I think the word "regressed" is bit wrong here. The degeneration of white matter in their brain basically simplified their brains. They lost more of the advanced connections in the brain, the things that develop with age and we call maturity.
By what I read it is a fucking awful way to go. You end up in vegetative state then just fade away after a while.
This is one of the diseases that is included in the discussion of assisted suicide and euthanasia.
I feel bad for those poor brothers and their family, it is an awful way to go.