Language Attrition and Reactivation in a Simultaneous Japanese–English Bilingual Teenager: A 6-Year Longitudinal Psycholinguistic and Neurolinguistic Case Study

Aims and objectives/purpose/research questions: This study examines long-term bilingual (attrition/reactivation) trajectories in a simultaneous Japanese–English bilingual adolescent during two consecutive cross-national moves. It asks the following: (1) To what extent do the data support the regression hypothesis on a multiyear timescale? (2) When patterns deviate from monotonic loss/recovery, do they align with complex dynamic systems theory (CDST)? (3) Does change in brain activation precede subsequent psycholinguistic attrition or reactivation? and (4) How susceptible is a simultaneous bilingual who moves postpuberty to environment-driven attrition relative to children versus adults? Design/methodology/approach: A 6-year longitudinal single-case design tracked a simultaneous Japanese–English bilingual who moved from the United States (birth–16) to Japan for 3 years of local schooling and then returned to the United States for 3 years of tertiary education. Annual psycholinguistic and neurolinguistic assessments were administered across both relocations. Data and analysis: Yearly behavioral (psycholinguistic) measures and brain-based indices (neurolinguistic activation) were analyzed to characterize language-specific change, cross-linguistic coupling, non-linear trends, and lead–lag relations between neural and behavioral indices across attrition and reactivation phases. Findings/conclusion: (1) A subset of outcomes supports the regression hypothesis, but over a considerably longer horizon than typically reported, indicating multiyear rather than monthscale dynamics. (2) The remaining outcomes exhibit non-linear, context-sensitive fluctuations consistent with CDST. (3) Shifts in brain activation preceded and predicted later behavioral attrition/reactivation, suggesting neurofunctional reconfiguration as a precursor of performance change. (4) Despite simultaneous bilingualism, the adolescent was not impervious to postpubertal, environment-driven attrition; susceptibility appeared attenuated relative to younger children and closer to adult-like resilience. Originality: The study integrates annual psycholinguistic and neurolinguistic evidence across two transnational moves, demonstrating that neural change can foreshadow behavioral attrition/reactivation in a simultaneous bilingual adolescent. Significance/implications: Findings refine models of bilingual maintenance and loss by linking long-horizon trajectories to both regression hypothesis and CDST accounts, and by positioning neural reconfiguration as an early marker for forthcoming behavioral change—informing timing and targets for support during educational transitions.