By Anastasia Alenova
Due to globalisation, many cultures and languages are mixing, leading to an inevitable rapid rise of bilingualism. Nowadays, the majority of people speak more than one language over their lifetime, with many sources claiming that bilingual language learning confers unique patterns of neurofunctional activity and anatomical changes. Such a claim is unsurprising, as the brain is a very adaptive organ which can change functionally and reconfigure its structure in responses to environmental stimuli, cognitive demand and behavioural experience (Li et al., 2014; Klein et al., 2014). However, little is known about the specific changes in the brain mediated by learning a second language, and why such changes occur (Osterhout et al., 2008).
Understanding language is an extremely complex feat, which requires the ability to analyse and integrate the phonological, syntactic and semantic information of a sentence within milliseconds (ms). Processing syntactic and semantic information is defined by separate frontotemporal networks in the left hemisphere, while the right hemisphere is involved in the prosodic process, such as understanding information from intonation in sentences. Both hemispheres communicate via strong anatomical connections of the corpus callosum to ensure understanding (Friederici et al., 2010). More specifically, different areas in the left hemisphere are involved in the various aspects of the neurobiology of language, and carry different weight depending on the focus of the language. For instance, the superior temporal gyrus is responsible for acoustic phonetic and phonological processing, which carries high importance for understanding Chinese tones for instance, while the middle frontal gyrus plays a role in word meaning selection. Other areas, such as the hippocampus, are important for memory formation geared towards lexical semantic associations (Li et al., 2014).
Learning and retrieving information to understand language may take different forms. A very common method is that of children, who learn a language by first memorising a combination of words, then understanding the syntactic rules via a grammaticalisation process which may take a few months. The integration of grammatical knowledge into real-time language processing is supported by the following evidence. Syntactic anomalies, or wrong grammar, elicit a positive electrophysiological response after 600 ms (P600), while a semantic anomaly, such as a nonsense sentence, lead to a negative response after 400 ms (N400) (Osterhout et al., 2008). According to low resolution electromagnetic tomography, N400 distribution is mostly distributed in the posterior regions, such as the temporoparietal regions, while P600 has a more anterior distribution in the medial dorsal frontal lobe. Different mechanisms contribute to the analysis of language (Friederici et al., 2010).
However, second language (L2) learning and native language understanding do not necessarily involve the same brain areas. While similarities between native language (L1) and L2 favour syntactic learning of L2, and dissimilarities lead to a slower learning rate, both languages are associated with slightly different pathways (Osterhout et al., 2008). Language learning in adulthood is dependent on left hemisphere cortical structures and is characterised by fast processes in Broca’s area in the prefrontal cortex and Wernicke’s area in the temporal cortex (Friederici et al., 2010). Furthermore, although the N400 electrophysiological brain response is similar for both L1 and L2, P600 for L2 is mostly located in the left inferior frontal gyrus (Li et al., 2014; Osterhout et al., 2008).
Learning a second language leads to behavioural and neural changes in the brain. However, the degree of brain structure variation depends on multiple factors such as age of language acquisition and proficiency attained in L2. When L2 is acquired early, certain anatomic and neural brain changes may be approximated to patterns of native language learning, leading to a shared network for language production (Li et al., 2014; Friederici et al., 2010). When bilingualism is acquired from birth, L2 learning does not have any effect on brain development, such as cortical thickness levels. However, a late onset of the learning process is associated with a higher variability in the language network involved and increased cortical thickness in the left inferior frontal gyrus and left superior parietal lobe. At a more global level, age of language acquisition affects hemisphere involvement. If both languages are acquired before the age of six years old, both hemispheres are involved for both languages. However, learning L2 passed that age has been associated to an increased dominance of the left hemisphere for both languages (Klein et al., 2014). Age of acquisition also impacts how grammatical knowledge is stored, with early bilingualism leading to acquisition via frontal systems such as Broca’s area, while late bilingualism leads to learning via a temporally located neural system (Friederici et al., 2010).
The brain of a L2 learner is an extremely dynamic place from the very early stages of learning. Language learning leads to different types of structural property modification in the brain. Many claim that language learning is constrained to maturation and reduction of neural plasticity associated with increased age, however there is little evidence to support this view. So, it is never too late to start learning a new language! Much is still unknown, and there is a need to study the individual differences of learners to better understand structure-function-behaviour relationship in bilingualism and what makes learning successful (Li et al., 2014; Osterhout et al., 2008).
References:
Li, P., Legault, J. & Litcofsky, K. A. (2014) Neuroplasticity as a function of second language learning: Anatomical changes in the human brain. Cortex. 58 301-324. Available from: doi: 10.1016/j.cortex.2014.05.001.
Klein, D., Mok, K., Chen, J. & Watkins, K. E. (2014) Age of language learning shapes brain structure: A cortical thickness study of bilingual and monolingual individuals. Brain and Language. 131 20-24. Available from: doi: 10.1016/j.bandl.2013.05.014.
Osterhout, L., Poliakov, A., Inoue, K., McLaughlin, J., Valentine, G., Pitkanen, I., Frenck-Mestre, C. & Hirschensohn, J. (2008) Second-language learning and changes in the brain. Journal of Neurolinguistics. 21 (6), 509-521. Available from: doi: 10.1016/j.jneuroling.2008.01.001.
Friederici, A. D. & Wartenburger, I. (2010) Language and brain. WIREs Cognitive Science. 1 (2), 150-159. Available from : doi : 10.1002/wcs.9.
Imperial Bioscience Review 