Neuroplasticity in adult second language learning: a systematic scoping review of structural and functional neuroimaging evidence.

Background: Adult second language (L2) learning offers a valuable model for studying experience-dependent brain plasticity beyond early development. Neuroimaging studies have reported structural and functional changes associated with L2 acquisition, but findings remain heterogeneous and influenced by differences in study design, learner age, training intensity, and imaging modality. In particular, the relationship between structural remodeling and functional reorganization, and their relevance to aging and cognitive reserve, remains unclear. A narrative synthesis that maps and critically integrates this diverse evidence base is therefore needed to clarify current patterns, limitations, and open questions in adult L2-related neuroplasticity. Methods: We conducted a systematic scoping review of longitudinal and cross-sectional neuroimaging studies examining adult L2 learning-related plasticity. Following PRISMA 2020 guidelines, literature searches were performed in PubMed, Scopus, Web of Science, and PsycINFO up to June 2025. Fourteen studies on structural MRI, diffusion imaging, functional and MRI methodologies were included. Given substantial heterogeneity in study designs, populations, and outcome measures, findings were synthesized narratively, with particular emphasis placed on longitudinal and intervention-based evidence. Results: Across studies, adult L2 learning was associated with measurable neuroplastic changes at both structural and functional levels. In younger adults, intensive training was linked to gray matter alterations within frontal and temporal language regions and to white matter reorganization in fronto-temporal pathways. Functional imaging studies reported changes in task-related activation and intrinsic connectivity within language and cognitive control networks. In older adults, evidence for functional reorganization was more consistent than evidence for structural change, while support for L2-related cognitive reserve effects remains limited and preliminary. Conclusion: The available evidence suggests that adult L2 learning engages dynamic and use-dependent neuroplastic processes, characterized by an interaction between functional reorganization and more gradual structural adaptation. While the adult brain retains a notable capacity for learning-related change, the expression of plasticity appears sensitive to age, training intensity, and methodological factors. Functional reorganization may represent a relatively preserved marker of plasticity in later adulthood. However, claims regarding durable structural change and cognitive reserve require further longitudinal investigation. Systematic review registration: CRD420251140530.