The study of discourse is of particular interest for understanding the linguistic and cognitive difficulties of people with neurodegenerative diseases. Analyzing fluency and disfluencies (hesitations, reformulations, interruptions, errors, pauses, prosodic organization) in such studies is particularly crucial since they reflect a good command of language skills and allow us to distinguish between different types of impairments, in particular with regard to primary progressive aphasias (PPA). These are characterized by predominant deficits in language skills and are considered as atypical variants of Alzheimer's disease on the one hand and frontotemporal dementias on the other. A distinction is made between fluent - semantic and logopenic - and non-fluent variants of PPA. In an elderly population with or without neurodegenerative pathology (typical or atypical Alzheimer's disease and front-temporal dementias), this project aims to: 1) characterize the nature of the disfluencies observed during oral production of discourse and during reading; 2) specify the understanding of their neural and cognitive causes according to the type of disorders; 3) investigate the impact of a potential history of oral or written language developmental disorders on their manifestations.