My grandfather with dementia, in a moment of clarity, asked to go through an old photo book to point out everybody who was dead and laugh

I believe that is called the "window of lucidity" and, somewhat ironically, confuses people unfamiliar with dementia and Alzheimer's.

My grandfather never lost his sense of humor, but as his Alzheimer's increased, it got lumped in with all the other weird behavior. I got it, though, and I told him I understood. I really treasure that.