Morphological processing in Alzheimer’s disease

Abstract This study investigated the degree to which cognitive mechanisms support word recognition and word inflection in aging and how this changes in Alzheimer’s disease (AD). We tested competing hypotheses regarding the functional organization of language within the broader cognitive system. One set of hypotheses, derived from dual-system theories like the Declarative/Procedural (DP) model, predicts a functional architecture segregated by linguistic function. An alternative set of hypotheses posits a more integrated architecture, organized by task demands and resource availability. We analyzed participants’ performance on a lexical decision task and a word inflection task, alongside neuropsychological tests, using both behavioral and network analyses. In healthy controls (HC), the network analysis revealed a highly integrated architecture where language tasks were clustered by functional demands (e.g., speed vs. accuracy) rather than segregated along a strict lexicon/grammar divide. In the AD group, behavioral results showed a classic dissociation, with disproportionate impairment on irregular word inflection — a pattern traditionally seen as evidence for a modular memory failure. However, our network analysis revealed a different underlying mechanism. We observed a dramatic network reorganization where a core declarative memory module became functionally isolated, causing language tasks to form new, compensatory alliances with remaining frontal-executive resources. This provides clear evidence of a shift where executive functions are recruited to support language abilities when dedicated memory systems decline. These findings suggest that the cognitive substrate for language is not static but adapts dynamically in neurodegeneration, shifting its reliance from failing declarative memory systems to domain-general executive control pathways.