A dependency-network study of syntactic connectedness in the connected speech of Alzheimer’s disease

Background Research on discourse impairment in Alzheimer’s disease (AD) has emphasized coherence, with less attention to how syntactic relations are organized.Aims To examine syntactic connectedness in AD discourse and interpret network differences through lexical and grammatical resources.Methods & Procedures Cookie-Theft descriptions from 150 Mandarin speakers [50 persons with AD, 50 healthy older adults (HCO), and 50 healthy young adults (HCY)] were converted into dependency networks. Analyses examined global network properties and size-adjusted participant-level density. Additional analyses examined lexical diversity (MATTR) and the association between network density and MMSE scores within the AD group. Centrality of shared words and max k-core composition were compared across aggregated group networks.Results All groups showed small-world organization and similar degree-distribution patterns. Adjusted density was significantly higher in HCO than in AD and HCY, with no significant AD–HCY difference. Within the AD group, MMSE was not significantly associated with adjusted density. Both older groups had lower MATTR than HCY, without a significant AD–HCO difference. In the aggregated networks, betweenness was significantly lower in Net-AD than in Net-HCO for shared function words and than in both control networks for shared content words. Function-word proportions were highest in Max k-coreAD, intermediate in Max k-coreHCO, and lowest in Max k-coreHCY, with a significant difference only between the AD and HCY cores.Conclusions The findings suggest more restricted syntactic combinations in the AD group than in HCO, alongside possible difficulties in referential organization and event integration. Both older groups may share compact network organization around common grammatical frames. The graded increase in core function-word proportions across groups may reflect a compensatory trade-off with content words, helping older adults with and without AD sustain syntactic organization when lexical retrieval is constrained.