Cortex-Wide Language Organization in Bilingual Brains Reflects Individual Language History

Overview & Synopsis

Experience shapes the brain, and bilingualism offers a valuable case for studying this relationship. Yet bilingual neuroimaging research has mostly focused on group-level differences, leaving unclear how a speaker’s two languages are distinguished across the cortex within individuals. We addressed this question using functional magnetic resonance imaging (fMRI) data from 77 sequential Chinese (L1)-English (L2) bilinguals performing a cued picture-naming task. Single-trial beta estimates were projected onto the Schaefer-800 parcellation and analyzed at two spatial scales using cross-run multivariate logistic regression classifiers. Local decoding was summarized as cortical extent, and cortex-wide decoding as cortical strength. The two measures were positively related, but captured different aspects of cortical language differentiation: local fine-grained configurations versus broader parcel-mean activity patterns. Decomposition of the cortical measures converged in showing relatively weak contributions from visual and limbic systems, and stronger contributions from language-semantic-responsive, control, and salience systems. Language identity was decoded more strongly than trial-switching status, although language decoding was weaker following a switch. Finally, cortical decodability predicted second-language history in held-out participants, with stronger and more extensive differentiation in later and less proficient bilinguals. Our findings reveal a distributed, multiscale cortical signature of bilingual language production that varies meaningfully across individuals, pointing to the importance of individual differences in understanding how bilingual brains represent their languages.