Language learning is a hallmark of human ability. This ability has traditionally been associated with the remarkable brain plasticity in pre-puberty childhood. Recent studies have suggested that the language-learning brain is plastic not just in childhood but across the lifespan, which enables even older adults to learn a second language with success. Being able to speak a second language (L2) confers obvious socioeconomic benefits in our globalised world, but L2 learning also leads to brain changes not always obvious to the naked eye: it allows the language experience to act as a mental exercise machine to remodel our brain, and this remodeling helps to build a better brain. I focus on ‘remodeling’ here because we usually learn our L1 (native language) before L2; in reality, multiple languages can be learned at the same time. The interesting question is: what are the features of a ‘better brain’ after the ‘remodeling’?
Thanks to neuroimaging methods such as magnetic resonance imaging (MRI), we can now precisely identify the remodeling effects of L2 learning. First and foremost, our brain becomes more responsive to the new auditory-visual features in the new language as learning progresses. For example, English speakers, when learning Chinese as an L2, find lexical tone a difficult aspect to learn. But when they become better in Chinese, functional MRI images show that their brain’s superior temporal gyrus (STG) in the left hemisphere responds more strongly to lexical tone differences. This is the neural signature that they treat Chinese tones not simply as acoustic signals (i.e., variations in pitch) – as they would before learning Chinese, but as important linguistic information. Not incidentally, the left STG is the area involved in lexical tone processing by native Chinese speakers; furthermore, if the tones were perceived as acoustic variations only, the right instead of the left hemisphere would be more active.
Language learning leads to more integrated brain activation
Secondly, brain regions like STG often work together with other regions to handle new language features, especially as the learner becomes better in the L2. Functional MRI network analysis has enabled us to reveal the underlying patterns of how different regions work in a network for successful L2 learning. For example, we showed that American students learning Chinese for only six weeks displayed a more integrated brain network that connects the STG with the frontal and the parietal cortex, and this contrasted with students who did not learn Chinese within the same period. Moreover, successful learners developed a stronger multi-path brain network implicated in both language and memory (e.g., dorsolateral prefrontal cortex), suggesting that L2 learning success is correlated with the individual’s cognitive capacity.
Interestingly, individual differences were present even before learning took place, allowing us to use network patterns to not only distinguish between learners but also predict who might be the more successful. We further examined how brain networks evolved over time as a function of L2 proficiency: early on when L2 proficiency was low, learners relied more strongly on the frontal-striatal network but as their L2 proficiency increased, they made more use of the frontal-temporal network. This shift in the engagement of different networks is likely due to the reduction in effort needed to monitor the competing languages, accompanied by increases in automaticity and efficiency in the L2. The dynamic engagement of the frontal-temporal (and frontal-parietal) networks due to bilingual experience helps to define a better brain, because such engagement might delay the onset of cognitive ageing (e.g., the over-reliance on anterior regions at the expense of posterior regions).
Second-language learning also changes the brain’s structure
Finally, L2 learning remodels not only the functional network, as discussed above, but also the anatomical structure of the brain. Because of their roles involved in learning, the relevant brain regions become strengthened – this is reflected in the increase of volume in grey and white matter. For example, the anterior cingulate cortex has been found to increase in size because it plays an important role in monitoring which language is being spoken and in resolving the conflict arising from the intrusion of the unintended language. Work from our lab has also provided the first evidence that anatomical changes could result from L2 learning in different contexts (e.g., 2D textbooks vs. 3D virtual environments).
Remodeling means the reorganisation of the brain, and the learning of a new language, even in late adulthood, provides a powerful experience for this reorganisation to occur. But how does L2 learning differ from other non-linguistic experiences (e.g., spatial navigation) in remodeling the brain? What brain difference does it make to learn distant vs. similar languages? And how can we use brain-behavior correlation data to fully account for individual differences in language learning? These are only a few of the open questions that will define exciting new research in the future.