Formulaic and novel language dissociate in Alzheimer’s clinical syndrome: evidence for the Dual-Process Model

INTRODUCTION: Spontaneous speech is commonly disrupted in persons with Alzheimer's disease (AD) and/or Alzheimer's clinical syndrome (ACS). Importantly, different aspects of speech (e.g. formulaic versus more novel or flexible speech) place different demands on distinct cognitive systems. Formulaic language may rely on automatized procedural processes, while more novel or diverse speech requires more flexible lexical-semantic processes associated with the subsystems of declarative memory. Given that AD/ACS are associated with impaired declarative processes and relatively spared procedural processes, we predicted that individuals with ACS may show increased reliance on formulaic language along with reduced diversity in speech. METHOD: We analyzed the spontaneous speech of 81 individuals with ACS (aged 56-88) and 61 healthy controls (aged 47-80) who completed a picture description task using computational tools for the analysis of formulaic language (operationalized as proportion of frequent trigrams produced and mutual information score of trigrams) and novel language (operationalized as root type-token ratio, measure of textual lexical diversity, and semantic diversity). RESULTS: Across all measures, individuals with ACS produced significantly more formulaic language than control participants and significantly less novel language than control participants, with small-to-medium overall model effect sizes. Machine learning classifiers trained on these patterns of formulaic and novel language distinguished between controls and individuals with ACS with reasonable accuracy, sensitivity, and specificity. CONCLUSION: The spontaneous speech of individuals with ACS contains more formulaic language and less novel language than that of healthy controls, consistent with the Dual-Process Model. These differences may have clinical relevance and warrant further investigation. Keywords: Alzheimer's Disease, Alzheimer's clinical syndrome, formulaic language, lexical diversity, spontaneous speech.