Recently revised Alzheimer Disease (AD) diagnostic1described nonamnestic presentations: 1/ language presentation (logopenic progressive aphasia) 2/ visuospatial presentation (posterior cortical atrophy or PCA) and 3/ executive dysfunction. AD pathological changes may precede the clinical diagnosis of dementia of AD type for a while2. Biomarkers have been developed: biomarkers of brain amyloid-beta (Aß) (CerebroSpinal Fluid CSF concentration ßamyloid, molecular imaging with amyloid targeted PET ligands), biomarkers of neural degeneration (MRI hippocampal volume, regional metabolism as assessed by PET with [18F]-FDG) and may be used to made early detection of the neuropathology associated with AD Even if CSF biomarkers (tau, p-tau and β amyloïd are interesting to improve diagnosis of AD, they cannot provide topographic information. PET tau imaging seems to be promise to evaluate quantitative and spatial assessment of tau lesions both in AD and fronto-temporal lobar dementia.

The hypothesis of the research is that it exists a different regional pattern of tracer retention across brain regions according to clinical symptoms : temporal for logopenic aphasia and occipital for posterior cortical atrophy.

Inclusion Criteria:

• 50 years old and more
• native langage: french
• study level upper (or equal) than 7 year (considering first year of grammar-school as start)
• correct sensory abilities (auditive device allowed) for tests
• affiliation to social security
• Informed, written consent form
• for Alzheimer disease group: people with Alzheimer Disease defined as National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer's Disease and Related Disorders Association (ADRDA) standards: Light to mild AD defined by Mini-Mental State Examination (MMSE) score between 15 and 25 (included)
• for Benson disease group: Benson disease following Mendez et al (2002) and Tang Wai et al (2004) criteria
• for healthy volunteer group: normal MMS score (more than 26 for bachelor level)

Exclusion Criteria:

• history of disease with consequances on cognitive functioning (tumor, stroke, head trauma, etc.), cerebral surgery
• use of alchohol and/or drug
• anormalies in neurological exam (focal deficit) not included in the classic symptoms
• contraindication to magnetic resonance imaging (RMI)
• contraindication to PET: people with prolongation of QT interval or taking medication that can lead to "torsades de pointe".
• claustrophobia
• person with legal protection
• exclusion period because of participation to another experimental protocol and actual participation to an experimental protocol
• pregnant or lactating woman or able to procreate and without contraception