Enhanced efficiency in the bilingual brain through the inter-hemispheric cortico-cerebellar pathway in early second language acquisition

The bilingual experience has a profound impact on the functional and structural organization of the brain, but it is not yet well known how this experience influences whole-brain functional network connectivity. We examined a well-characterized large sample (151 participants) of monolinguals and bilinguals with varied age of second language acquisition, who underwent resting-state functional magnetic brain imaging. We constructed comprehensive functional brain networks for each participant, encompassing cortical, subcortical, and cerebellar regions of interest. Whole-brain analyses revealed that bilingual individuals exhibit higher global efficiency than monolinguals, indicating enhanced functional integration in the brain. Moreover, the age at which the second language was acquired correlated with this increased efficiency, suggesting that earlier exposure to a second language has lasting positive effects on brain functional organization. Further investigation through the network-based statistics (NBS) approach indicates that this effect is primarily driven by heightened functional connectivity between association networks and the cerebellum. This work shows that early learning enhances global whole-brain efficiency and that the timing of learning of two languages has an impact on functional brain organization. Significance statement Long-term learning impacts brain organization at different spatial scales, and this may be particularly enhanced during early stages of life. Bilingualism offers a unique opportunity to test long-term learning effects in the human brain, given that exposure to a second language can occur from birth or later in life, and can be maintained over long periods of time. We found that second language acquisition in early childhood (before five years of age) enhances brain network efficiency, and that this effect goes beyond the language and cognitive control regions, in fact, the interhemispheric cortico-cerebellar circuit plays a key role. This work shows that the timing of bilingual learning experience alters the brain functional organization at the global and local levels. ### Competing Interest Statement The authors have declared no competing interest.