MAPT and Cognitive impairment: Tau-PET observations are in line with neuropathological findings and NFT Braak stages in human brain aging and AD and confirm that: a) tau pathology precedes by several decades the appearance of β-amyloid in brain aging without cognitive impairment; b) tau pathology may be found in some individuals suffering from cognitive impairment without concomitant β-amyloid deposition, and; c) tau pathology, rather than β-amyloid pathology, correlates with progressive cognitive decline in sAD.