Syntactic complexity in Mild Cognitive Impairment due to Alzheimer’s disease (MCI-AD): long-distance dependencies and working memory in comprehension of relative clauses

Language impairments are often observed in neurodegenerative disorders, but the role of short-term phonological and working memory in sentence processing remains unclear. This study assessed sentence comprehension in 18 individuals with Mild Cognitive Impairment due to Alzheimer’s disease (MCI-AD) and 18 age-matched controls using a sentence-to-picture matching task. The task involved relative clauses with syntactic complexity manipulated through sentence type, branching direction, and linear length. Cognitive and memory skills were also evaluated. Participants with MCI-AD showed greater difficulty comprehending object-relative clauses and center-embedded structures, with no effect of linear length. The asymmetry between subject- and object-relative clauses confirms that filler-gap dependencies across an intervening noun phrase significantly affect sentence comprehension and that centre-embedding structures built on the subject increase the processing load. This is arguably because the matrix subject is separated from the main verb with which it agrees. Our study also highlights the role of the working memory resources necessary to compute filler-gap dependencies in structures of intermediate complexity, such as centre-embedding SRs. These findings indicate that working memory limitations modulate sentence processing difficulties in MCI-AD, and that their role is visible depending on the grammatical complexity of the structures involved.