There is a good amount of evidence to suggest that learning and using more than one languages can affect the function of the brain, with some researchers even proposing a general cognitive advantage for bilinguals, especially in older age. Until recently less was known about how these cognitive effects might be reflected in structural changes in the brain. However, since the seminal paper by Mechelli et al. (2004), and with considerable advances in MRI technology over the past 15 years, a growing number of recent studies have repeatedly shown the effects of bilingualism in both major parts of the brain, the grey and the white matter.
The grey matter consists of the cell bodies of our brain neurons, where most of information processing takes place, which are organised around the surface of the brain. The white matter on the other hand consists of the axons of our neurons, i.e. the part of the cell that provides communication with the surrounding neurons. The axons converge and interconnect underneath the grey matter, forming the white matter. We call it white matter because the axons are wrapped in a fatty layer, the myelin, which ensures better neuronal communication – the way information is transferred around the brain. The myelin functions as an “insulation” that prevents information “leaking” from the axon during transfer. Any structural changes in the grey matter are typically interpreted as a reorganisation of the affected brain region in order to accommodate and process new information, whereas any changes in the white matter are interpreted as reinforcement of white matter tracts in order to provide more efficient communication between brain areas, esp. under increased cognitive demands.
The experience of bilingualism ticks both boxes: a) It involves learning and processing of two, instead of one, linguistic systems, including phonological, semantic, lexical, grammatical and pragmatic information; b) it involves increased cognitive demands, esp. since both languages are now thought to be active at any time, so using one language entails actively suppressing the other one. None of these processes apply to monolingual speakers of a language, so it is only reasonable to expect that bilinguals will demonstrate structural changes in both the grey and white matter of the brain, when compared to monolinguals.
This is indeed the case, as demonstrated by research published in the last decade or so (for comprehensive review papers, see here and here). Recent work at the Bilingualism in the Brain lab at the University of Reading has provided even more evidence that learning and using a second language has an impact on the structure of the brain. A 2014 study revealed that bilinguals have greater grey matter volume in the cerebellum, a part of the brain involved in many cognitive functions, including linguistic ones. Moreover, the amount of grey matter in the cerebellum of bilingual participants correlated positively with the speed by which they performed a grammatical task; in other words, the bigger the grey matter volume in the cerebellum, the more efficiently they performed the task. No similar effects were observed in monolingual speakers, suggesting that the cerebellum might be a key area in the acquisition and processing of a non-native grammar.
A more recent study (Pliatsikas et al., 2015) showed that bilingualism also affects the integrity of the white matter of the brain. More specifically, it was shown that highly proficient non-native speakers of English who had lived and worked in the UK for an average of 7.5 years, had enhanced structure (expressed as increased myelination) in a number of white matter tracts that are involved in linguistic processing, among other functions. It is very likely that enhanced white matter facilitates cognitive processing, so the observed structural effects might be linked to the suggested cognitive benefits of bilingualism; however, that study did not include any cognitive tasks that would answer this question. Nevertheless, it is important to know that the observed structural effects almost mirror the pattern of changes observed in a group of elderly lifelong bilinguals in a previous study. This suggests that enhancement of white matter, and any benefits that may come with it, do not necessarily require lifelong bilingual experience, but immersive experience, i.e. active usage of both languages.
It is obvious that this is still a young but promising field of research. A thorough overview of the existing evidence reveals that factors that have not been systematically controlled yet might be crucial to explain the observed effects. These factors include language proficiency, immersion in a bilingual environment, age of acquisition of the additional language, socioeconomic factors, and so on. Currently ongoing and planned projects in our Bilingualism in the Brain lab look at exactly those factors, while at the same time aim to provide a link between the effects of bilingualism on cognition and on brain structure. This will allow us to eventually build a comprehensive theory about the potential benefits of bilingualism, especially in older age.