A dependency distance approach to the syntactic complexity variation in the connected speech of Alzheimer’s disease

Previous research on the syntax complexity variation in individuals with Alzheimer’s disease primarily focuses on isolated language features like sentence length or syntactic structure, with limited exploration of their connection to memory and cognitive deficits. This study investigated the syntactic complexity variation in AD by associating working memory load with fine-grained syntactic features adopting a dependency distance approach. The findings include: (1) AD patients had shorter mean dependency distances (MDD) and more head-final dependencies compared to the HC group; (2) The patients still produced their speech under the DDM tendency but exhibited a stronger preference for simple syntactic structures; (3) Sentences produced by AD patients were statistically shorter than the HC group, and the patients exhibited greater difficulty handling syntactic structures as sentence length increases; (4) The patients could employ different syntactic structures but perform worse in extending them, particularly in nominal groups and hypotactic constructions; (5) The patients showed stronger reliance on sentence-initial adjuncts and inverted constructions. These findings suggest that syntactic impairments in AD exhibit a potential hierarchical decline along the continuum of syntactic complexity, with working memory deficits being a critical factor.