MADRID, 21 (EUROPA PRESS)
A team of researchers from the National Institutes of Health (United States) and the University of Cambridge (United Kingdom) has discovered that the brain's body map remains intact after limb amputation, contradicting the previous belief that it reorganizes to compensate for the loss.
The study, published in the journal Nature Neuroscience, has implications for both the treatment of phantom limb pain and for facilitating the control of robotic prostheses. This study compares the brain activity of three people scheduled for hand amputation, both before and after the procedure.
These maps are responsible for processing sensory information such as touch, temperature, and pain, as well as body position. If you touch something warm with your hand, a specific region of the brain is activated; if you stub your toe, another region is activated.
“From our previous work, we suspected that the brain maps would remain largely unchanged, but the extent to which the map of the missing limb remained intact was astonishing… Given that the somatosensory cortex is responsible for interpreting what is happening inside the body, it seems surprising that it doesn’t seem to know the hand is no longer there,” said Tamar Makin, one of the study’s lead authors and a professor at the University of Cambridge.
During the research, brain scans were used to construct maps of each individual's hand and lips, as the lips are positioned close to the hand on these maps. After examining the maps before and after amputation, the scientists found that the corresponding brain region was activated almost identically.
NO SIGNS OF REORGANIZATION OF THE BRAIN BODY MAP
The scientists acknowledged that if they hadn't known when the data was collected, they wouldn't have been able to distinguish the two maps. In contrast, a machine learning algorithm "had no trouble" distinguishing which phantom finger was moving before and after the amputation.
"We didn't see any evidence of the reorganization that classical thinking assumes occurs. The brain maps remained static and unchanged," said Hunter Schone, the study's first author and a researcher at the University of Pittsburgh (United States).
It is worth noting that this is the first study to analyze hand and face maps of individuals before and after amputation. Previous work was based exclusively on analyses performed after amputation, which is why it was believed that, after the amputation of a limb, the neighboring regions reorganized and occupied the area previously assigned to the missing limb.
Since this doesn't happen, it can be explained why most people who have undergone an amputation report sensations such as pain or itching in limbs that are no longer in place.
"The remaining parts of the nerves, still within the stump, are no longer connected to their terminal receptors. They are completely disconnected from the sensory receptors that have constantly transmitted signals to them. Without terminal receptors, the nerves can continue to grow, forming a thickening of the nerve tissue and sending confusing signals to the brain," Schone emphasized.
Furthermore, researchers have concluded that the boundaries within brain maps are not well-defined, and that each area is not limited exclusively to a single part of the body.
To complement the findings, these results were compared with 26 participants who had suffered upper limb amputations an average of 23.5 years previously, also showing similar representations to the three subjects studied, suggesting the long-term stability of the representations of the amputated limbs.
PROMISING FINDINGS FOR THERAPIES
Dr. Schone emphasized that these discoveries will enable new technologies to advance toward accessing finer details of the hand map, such as distinguishing the fingertip from the base, as well as restoring qualitative and complex aspects of sensation, such as texture, shape, and temperature.
"This study is a powerful reminder that even after limb loss, the brain clings to the body, waiting for us to reconnect... The most promising therapies involve rethinking how amputation surgery is actually performed—for example, by grafting nerves into new muscle or skin so they have a new place to attach," he added.
In fact, of the three participants, one had significant pain in the limb before the amputation, but underwent a complex procedure to graft nerves into new muscle or skin, and now feels no pain. The other two participants received standard treatment and continue to experience phantom limb pain.
"If the brain were rewired after an amputation, these technologies would fail. If the area that previously controlled the hand was now responsible for the face, these implants simply wouldn't work. Our findings offer a real opportunity to develop these technologies now," said study co-author Chris Baker, a member of the National Institutes of Health.
DECIPHERING OR REDEFINING CORTICAL REORGANIZATION
On the other hand, the head of the Experimental Neurophysiology Group at the National Paraplegic Hospital's Research Unit, Juan de los Reyes Aguilar, explained in statements sent to SMC Spain that this work could contribute to deciphering "and even redefining" the complexity of the cortical reorganization phenomenon.
"Given the data on perceived pain shown in the article, it could be argued that the concept of cortical reorganization after amputation or spinal cord injury is somewhat more complex than what is currently presented in the literature, which is reduced to an expansion of the activated cortical area, and could focus on alterations in sensory perception," explained De los Reyes, a member of the Castilla La Mancha Health Service (SESCAM), the Castilla La Mancha Health Research Institute (IDISCAM), and the Spanish Society of Neuroscience (SENC).
