But Dr Apkarian said other research in rats had shown that spinal cord neurons die, which suggests the brain changes could be irreversible.

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While nerves in the arms, legs and peripheral organs can sprout back at the rate of up to 1 millimeter per day, adult neurons in the brain or spinal cord, the so-called gray matter, were long assumed incapable of growing back after an injury.

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This, Dr Silver is convinced, is because the activity in the light-sensitive neurons somehow activated a latent nerve pathway that spans both sides of the spinal cord, allowing them to synchronise.

ECONOMIST: Neurology