As monstrous as neurodegenerative disease is, its powers of identity theft have been greatly exaggerated. Remarkably, a person can undergo significant cognitive change and still come across as fundamentally the same person.
What makes us recognizable to others resides almost entirely within a relatively narrow band of cognitive functioning. It is only when our grip on the moral universe loosens that our identity slips away with it.
Your Brain, Your Disease, Your Self - The New York Times