Many cognitively healthy older adults have, upon post mortem evaluations, been found to have varying amounts of neurofibrillary tangles (tau) and beta-amyloid plaque deposits, which are the hallmark brain pathologies known to be associated with Alzheimer's disease and various other dementias. While some with these pathologies may not clinically express cognitive decline or dementia in their lifetime, human post-mortem studies suggest that increasing neurofibrillary tangle density correlates with neurodegeneration and cognitive impairment.