The topic of neural plasticity is a complex and expansive one, which has been of great interest to cognitive neuroscience researchers for previous decades as it unlocks answers to the most convoluted processes the brain undertakes to sustain organisms as astute as humans. The study of bilingualism serves as an excellent model for neural plasticity, as the physiological effects of bilingualism show a dramatic increase in neural plasticity in different anatomical measures depending on several aspects of the bilingual experience. Previous studies have demonstrated increased plasticity, especially in those who had an earlier age of acquisition (AoA) of the language. The two articles Early bilingualism, language attainment, and brain development by Jonathan A. Berken et al., as well as Language experience modulates bilingual language control: The effect of proficiency, age of acquisition, and exposure on language switching by Michela Bonfieni et al. both allocate a great amount of focus on the impact AoA has on the anatomy of the brain and the individual's bilingual experience showing similar findings from their experimentation, but also diverge in several ways throughout their studies as we will see below.
Berken's review paper, Early bilingualism, language attainment, and brain development, is compromised of several separate studies performed by Berken et al. which piggyback on each other to reach well-grounded conclusions on the effects early versus late learning experiences have on the brain, using bilingualism as the model. The researchers initially divided the participants of this experiment into three groups; the simultaneous bilinguals (those who learned both languages before the age of 5), the sequential bilinguals (who learned their L2 after turning 5 years old), and monolinguals (the control group, speaking only one language). The researchers also set very concise standards and requirements for each of the groups, to check for as many controls as possible. For example, they set specific standards for the cutoff age of simultaneous bilinguals, and they issued questionnaires and individuals to assess language proficiency by evaluating vocabulary, syntax, fluency, as well as accent. This way, they could truly assign differences in brain structure and function found in the studies to different AoA stages as all different stimuli would be eliminated.
In one of the first experiments described, Neural Activation in Speech Production and Reading Aloud in Native and Non-Native Languages, Berken and his team aimed to determine the influence of age of acquisition on the brain regions known to be involved in speech production and reading. By the means of fMRI testing, the researchers drew several conclusions after recording brain activity while participants were reciting meaningful sentences in the languages French, and English. The maps showed that areas such as the motor cortex, Broca's area, and superior temporal areas, were activated in both bilingual groups. Moreover, the monolinguals recruited several other brain areas including those areas to read in French, as they only spoke English and required more cognitive work to achieve this task. Further testing revealed that late learners had more extensive activation in the premotor cortex, the IFG, and the fusiform gyrus than simultaneous bilinguals. Another important finding of the study revealed a positive correlation between AoA and BOLD signal, meaning there was more effort required in individuals who had a later AoA. The second study analyzed, Effects of Early and Late Bilingualism on Resting-State Functional Connectivity, studied participants at rest to find useful information on connections between brain areas in bilinguals. The researchers seeded the left IFG to look for connectivity between brain regions and found there were connections between the left IPL, the right IPL, as well as the right IFG with the left IFG. Furthermore, they found a negative correlation between AoA and the strength of these connections, meaning that connections were not as strong in participants who learned L2 later in life. This finding had major implications for the efficiency of learning at different stages of life, as all the areas showing connectivity with the IFG were areas of the brain that are well-associated with cognitive functions such as cognitive control. As efficiency is a major goal scientists try to comprehend in these mental processes, the study performed by Bonfieni is a good correlate to Berken's studies, as her team shows that daily exposure long after AoA produces higher language proficiency, and less mental effort to speak different languages.
In Bonfieni's study, her research team examined 83 participants divided into two groups. The participants were, similarly to Berken's experiment, assessed via a questionnaire to determine group placement. The participants were then asked to name pictures they were shown as quickly and accurately as possible. The researchers also controlled for the language being tested by acquiring individuals who were fluent in different languages, in order to see if this played a role in their findings. As such, they could determine participants' abilities to control languages (specifically their ability to switch between languages), and how that is shaped by linguistic experience. Based on the tasks given, individuals were assessed and it was found that the ability to switch between languages was modulated heavily by L2 proficiency. Furthermore, the data showed that persistent exposure to L2 showed greater abilities to switch between languages as well. While no brain analyses were run to determine brain activity during these experiments, the findings in Bonfieni's study seem to align with the findings from Berken's study. However, further experimentation could be done to confirm said findings by measuring specific brain activity. Both of these studies lead us a step closer to determining the most efficient methods of learning that are established in our brains based on our AoA, environmental stimuli, and many other factors impacting our learning experience.
References
Berken, J. A., Gracco, V. L., & Klein, D. (2017). Early bilingualism, language attainment, and brain development. Neuropsychologia, 98, 220–227. https://doi.org/10.1016/j.neuropsychologia.2016.08.031
Bonfieni, M., Branigan, H. P., Pickering, M. J., & Sorace, A. (2019). Language experience modulates bilingual language control: The effect of proficiency, age of acquisition, and exposure on language switching. Acta Psychologica, 193, 160–170. https://doi.org/10.1016/j.actpsy.2018.11.004
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