Hand transplants are eventually “accepted” by the brain, a study shows, raising the prospect of full movement being recovered. Surprisingly, it seems that in right-handed people, the left hand is accepted sooner.

The motor cortex – the part of the brain responsible for muscular movement – maintains a physical map of the body, with different areas registering sensations in different body parts. When the brain is deprived of sensory input from a limb, such as after a hand amputation, that region goes unused. To stop prime real estate going to waste, the brain rewires itself, with areas representing the face and upper arm “creeping in” to take over the region formerly dominated by the hand.

To find out if a transplanted hand can reclaim these brain regions, Angela Sirigu and colleagues at the Institute for Cognitive Science in Lyon, France, used magnetic pulses to stimulate these areas in two people who had undergone double hand transplants. They found that muscles in the new hands responded to the stimulation, suggesting that the brain had fully accepted them.

Previous research had shown that stroking a transplanted hand triggered brain activity in the same region as in non-amputees, but this is the first demonstration that the hand muscles are actually represented in the brain. “We can see the brain directly activating the new transplanted muscles,” says Sirigu.


Head start

In both patients, the left hand was quicker to get this space back – and regain movement – than the right. In one case, the left hand re-acquired a significant “presence” in the brain after 10 months; the right hand took 26 months.

One explanation, say the researchers, is the varying flexibility of the brain regions responsible for each hand.

They believe that because both subjects were right-handed, the brain regions dominated by the right hand were more active prior to amputation and therefore not as flexible to rearrangement. In contrast, the areas corresponding to the left hand were commandeered to a greater extent by other body parts. This may have led to greater flexibility in the left-hand region, Sirigu says, allowing signals from the transplanted left hand to be integrated faster.

Amputees waiting for a transplant should still use prosthetic limbs, though. Before the transplant, both patients had prosthetics, which Sirigu believes helped to keep the original brain representation of the hand alive. “Prosthesis reduces the chronic pain experienced by patients so we can’t ask them to go without,” she says.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073_pnas.0809614106)