Intraindividual variability and language instability in post-stroke aphasia: Effects of working memory demand and associations with language performance

• Language performance was more variable in people with post-stroke aphasia than in controls. • Intraindividual variability increased under high working memory demand in aphasia. • Greater performance variability was linked to poorer language outcomes. • Variability provides additional insight beyond mean scores in language performance. • Mixed-effects models effectively captured intraindividual variability. Intraindividual variability (IIV) – within-person fluctuations in performance – has been linked to cognitive vulnerability and poorer outcomes in aging and clinical populations but remains understudied in post-stroke aphasia, despite its potential to reveal functional instability and sensitivity to task demands beyond average performance measures. This study examined whether IIV in language performance, measured between sessions (BS-IIV) and within sessions (WS-IIV) across phonological, lexical, and semantic tasks, differs between individuals with aphasia and healthy controls, and how it relates to working memory (WM) demand and standardized language measures. Fifty-five individuals with aphasia following left-hemisphere stroke and 17 neurotypical adults completed three auditory two-alternative forced-choice tasks under low and high WM demand, across four consecutive days. WS-IIV was significantly higher in the aphasia group across all tasks and conditions. BS-IIV was also elevated in aphasia, except in the low WM condition of the lexical and semantic tasks. Within the aphasia group, IIV increased under high WM demand for lexical and semantic tasks – a pattern not observed in controls. Most correlations between IIV and standardized language scores were negative, suggesting that greater variability relates to poorer language outcomes, although only a subset reached statistical significance. For instance, BS-IIV in the semantic task (low WM) was significantly negatively correlated with the WAB-AQ, while WS-IIV in the lexical task (low WM) showed a similar association. These findings suggest that WM demand contributes to language instability in aphasia, and that IIV may serve as a complementary marker to mean performance measures.