Theory of mind deficits in non-fluent primary progressive aphasia

Theory of Mind (ToM) is a complex socio-cognitive subdomain that is under-researched in neurodegenerative disorders, particularly in persons with primary progressive aphasia. We studied 14 persons with non-fluent/agrammatic variant primary progressive aphasia (nfaPPA), and asked two questions: (1) whether persons with nfaPPA have intact or impaired ToM, with emphasis on their false belief attribution abilities, relative to healthy controls; and (2) whether false-belief attribution (a component of ToM) is associated with their syntactic and executive function (EF) abilities. False belief understanding was tested through nonverbal videos, with participants deciding whether the story ending was an appropriate end of each video scenario or not. Syntactic production abilities were measured through repetition of syntactically simple and complex sentences (comprising length-matched complement and adjunct sentences), and EF tasks, specifically, a digit-back and an attention-shifting task. Persons with nfaPPA were less accurate than controls in adapting their reasoning to the false beliefs of other agents in the nonverbal videos of the false belief attribution task. Their false belief attribution performance was significantly predicted primarily by their syntactic production, followed by their EF. The overall findings suggest that persons with nfaPPA may have impaired performances in ToM tasks, due to impairments in basic non-social cognitive functioning, such as syntactic and EF abilities.