The flexible, adaptable human brain rewires itself to accommodate a double hand transplant, even in patients who waited years for their new hands, according to a new study. But in a perplexing result, researchers found that the two right-handed patients that they studied both formed quicker connections between their brains and their new left hands.
The researchers studied the motor cortex, which devotes different areas to 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, [researchers] 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 [New Scientist], suggesting that the brain had rewired itself once again to accommodate the new hands.
In the study, published in the Proceedings of the National Academy of Sciences, researchers looked at two men, one age 20 and the other 42, who both lost their hands in work injuries and waited three or four years for transplants. After the younger man received his transplants, he was checked periodically and the researchers found his brain had re-established nerve connections to control the left hand by 10 months, while it took 26 months to complete the rewiring needed for the right hand. “Interestingly, despite that LB was right-handed, and that after his amputation he used his prosthetic device mostly with his right hand, hand preference shifted from right to left after he had the graft,” the researchers reported [AP]. The older man had even more dramatic preference: 51 months after his surgery, his brain had established a strong connection to his left hand, but not yet to his right.
The results could mean that because the right hand is more dominant in these men, its representation in the brain is more rigid than the left–and thus the brain is less able to rewire control of it–says co-author Claudia Varga [ScienceNOW Daily News]. However, there are many other possible explanations, ranging from slight differences in how each hand was reconnected to the arm (researchers note that a different surgeon worked on each hand). It could also relate to the advanced prosthetic right hands the two men used while waiting for their transplants. The motor cortex may have reorganized to accommodate the prosthetic, and this may have slowed its ability to then accept the new donor hand [ScienceNOW Daily News].
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Image: National Academy of Sciences, PNAS