Untangling the Temporal Dynamics of Bilateral Neural Activation in the Bilingual Brain

A persistent unanswered question in cognitive neuroscience has been what are the neural origins of human brain lateralization? Language is strongly lateralized to the left-hemisphere, however, lateralization varies with language experience. Bilinguals demonstrate a greater extent and variability of right-hemisphere involvement for language relative to monolinguals. Here, bilingualism is used as a lens into the conditions that drive brain lateralization. Why does bilingual language processing yields more robust bilateral neural activation relative to monolingual language processing? Neural activation and functional connectivity were measured to test hypotheses about the temporal dynamics of hemispheric recruitment during language processing in monolingual and bilingual children with varying ages of first bilingual language exposure. Hypothesis (1), The human brain is strongly left-hemisphere lateralized for language, but, when faced with the demands of two languages, additional right-hemisphere resources are recruited. Hypothesis (2), The human brain has the potential for enhanced dual hemispheric language processing that can be either potentiated or not based on early life bilingual versus monolingual language experience. If dual language experience places increased cognitive demands on the bilingual brain requiring additional right-hemisphere resources, asynchronous neural activation in left and right hemispheres was predicted. If dual language experience potentiates dual hemispheric language processing, synchronous neural activation in left and right hemispheres was predicted. Furthermore, only early-exposed bilinguals but not later-exposed bilinguals or monolinguals, would show synchronous neural activation across the hemispheres. Early experience with one language (monolinguals) or two languages at different times during a child's development (early-exposed bilinguals, later-exposed bilinguals) revealed differences in the time-course of activation across the two hemispheres' language areas, supporting Hypothesis (2). Monolinguals and later-exposed bilinguals showed asynchronous activation between the hemispheres. Early-exposed bilinguals showed synchronous activation between the hemispheres. The results provide a new view on how different experiences can drive lateralization in development and reveal the neural basis of bilateral activation in the bilingual brain. Synchronous temporal accessing of the hemispheres in bilinguals suggests early life bilingual language experience may support more equal and efficient hemispheric involvement, and, in turn, constitute the brain-based mechanism that makes possible the widely observed linguistic and cognitive advantages in young bilinguals.Ph