Maps of the brain typically mark areas as being generally responsible for particular body parts. This view of neural architecture may need updating after functional MRI scans of people born with only one hand showed that parts of the brain usually assigned to the missing hand become responsible for body parts used to fill in.
When parts of the body are used or touched, specific areas of the brain light up in MRI scans, which has been taken as evidence for a one-to-one responsibility, confirming evidence seen from strokes or brain injuries. Regions responsible for sight can be repurposed for language in those blind from birth, but it is unclear how generalizable this is.
Dr Tamar Makin of University College, London, examined the behavior of areas of the brain usually associated with a certain hand in people who were missing that hand from birth. Makin then asked them to conduct tasks in which most people use two hands (e.g., wrapping a present and food handling) in a functional MRI machine, allowing her to study this quite large brain area in the process.
In Current Biology, Makin reports witnessing responses to other body parts, such as feet or the mouth, in the area normally devoted to the missing hand. This occurred even with body parts whose ordinary brain region is not nearby.
“Scientifically, I think one way to put our results in context is to say, what if the hand area is not the hand area per se, but just the part of the brain in charge of function ‘normally’ carried by that hand?” Makin said in a statement.
“In intact participants, all this is carried by the non-dominant hand,” she continues. “But the fact that we see such a striking different representation in that area in congenital one-handers might suggest that this is not actually the hand area. If true, this means we’ve been misinterpreting brain organization based on body part, rather than based on function. It’s kind of mind blowing for me to think we could have been getting this wrong for so long. The implications, if this interpretation is correct, are massive.”
Makin stresses that more evidence is needed before we completely rework our ideas of how the brain is structured. In particular, she suggests looking at the brains of children as they learn certain functions, but argues further research could prove highly valuable. “If we, as neuroscientists, could harness this process, we could provide a really powerful tool to better healthcare and society,” she said, citing examples such as controlling prosthetic arms.
The discovery of just how flexible the brain can be, known as neuro-plasticity, has been one of the big scientific developments of recent decades, as it has been shown that brain parts can take on new roles to compensate for damage. Nevertheless, Makin's discovery could take that plasticity to a new level.