I've read some articles and science papers about Parkinson's disease and found out that many patients often have cognitive difficulties, if not all of them. For memory difficulty, medications to treat Alzheimer's are used, but is there something for other cognitive difficulties, e.g., for focus and executive dysfunction? No medication was mentioned in some papers I've read, so I wonder if there is any medication that helps with those problems at the moment?
Also, what is the actual damage in the brain after having Parkinson's for about 15 years? Is memory irreversibly damaged, i.e., can't be retrieved with medications?