If you asked me what the hardest thing about living in Paris has been, my answer would be simple – the language barrier. Before leaving for Paris, I didn’t know any French besides how to say hello and goodbye. While I have picked up a few useful phrases in the past 4 weeks, it has still been very difficult to remember what I’ve learned. I began to wonder why I was having such a hard time with French, especially based on my previous experiences with language. When I was a young child, my mother used to teach me Chinese words and phrases. While I am nowhere near fluent in Chinese, I can still easily remember names of words and recognize phrases that I learned many years ago. On the other hand, learning French has been quite the struggle. I can spend a while reading my French traveler’s guide and practice my accent, yet hardly remember what I practiced the next day. Language is a very important field in neuroscience, so this experience led me to ask several questions: Why is it more difficult to learn a second language as we get older? Are there differences in anatomy of language areas in the brain depending on what age you learned a second language? While it is generally well known that children are able to learn languages much more quickly than adults (Johnson et al., 1989), I wanted to look further into how the age of learning a second language affects brain structure.
In 2014, Klein et al. published a study that examined how the age at which a second language is learned shapes brain structure. This study used four groups of participants: monolinguals who spoke only one language (monolinguals), bilinguals who learned two languages either simultaneously from birth or up until age 3 (simultaneous bilinguals), bilinguals who learned their second language from early childhood ages 4-7 (early sequential bilinguals), and bilinguals who learned their second language during late childhood ages 8-13 (late sequential bilinguals). All participants were interviewed and given questionnaires about their language background to determine which group they belonged to. It’s important to know that monolinguals were considered fluent only in their native language even if they received some formal training of another language, so taking a few years of Spanish in school doesn’t count as being bilingual. This study used magnetic resonance imaging scans (MRI), which allowed researchers to take an image of the brain and compare anatomical differences between participants’ brains.
First, researchers tested for general differences in cortical thickness (how thick the outer layer of tissue in the brain was) using MRI between monolinguals and the different groups of bilinguals. They were interested in measuring cortical thickness to see exactly how being bilingual affects growth in language areas of the brain during development. A thicker cortex meant that there was more neuronal (cells in the brain) development in that brain region. Researchers found that there was a significant difference in cortical thickness between the groups in a brain region called the left inferior frontal gyrus (LIFG). The LIFG is very important for phonological and syntax processing in language (Vigneau et al., 2006). Phonological processing means using sounds to understand language, and syntax refers to understanding the order of words to form sentences. Researchers found that the LIFG was much thicker in the early and late sequential bilingual groups compared to the monolingual group. Put more simply, the LIFG was much thicker only in bilinguals that learned their second language after early childhood compared to monolinguals. These differences in cortical thickness were not surprising, since the LIFG is a key brain area involved in language processing. These results demonstrated that learning a second language after becoming fluent in the first language changes brain structure during development. This was very significant finding, because it shows the “plasticity” of the brain, or the brain’s ability to reorganize itself and form new connections in different environments! To explain why the cortex becomes thicker in early and late sequential bilingual groups, researchers suggested that learning a second language after early childhood causes neurons and connections between neurons to grow in brain areas involved in language.
MRI scans showed that there was no difference in cortical thickness between the monolingual group and the simultaneous bilingual group. This was another very important finding, because it showed that being bilingual only affects brain development when a person learns their second language after early childhood. Researchers reasoned that these differences in cortex thickness might mean that there are different learning processes involved in first and second language learning only when the languages are learned separately after early childhood. These different learning processes might cause the cortex in language areas to become thicker as neurons and their connections grow. These results also show that the age when learning a second language is very important for setting up the brain structures involved in language.
Once researchers determined general differences in cortex thickness between monolinguals and bilinguals, they wanted to further study the relationship between brain structure and age of language learning in the bilingual participants. They found that the later a second language was learned after an individual learned their first language, the thicker the cortex was in the LIFG. Based on these results, researchers suggested that that the thicker cortex associated with later second language learning might reflect the brain using less than optimal neural circuits for language learning. An easier way to think about the brain is by thinking of it as a huge switchboard with lots of connections between each area of the brain. A neural circuit is like a path that information follows to get from one part of the brain to another. There are neural circuits that are direct and very quick, but there are also more roundabout ways to send information from one area to another. As we mature, our brain begins to solidify its connections, so the neural circuits used when a second language is learned at a later age may not be as direct and quick. Using suboptimal circuits could contribute to the cortex becoming thicker, as neurons increase their connections to follow a roundabout path. Learning both languages at the same time during early childhood appeared to use optimal neural circuits for language learning, because there were no differences in thickness between monolinguals and simultaneous bilinguals.
I found this study to be very interesting because it showed that there are anatomical differences in language regions of the brain that depended on what age a participant learned their second language. It was also very informative because it shows that the brain isn’t a set in stone structure, and our environment can significantly contribute to our development. As a follow up for more concrete conclusions about neural circuits involved in language learning, I’d like to see a study where researchers measure activation of the LIFG rather than just differences in cortex thickness. For example, functional magnetic resonance imaging (fMRI) measures brain activity by detecting blood flow to specific brain regions. Participants could read or listen to their native language followed by their second language in an fMRI machine to measure and compare how much language areas of the brain are active. Results from this would be even more informative in understanding how the age at which a second language is learned plays a role in language processing. For example, variation in brain activity could confirm differences in optimal and suboptimal neural circuits depending on what age the second language was learned. This would allow researchers to understand more about how neural processing, rather than just anatomy, is affected in language areas by learning a new language.
Until next time,
Johnson JS and Newport EL (1989). Critical period effects in second language learning: The influence of maturational state on the acquisition of English as a second language. Cognitive psychology, 21(1), 60-99.
Klein D, Mok K, Chen JK, & Watkins KE (2014). Age of language learning shapes brain structure: a cortical thickness study of bilingual and monolingual individuals. Brain and language, 131, 20-24.
Vigneau M, Beaucousin V, Herve PY, Duffau H, Crivello F, Houde O, and Tzourio-Mazoyer N (2006). Meta-analyzing left hemisphere language areas: phonology, semantics, and sentence processing. Neuroimage, 30(4), 1414-1432.
Cerebral cortex image (Creative Commons): http://www.neuroscientificallychallenged.com/blog/know-your-brain-cerebral-cortex
Left inferior frontal gyrus animation (Creative Commons): https://commons.wikimedia.org/wiki/File:Inferior_frontal_gyrus_animation_small.gif
Neural connection image (Creative Commons): http://maxpixel.freegreatpicture.com/Network-Brain-Cells-Brain-Structure-Brain-Neurons-1773922
French Phrasebook Image: https://images-na.ssl-images-amazon.com/images/I/51pqTbOV1qL._SX350_BO1,204,203,200_.jpg
Figure 1 from Klein et al., 2014