Question: why do some people with dementia live in the past? what happens to the brain cells? do they just lose thread or do the cells die?

Susan Cartwright answered on 21 Jun 2015:

To answer the last question first, the brain cells die. IN Alzheimer’s disease, the commonest form of dementia, abnormal structures called neurofibrillary tangles and amyloid plaques form in the brain, damaging connections between brain cells and eventually causing cell death. These structures are made by proteins that do occur normally in the brain, but in these cases are folded up wrongly and cause damage. A misfolded protein, albeit a different one, is also the cause of Creutzfeld-Jakob disease (mad cow disease).

The reason that patients with dementia often seem to live in the past is that for some reason these damaging structures form preferentially in the regions of the brain associated with learning new facts and laying down new memories. Recent memories are stored preferentially in certain parts of the brain, particularly parts called the hippocampus and entorhinal cortex, whereas older memories are stored in a much more distributed fashion over the whole of the cortex (the outer layer of the brain, which is the bit you think with). Alzheimer’s disease strikes first in the region of the hippocampus, so it tends to destroy recent memories and the ability to form new memories and learn new tasks, while initially leaving older memories reasonably intact. Since the only memories the person can reliably recall are old ones, the person feels as though he or she is living at the time those memories were laid down – so, for example, your grandmother may forget that your grandfather is dead, or mistake your father for your grandfather (because in her accessible memories, her husband is still a younger man).