The Bookcase Theory
There is a theory called the book case theory for a dementia person. This is described as having two bookcases, one is made out of solid oak and this one is full of your emotion. The other bookcase is made out of cardboard, and this one is full of your memories. Then you think of dementia like a earthquake, and when it hits it knocks one of the books of the cardboard bookcase falls of and the person loses that memory. Yet the oak bookcase, stays standing tall and the person still gets to keep their emotions. This explains when I used to see my grandma who had sever dementia, she would always be happy to see me, but didn't know my name.
I drew this picture as a visual representation of what I would imagine it to look like in the spoofed of the mind palace. This one was designed after my grandma as its from her time line, as you can see she still keeps all of her emotions, but is missing a fair few of her memories. Especially her most recent memories are gone. That’s how dementia works it erases your most recent memories first, and is like there going back in time in their own heads.