People with disabilities often report the phenomenon of "phantom pain," or "phantom limbs," when they sense the presence of missing fingers, hands, feet, or legs, and sometimes even feel pain in once amputated limbs. Until now, science could not explain this phenomenon in any way. But now, using ultra-high-resolution images, scientists at Oxford University were able to study the brains of amputees and see how their brains change after the loss of an arm. Brain detail at such a high level revealed for the first time an amazing thing: the amputee's brain retains an incredibly detailed map of the missing hand and individual fingers.
The existence of this detailed map of the hand in the brain - even decades after amputation - may partially explain the phantom limb phenomenon.
Sensory deprivation in people who have experienced blindness, deafness, or amputation, for example, has long been a fertile field for the study of brain plasticity. Lead researcher Sanna Kikkert and her colleagues at the Hands and Brain Laboratory, led by Associate Professor Tamara Makin, took as a basis for the study one aspect of the phantom limb phenomenon, when amputees not only sense the presence or feelings in the missing limb, but can also “control” their phantom hand. By asking people to move their phantom fingers individually while scanning their brains in parallel, the scientists were able to draw up a detailed map of the phantom hand representation in the brain.
Previous research has shown that moving the phantom hand creates activity in the amputee's brain, but until now it has been difficult to tell what exactly this activity is. For example, it is difficult to prove that brain activity indicates the existence of a missing hand card rather than some abnormal activity caused by amputation.
Kickert's research shows that patterns of phantom hand activity contain important features of the “normal” representation of the hand, such as the spatial arrangement of the fingers in relation to each other. The team was able to demonstrate that the phantom limb hand maps were within the range indicated in a control sample of two-handed participants. Considering the subjects lost their arms 25 to 31 years ago, this is incredible.
In a paper published in the journal eLife, scientists were also able to refute several other, more trivial explanations for phantom brain activity. They showed that phantom arm activation is not a simple result of muscle or nerve activity in the stump remaining after amputation. For example, in amputees who lost their muscles (due to amputation above the elbow), the hand maps remained the same as in those who could not send or receive signals from the limb at all (due to nerve damage). Nevertheless, it is still a mystery whether the stored hand map in the brain causes phantom sensations of the limb, or whether the sensations themselves preserve the hand map in the brain.
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How the brain sees the body
These results are doubly interesting because they contradict conventional wisdom regarding how the brain forms and maintains the sensory map of the body. This sensory map is called the somatosensory homunculus (from the Greek for "little man") and has long surprised scientists with its highly organized structure. Organized in that the brain folds body parts very similar to how they are located on the body:
It has long been thought that this map needs a constant stream of sensory input from the body to keep it organized. This thought has been supported by a significant number of animal studies showing that when a limb is amputated, body regions nearby on the homunculus invade and overwrite the missing limb's territory.
A similar reorganization was found in humans. A 2013 study by Tamar Makin and colleagues found that after amputation, the brain takes away the remaining territory from the missing arm. Their study also showed that this grip was related to the way the subjects used their bodies: the more the amputee used the remaining arm for daily activities, the more that arm took up the brain resources of the missing arm, possibly to support the overuse of the intact arm.
Kickert found a similar reorganization in her group of amputees in the area of the missing arm in the brain, as well as detailed hand maps. This means that after amputation, this area of the brain not only remains functional, but is also maintained, despite the subsequent reorganization - this fact has not been previously recognized.
In fact, it can be used to create amazing technologies specifically for the crippled and disabled: "neuroprostheses" that are controlled directly by the brain, usually through electrodes implanted in the cerebral cortex. The hand map saved after amputation can be used to drive individual fingers in these neurocomputer interfaces.
ILYA KHEL