After a limb is amputated, the neurons that once controlled it are assigned new tasks. Turns out this process might be reversible: patients who received double hand transplants can regain lost neurological control systems, according to French researchers. The team closely followed two patients to watch how they relearned to use their hands, measuring regions of the brain that control muscle movement, Wired reports. They found the patients’ brains detected the hands and could manoeuvre them years after the transplant, largely due to the fact that—after surgeons reattach nerves from arm stumps to the donated hands—the nervous system begins regenerating nerves at the rate of about 1 mm a day. Months later, control and sensation returned to the hands. “When the hand is transplanted and the original connections, to some extent, are being re-established, you’re reversing that process of plasticity and going back to something closer to the original organization of the brain,” said Jon Kaas, a neuroscientist at Vanderbilt University who reviewed the study for the Proceedings of the National Academy of Sciences.