Scientists believe they may have discovered how a man made a dramatic recovery after spending 19 years in a near-vegetative state.
Terry Wallis was in a vegative state for 19 years
Analysis suggests Terry Wallis, who was involved in a car accident when he was 19, has re-grown brain tissue.
The finding holds out hope of a greater understanding of brain damage recovery.
However, the Journal of Clinical Investigation study could not pin down the exact type of tissue regeneration that had taken place.
Three years ago, Mr Wallis uttered his first word, "Mom", and has shown continual, although limited, improvement.
Since speaking his first words, Mr Wallis's speech has improved and he has regained some movement in his legs, but his short-term memory is very poor and he does not understand what has happened to him.
To investigate how Mr Wallis has made this recovery, a US/New Zealand team of researchers scanned his brain using a technique called diffusion tensor imaging (DTI).
Using this method, the scientists were able to look at Mr Wallis's brain in detail, so as to see any damage or reorganisation of his white matter.
White matter is the part of the brain that contains nerve fibres wrapped in an insulating fatty substance, called myelin. It is responsible for transmitting information in the brain, whereas grey matter processes it.
The first scan took place eight months after he spoke his first words.
Henning Voss, associate professor of physics and radiology at Cornell University, New York, and the lead researcher, said the scan revealed that Mr Wallis had suffered severe damage to his white matter, but in one region of his brain there was an increase in tissue bulk.
The DTI scan revealed information about Mr Wallis's white matter
In a second scan 18 months later, the scientists could see increased white matter in a second area of his brain, in a region associated with movement and co-ordination. Dr Voss said this was also in accordance with his progress.
The researchers believe the most likely explanation is that axons, the long thin connections that make links between different brain cells, have re-grown.
Dr Voss said: "We are thinking there must be a reason why he came out of the minimally conscious state at some point, and our interpretation is that there was a very slow and ongoing self-healing process in the brain."
He added that this case was unique, and cautioned that this process may not occur in all minimally conscious state patients.
But some scientists have said there could be other explanations for the finding, such as the re-growth of the sheath that surrounds the nerve fibres, a process called re-myelination, rather than axon re-growth.
Dr Adrian Pini, a neuroscientist from King's College London, said: "They definitely have evidence of re-myelination, which would improve the speed of conduction and connectivity of different cells, improving function.
"But the distinction is whether or not the axons were already there, and whether they had been lying about but without any myelin, or whether axon re-growth has occurred and appropriate connections have started to sprout across the brain.
"It is difficult to be sure using this technique."
In an accompanying article, Steven Laureys, of the University of Liege, Belgium, said: "The findings of Voss and his co-workers will increase our understanding of severely brain-damaged patients and their 'miracle' recovery of consciousness."