A corpus-based dependency study of the syntactic complexity in the connected speech of Alzheimer’s disease

Abstract

ABSTRACT Background While numerous studies observed notable changes in syntactic complexity among individuals with Alzheimer’s disease (AD), these studies predominantly concentrated on isolated internal structures of language but rarely defined the syntactic complexity variation in AD on a continuum. Given that working memory load exhibits a continuous rather than binary pattern in populations and plays a crucial role in syntactic processing and generation, research on syntactic complexity changes in AD could benefit from expanding to include this perspective. Aims To examine the probabilistic distribution of syntactic complexity in AD under controlled sentence length conditions from the perspective of working memory, and to develop a comprehensive linguistic profile of syntactic complexity variation in AD by analyzing fine-grained syntactic features. Methods & procedures The corpus materials consist of descriptions based on the Cookie-Theft picture component provided by 70 individuals with AD (mean MMSE = 20.97) and 70 cognitively intact elderly individuals (mean MMSE = 29.21) from the DementiaBank clinical language transcript dataset. We employed dependency distance to quantify working memory load and syntactic complexity. Additionally, three finer-grained dependency metrics, namely adjacent dependency distribution (1dd%), mean dependency distance (MDD) and dependency direction distribution, enable a deeper interpretation of the syntactic complexity variations in AD from different perspectives. Results (1) The distribution of dependency distances in AD was similar to that in the healthy control (HC) group, both following the Zipf-Alekseev model, and the variations of the parameters within the model were analogous between the two treebanks; (2) The performance of AD patients in adjacent dependency, MDD and dependency direction differed from that of the HC group across varying sentence lengths; (3) “Simplified” and “ungrammatical” structures were the primary syntactic features in AD. Conclusions These findings confirmed the presence of syntactic impairments in AD. The decline of syntactic complexity can be attributed to the failure of the “End Weight” principle due to working memory deficits in AD patients. The study contributes to the easier identification of AD patients and more effective working memory interventions, thereby improving language production in AD patients.