm or leg, during a highly unusual experience.
Scientists say that today the theory is rapidly gaining popularity among neurophysiologists that phantom pain occurs due to the fact that the brain "habitually" gives certain commands to the lost limb, expects to see their result of actions, which, of course, does not occur. This gives rise to painful sensations.
Accordingly, neuroscientists believe that this response can be suppressed by giving the brain something real to replace a lost arm or leg - for example, a cybernetic limb connected directly to the brain or to nerve endings in the stumps of the legs and arms.
Guided by this idea, Seymour and his colleagues gathered a group of ten disabled people experiencing phantom pain due to arm amputation or damage to the brachial plexus, and invited them to connect and learn how to use the cyber hand. Unlike other similar devices, this cyber-limb connected to the brain "wireless", reading signals from the center of movement in the brain using a magnetoencephalograph.
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The learning process, the researchers say, was two-way - the brain learned to use the hand, and the machine learned to decode its signals. Therefore, at first, the movements of the cyber-hand were extremely sloppy and inaccurate, and it took the participants a lot of time to learn how to use it as a “native” hand.
The art of self-deception
In a completely unexpected way, the scientists had to stop the experiment after a while - contrary to their expectations, the connection of the cyber hand to the brain of the disabled led not to weakening, but to a sharp increase in pain, and the more the participants in the experiments learned to control the artificial limb, the stronger the unpleasant sensations became.
This result surprised scientists, since connecting a cyber hand to the brains of healthy people has never caused such consequences. Trying to understand why this is happening, they changed the experiment by connecting a cyber manipulator to the part of the brain that controlled the healthy hand.
This time the result was the opposite - when the volunteers learned to operate the prosthesis, the pain magically disappeared for a while. This fact, according to the authors of the article, indicates the real source of pain - the so-called plasticity of neurons, the ability of the brain to "repurpose" nerve cells to solve new problems.
According to Seymour, amputation of an arm or leg leads to the fact that the nerve cells that controlled their movement lose their function. The brain adapts them to solve new problems, trying in vain to "fix" non-working limbs, and in some cases this leads to the development of phantom pain due to the fact that wrong connections arise between neurons.
This process, as shown by the experiments of the authors of the article, can be "redirected" to a healthier channel by forcing the brain to reconnect the neurons that controlled the amputated limb, this time in the correct way. The neurophysiologist hopes that the first clinical versions of such a therapy for phantom pain will appear in the next five years.