Alzheimer's disease may damage the brain in two distinct phases, based on new research using sophisticated brain mapping tools. According to researchers who discovered this new view, the first, early phase happens slowly and silently -- before people experience memory problems -- harming just a few vulnerable cell types. In contrast, the second, late phase causes damage that is more widely destructive and coincides with the appearance of symptoms and the rapid accumulation of plaques, tangles, and other Alzheimer's hallmarks.