An experimental form of gene therapy for Parkinson's disease has been shown to produce promising results.
Nerve cell activity was dampened down by the therapy
US scientists treated 12 patients with a virus genetically modified to carry a human gene which dampens down the nerve cells over-excited by Parkinson's.
Now brain scans have revealed significant improvements - which were still present a year later.
The Feinstein Institute for Medical Research study features in Proceedings of the National Academy of Sciences.
However, the work is still at an early stage. The main aim was to test whether the therapy was safe.
Scientists delivered the gene only to one side of the brain - that which controls movement on the side of the body most affected by Parkinson's - to reduce the potential risk.
It makes an inhibitory chemical called GABA that turns down the activity in a key part of the pathway which controls movement.
Motor network changes
The US team tested the impact of the therapy by using a form of brain imaging known as positron emission tomography (PET) to track changes in the brain.
They focused on two discrete brain networks - one that regulates movement, and another that affects thinking processes.
Only the motor networks were altered by the therapy - but this was all the researchers had hoped for.
The scans showed that the motor network on the untreated side of the body got worse, and that on the treated side got better.
The improvement was reflected in an improvement in patients' symptoms.
They began to show signs of improvement one month after starting therapy, and by six months movement had improved by an average of 30%.
One patient registered an improvement of 65%.
The brain scans also showed those patients who received the highest dose of the gene therapy registered the longest-lasting effect.
Lead researcher Dr David Eidelberg said: "Having this information from a PET scan allows us to know that what we are seeing is real.
"This study demonstrates that PET scanning can be a valuable marker in testing novel therapies for Parkinson's disease."
Parkinson's, which affects around 120,000 people in the UK, is caused by the loss of brain cells, which leads to a drop in chemicals required to regulate cell activity, and the connections they make with their neighbours.
Dr Kieran Breen, of the Parkinson's Disease Society, said the disease was likely to be caused by a mix of genetic and environmental factors.
"Because of this, there are many potential ways to treat or cure Parkinson's, and gene therapy is one potential route holding a lot of promise," he said.
"This study is important as it suggests that it was the therapy itself, rather than a placebo effect, that was having a positive impact on patients' symptoms."