Early this year Glen Campbell was diagnosed with Alzheimer's, though apparently he has problems with short-term memory for years.
Undaunted he has released a new album and is now on the road for a farewell tour.
He made a public announcement about the disease. He has had major substance abuse problems in the past and a short but high-profile relapse a decade or so ago, so the announcement might be partly to ward off suggestions that he's off the wagon if he seems disoriented. It also brings some public attention and awareness to the disease.
The neurologist who is treating him said
his musical skills have been relatively preserved as he’s undergone this global decline in [brain] function.... There’s a disconnect between language production related to conversational speech and the use of language when language is recruited for purposes of music. It’s a way to reach those recesses of the brain that otherwise are dormant, a way to stimulate one’s cognitive function.
http://latimesblogs.latimes.com/music_blog/2011/08/glen-campbell-alzheimers-disease.html
Apparently he still plays well, but on tour he has an autoprompt for lyrics.
Campbell had several big hits in his own name, and was a member of the legendary Wrecking Crew.