MAPT and Mental deterioration: Those processes are thought to begin 15 to 20 years before cognitive decline is observed, although it is still unknown: (a) which factors influence Aβ deposition, a critical early pathogenic step; (b) whether individuals accumulating Aβ will inevitably develop cognitive symptoms of AD dementia should they live long enough and, if so, what influences the timing of this; and (c) how Aβ interacts with other brain pathologies, including tau deposition and vascular pathology, to cause cognitive decline.