Advancing ERP-based tools for language monitoring in chronic aphasia: psychometric and clinical utility of the word-level N400.

The N400 is a potential biomarker for cognitive-linguistic recovery in people with aphasia (PWA), as several studies have demonstrated treatment-induced changes. However, the validity, reliability, and functional significance of the N400 as a repeated measure in this population remains unverified, whether elicited through sentence- or word-level paradigms. This study addresses these key questions required for the N400 to be utilized as a biomarker of recovery in PWA. First, we developed and validated word-level semantic and phonological tasks to elicit the N400, compared group responses between healthy controls and PWA, examined the reliability and stability of N400 across groups, and explored relationships between the N400 and behavioral language performance. Our findings demonstrate successful validation of semantic and phonological tasks that elicit the N400, and two control tasks that do not. These tasks elicited N400 responses in PWA and controls, indicating similar underlying processing, albeit with greater variability in PWA. Reliability and stability of the N400 were mixed, indicating that caution is warranted when using N400 as a treatment response measure. Critically, condition-specific N400 amplitudes were significantly associated with functional language measures, highlighting the behavioral relevance of this neurophysiological signal. Together, these results provide validated elicitation tools, clarify limits on N400 reliability, and identify meaningful brain-behavior relationships, advancing the N400 as a more precise and interpretable biomarker for tracking recovery and treatment outcomes in aphasia.