How the brain acquires language is a topic that has been of interest to researchers over the last few decades. Native level language proficiency has been attributed to the critical period after which one does not reach native-level proficiency. This is because the brain wires as per the regularities of the first language and commits to it. This is explained by the native language neural commitment hypothesis proposed by Patricia Kuhl, a researcher who studies language development in infants. According to this hypothesis, learning a second language(L2) outside the critical period plasticity of the brain becomes difficult compared to the first language acquisition during the critical period.
Despite this, some people do acquire native-level proficiency in L2. There has been a lot of debate regarding second language acquisition. The main would be whether L2 is facilitated by neuroplastic changes outside the critical period or the brain adopts different strategies for the same.
A recent study in 2020 studied L2 acquisition in Chinese-English bilinguals who are native Chinese speakers. They measured the structural and functional activity of the brain through imaging as they learned the L2. They found an increase in neuronal activity in areas responsible for cognitive control, thus showing that learners had to learn L2 effortfully. They also showed increased activity in the language areas of the brain. Interestingly, the increase in activity was positively correlated to the L2 proficiency exhibited by the learners. The learners had a larger gray-matter volume, which suggests that neuroplastic changes mediate the learning of L2. Another interesting finding is that there was increased activation in the right fusiform gyrus as they learned L2. The researchers suggest that it might be due to the Chinese characters' logographic nature. This indicates that the brain still retains the native language networks.
This study highlights that both L1 and L2 shape the brain through neuroplasticity mechanisms, but the influence of L1 circuits on the learning of L2 would be an interesting topic to look into!
Wang, R., Ke, S., Zhang, Q., Zhou, K., Li, P., & Yang, J. (2020). Functional and structural neuroplasticity associated with second language proficiency: An MRI study of Chinese-English bilinguals. Journal of Neurolinguistics, 56, 100940.