The brains of patients with Parkinson's disease who had deep brain stimulation treatment produced new stem cells, according to new research.
The discovery is the first step to unlocking how the treatment works.
The study, a collaboration between scientists at the universities of Auckland and Florida, showed one reason why Parkinson's sufferers' symptoms improved with the treatment.
Dr Maurice Curtis, a senior lecturer in anatomy at the University of Auckland's Centre for Brain Research, said the study showed treatment increased the level of plasticity in the brain.
"In this instance the brain has actually started to produce more new stem cells in response to the deep brain stimulator being implanted."
Stem cells regenerate brain tissue in the same way as they produce new skin cells when skin is cut.
"We always knew that when people had these electrodes implanted in their brains that their symptoms would improve, but we've never really known why that should make a difference."
Dr Curtis said the results were surprising because Parkinson's sufferers usually had a reduction in stem cells.
The study involved about 15 elderly American and New Zealand Parkinson's patients who had the tiny electrodes surgically implanted.
"What these electrodes seem to be doing is to actually increase the amount of stem cells that are present in the key areas of the brain that normally just have a small number of stem cells. So the hope is that those stem cells are actually doing something beneficial."
The research, published on the Public Libraries of Science website, pointed to improved symptoms though Dr Curtis said it was too early to say if it would slow progress of the disease or what caused it.
"We know that stem cells mount a major regenerative response but is that what really brings about changes in the brain that improves the symptoms? Even though this paper is definitely a step in the right direction it leaves lots of unanswered questions about why deep brain stimulation works the way it does."