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

How personal is a cognitive distinction? People performing the same task show similar brain activity, which group-level neuroimaging builds on. Yet brain organization differs reliably between individuals, and people bring different experiences to every task. Bilingual language use is a natural test case. We asked whether the distinction between a bilingual’s languages is common to everyone performing the task, or whether each person keeps a part of their own. We decoded cortex-wide fMRI activity within each of 77 bilinguals naming pictures in Chinese, their dominant language, or English, switching languages from one moment to the next. Within a person, the language in use can be classified on single namings (balanced accuracy 59.6%, chance 50%). Classifiers trained on one person also transfer to others (56.9%). To locate what transfers, we estimated the main axes of trial-to-trial task activity in other participants, without language labels. Transfer runs entirely along these axes, and the first 50 alone match the full cortical pattern (57.2%). Without these axes, transfer between people nearly disappears, yet decoding within a person is unchanged. The remaining activity is stable enough across runs to identify most individuals (74%, chance 1.3%). Age of English acquisition is reflected along the shared axes, accounting for individual differences in transfer (cross-validated R2 = 0.19), but not in the remaining activity. The distinction between a bilingual’s languages is thus partly carried by axes common to the group, and partly by a stable signature specific to each person.