Lighting up word retrieval: A scoping review of fNIRS studies on lexical access in healthy and neurodegenerative...

Lexical access is a fundamental process in language production, commonly investigated using verbal fluency (VF) and rapid automatized naming (RAN) paradigms. Despite its clinical relevance, especially in neurodegenerative disorders, cortical mechanisms supporting lexical access remain poorly understood. Functional near-infrared spectroscopy (fNIRS) enables the investigation of task-evoked cortical dynamics during overt speech, yet the literature has lacked a systematic mapping. This scoping review summarizes current research practices and findings to inform methodological refinement and clinical translation. A structured search of MEDLINE, Embase, PsycINFO, and Web of Science identified 30 eligible English-language original studies that used continuous-wave commercial fNIRS to study lexical-access tasks in healthy subjects and/or people with neurodegenerative disorders. Publication volume increased markedly after 2012 and was conducted predominantly in Asia. Verbal Fluency was the only paradigm employed, and no studies investigated RAN with fNIRS. Protocols predominantly used block designs with 30 or 60 s of active task blocks interleaved with rest and/or control blocks. Cortical coverage focused primarily on the prefrontal cortex, with less consistent sampling of broader configurations. Preprocessing and modelling pipelines showed substantial heterogeneity; HbO-based amplitude outcomes and block-averaging approaches predominated. The fNIRS literature on lexical access in healthy subjects and neurodegenerative cohorts is currently based on VF tasks and focused on the PFC, with limited methodological standardization. Future studies should diversify paradigms, expand network-level coverage, and harmonize methodologies and analysis practices to support clinical translation.