Patients with memory disorders are experiencing different trends which are difficult to predict. Moreover, the distinction between Lewy body disease and Alzheimer's disease is not easy as both diseases can present similar symptoms. Nowadays, routine examinations exist and can improve the diagnosis but there are not specific enough of one of those two pathologies.

Lewy body disease is characterized by the presence of particular structures in patient's brain, called "Lewy body", composed of a protein called "alpha-synuclein". The aim of this study is to measure the rate of alpha-synuclein in cerebrospinal fluid. This measurement could allow us to differentiate patient with Alzheimer's disease from those with Lewy body disease.

Inclusion Criteria:

• At least 45 years of age
• Subject agrees to participate in the study
• Subject meets the criteria Dubois et al., 2007. for diagnosis of probable AD.
• Subject with a syndromic presentation of mild cognitive impairment or mild dementia
• Subject has a study partner who is able to provide accurate information about the patient
• Subject affiliated to a social security scheme

Exclusion Criteria:

• Patient who meets both AD and Lewy Body disease criteria
• Contraindications to lumbar puncture
• Contraindications to an MRI scan
• Subject who is not affiliated to a social security scheme
• Subject under guardianship or curatorship
• Subject under judicial protection
• Subject refuses to participate in the study
• Subject has evidence of clinically relevant neurological disorder (fronto-temporal dementia, brain tumor, stroke, …) that led to persistent cognitive disorders
• Subject has evidence of clinically relevant psychiatric disorders (schizophrenia, major depression, …)
• Pregnant or attempting to become pregnant women
• Subject's vision and audition is sufficient for neuropsychological tests's assessment, based on the investigator's judgment.