From my experience of treating people with stroke and dementia, its not their true selves that are revealed, rather a disinhibition of behaviour. They may have always found the next door neighbour irritating but now they will tell them so or if something annoys them, instead of stopping at words, they will attack or throw things.
I did treat one lady who, we believed had had a TIA (transient ischaemic attack)...kind of a temporary stroke. She had been admitted through A and E...a concerned neighbour had called the police when her milk was left on her doorstep (this was mid 70's) and she had been found collapsed with typical one sided weakness. When I met her she was perfectly well and fit...a friendly outgoing chatty old lady....Then her vicar visited and asked privately if mrs xxx was ok. We couldn't tell him stuff of course but he could tell us stuff so I asked why and he said all the time she had known her, she had been very quiet and withdrawn. A regular churchgoer who never stopped for coffee after the service or said any more than good morning to any of the congregation. When I took her on a home visit, the neighbour who had called the police dropped in and was greeted cheerfully and offered tea which mrs xxx made herself. It was a very pleasant if unecessary assessment. Then as we left the neighbour made the same comment about how changed mrs xxx was. She went home and as she was perfectly fit, we passed monitoring her health back to the GP and I never heard of her again. All we could conclude back at the hospital was that the stroke had disinhibited her, only on her case it had made her more outgoing and social.