From what I understood from Terry's letter, he has a form of this that I had never heard of before:

I have posterior cortical atrophy or PCA. They say, rather ingenuously, that if you have Alzheimer’s it’s the best form of Alzheimer’s to have. This is a moot point, but what it does do, while gradually robbing you of memory, visual acuity and other things you didn’t know you had until you miss them, is leave you more or less as fluent and coherent as you always have been.

I spoke to a fellow sufferer recently (or as I prefer to say, ‘a person who is thoroughly annoyed with the fact they have dementia’) who talked in the tones of a university lecturer and in every respect was quite capable of taking part in an animated conversation.

Nevertheless, he could not see the teacup in front of him. His eyes knew that the cup was there; his brain was not passing along the information. This disease slips you away a little bit at a time and lets you watch it happen.

It sounds like he's going to be in there knowing what's happening the whole damn time. It sucks.