Neural hemispheric organization in successful adult language learning: Is left always right?
Abstract Learning a new language during adulthood is a markedly difficult and heterogeneous process. Whether language learning engages the same left lateralized language network as native language processing or a more distributed network is currently unknown. One approach to address this question is to evaluate the contributions of each cerebral hemisphere in successful language learning. The current review adopts an individual difference approach and provides a systematic overview of (1) the neural factors that predict various language learning outcomes and (2) neuroplastic effects of successful language learning. Our analysis shows that, prior to learning, the neural characteristics of the left hemisphere predominantly predict future speech sound learning. However, more higher level learning tends to be predicted by a more distributed network including the right hemisphere and bilateral brain structures. Over the course of language learning, both hemispheres show structural and functional malleability. We argue that a dynamic bilateral framework involving neural correlates both within and between the two hemispheres underlies the ultimate success of language learning. Learners' native language network (the leftward functional organization for language processing) is related to learning success at the speech sound and word levels. However, when learning involves greater complexity, the initial recruitment of the right hemisphere and the subsequent functional shift from right to left and bilateral hemispheres are essential to ensure successful attainment.