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.

"Unlike with Alzheimer's and other dementias, patients with Parkinson's don't lose their memory. Instead, they may develop trouble with planning, making decisions and controlling their emotions, and often exhibit changes in personality as a result. About one-third to one-half of Parkinson's sufferers exhibit some signs of cognitive impairment at the time they are diagnosed, but over time virtually all patients will experience substantial cognitive decline.
With Alzheimer's disease, the patient often stops recognizing family. "With Parkinson's, it's like the family doesn't recognize [the patient] anymore," says Thomas Montine, a neuropathologist who heads the Parkinson's disease research center at the University of Washington in Seattle."

i like this opinion. they are only the same if you have both diseases. IMHO