It had been thought that when a limb is lost, the parts of the brain responsible for sensing and moving that particular appendage soon “forget” the jettisoned member, and become reprogrammed to process other body parts. However, a study published this month in the journal eLife indicates that this is not the case, and that the brain does indeed remember amputated limbs, even several decades later.
In a statement, study co-author Sanne Kikkert from the University of Oxford explained how the discovery that amputation “does not erase the original function of a brain area” is of vital importance, since it “remove[s] a barrier to neuroprosthetics – prosthetic limbs controlled directly by the brain.” In other words, it means that if artificial limbs can one day be connected up to the central nervous system, the brain should be able to interact with them just as it did with the original body parts.
To conduct their research, the study authors recruited three people who had each had their left hand amputated between 25 and 31 years ago, all of whom reported ongoing “phantom” sensations – meaning they often “feel” their missing hand. Using an ultra high-field MRI scanner, the researchers observed the activity in the somatosensory cortex of participants’ brains as they were asked to mentally “move” each of their phantom fingers one by one.
Prosthetic limbs could one day be controlled by the brain, just like real limbs. sportpoint/Shutterstock
In particular, the study authors focused on a brain region described as the “hand area of the primary somatosensory cortex,” which “contains detailed finger topography,” meaning it has a map of the hand encoded into it, with each finger represented by a particular group of neurons. It had been believed that this neural organization is maintained by regular sensory input from the hand itself, and that when this stops, brain connections become reorganized, resulting in these neurons being reassigned to a new task.
When scanning the amputees’ brains, the researchers discovered that the activity in this hand area was the same as that seen when right-handed people are asked to move fingers on their left hand, suggesting that the phenomenon of phantom hands is probably caused by the fact that this mental hand map is maintained even decades after amputation.
Though this activity was slightly weaker in the brains of long-term amputees moving their phantom hands than in two-handed people moving their less dextrous hand, the fact that this connectivity remains intact effectively rewrites the conventional wisdom regarding limb representation in the brain.
