Beyond cognitive slowing: potential evidence for specific lexical-semantic impairment in multiple sclerosis

Background Expressive language difficulties are frequently reported by people with multiple sclerosis (PwMS), yet they often go undetected during routine neuropsychological assessments. This study examined expressive language performance in MS group as compared to healthy controls (HC), focusing on lexical access and semantic processing, to determine whether these deficits stem from generalized cognitive slowing or reflect an independent lexical-semantic impairment. Methods Performance level and reaction times (RT) were evaluated in both groups across specialized naming and semantic judgment tasks using the Cambridge Language Battery. Robust regression models were conducted to assess the predictive value of information processing speed (IPS), working memory (WM) and verbal fluency. Results Both groups showed high levels of accuracy, but PwMS had significantly longer reaction times across all tasks with large and consistent effect sizes. Robust regression analyses indicated a clear dissociation between the domains. General cognitive factors, particularly baseline IPS, explained a substantial proportion of variance (22% to 51%) in reaction times, largely reducing between-group RT differences after adjustment in all tasks. Accuracy in the Synonym Judgement Task was primarily related to semantic and phonemic fluency and, when their influence statistically accounted for, between-group differences were largely reduced. Conversely, even the best combination of these cognitive predictors explained less than 10% of the variance in accuracy for the naming tasks, where group differences remained largely unchanged. Discussion These findings demonstrate a robust dissociation between accuracy and speed in PwMS. While general cognitive slowing drives delayed responding, it fails to account for reduced accuracy on demanding language tasks, suggesting that subtle lexical-semantic dysfunction represents a partially independent feature of MS rather than a simple consequence of generalized cognitive slowing.