Have you been watching The Alzheimer's Project on HBO? I think it's still in rotation (if not, it's available on HBO OnDemand). I'll paste below, too.
In the first episode, a 78-year-old man named Woody Geist reunites with the Grunyons, his a cappella singing group. Geist has been suffering from Alzheimer's for 14 years, and often doesn't recognize his own family. But the filmmakers capture him singing along with the Grunyons—the lyrics and vocal arrangements embedded in some untouchable corner of his mind. Clip is here, 49 minutes in. Heartbreaking.
Thank you to Mark for the tip.