http://online.wsj.com/article/SB1000...googlenews_wsj
With this experiment, Dr. Palmiter hopes to determine whether it's the amount of dopamine in the brain or the lack of dopaminergic neurons that affects cognition in Parkinson's patients.

While there are limits to what can be gleaned from such animal studies, the findings should provide insight into how critical it is in treating the disease to keep the neurons from dying, rather than just boosting dopamine in the brain, according to Dr. Palmiter.