So I am going to be talking about hallucinations and a particular sort of visual hallucination, which I see among my patients. A few months ago, I got a phone call from a nursing home where I work. They told me that one of their residents, an old lady in her 90s, was seeing things, and they wondered if she’d gone bonkers or, because she was an old lady, whether she’d had a stroke, or whether she had Alzheimer’s.

https://www.ted.com/talks/oliver_sacks_what_hallucination_reveals_about_our_minds