How the brain distinguishes a bilingual's languages is partly shared across people and partly their own

Abstract

Cognitive neuroscience often seeks brain activity that is consistent across people. Individual differences in brain organization have received less attention. Bilinguals, in particular, remain understudied at the individual level, as neuroimaging has mostly approached them as a group. We analyzed functional magnetic resonance imaging (fMRI) data from 77 Chinese-English bilinguals. We decoded each person's current language state, Chinese or English, from cortex-wide activity and assessed cross-person transfer across participant pairs. We identified a low-dimensional population representation, derived outside each tested pair, that was sufficient to support transfer, with performance nearly matching that of the full cortical pattern. Variation in this transfer tracked how early participants had learned English. Removing these dimensions barely changed within person accuracy, but transfer progressively declined until it collapsed. Our findings reveal two levels of organization: a common component that supports transfer between brains and an individual component that remains reliably stable within a person across sessions.