Tau Brain Imaging in Typical and Atypical Alzheimer's Disease (AD)

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