Inclusion Criteria: Over the age of 21 Must have an informant who will be able to provide independent evaluation of functioning English is primary language All subjects must have insidious onset, report progression of their symptoms, and meet current clinical diagnostic criteria for Logopenic Aphasia (LPA) or Posterior Cortical Atrophy (PCA). All subjects with Logopenic Aphasia (LPA) must present with early and dominant impairments in language. Exclusion Criteria: If you have had a stroke or tumor that could explain your symptoms Subjects that present with early episodic memory impairment or meet clinical criteria for mild cognitive impairment will not be recruited into the study Subjects that meet specific criteria for another neurodegenerative disorder, including behavioral variant frontotemporal dementia, semantic dementia, primary progressive apraxia of speech, probable corticobasal syndrome, or progressive supranuclear palsy, will be excluded Subjects will be excluded if they have poor vision (20/400) Women that are pregnant or post-partum and breast-feeding will be excluded Subjects will be excluded from the study if they are unable to undergo the tau-PET scan due to a prolonged QT interval on ECG, or if they have any of the following genetic conditions which can increase the chance of cancer: Cowden disease, Lynch syndrome, hypogammaglobulinemia, Wiskott-Aldrich syndrome, and Down's syndrome Subjects will also be excluded if MRI is contraindicated (metal in head, cardiac pace maker, e.t.c.), if there is severe claustrophobia, if there are conditions that may confound brain imaging studies (e.g. structural abnormalities, including subdural hematoma or intracranial neoplasm), or if they are medically unstable or are on medications that might affect brain structure or metabolism, (e.g. chemotherapy).