Bilingualism and cognitive functions in brain diseases: from dementia to stroke

Dr Thomas BakThomas H Bak is a reader in Human Cognitive Neuroscience at the University of Edinburgh. In addition to his work with Bilingualism Matters, he is a member of the Centre for Cognitive Ageing & Cognitive Epidemiology (CCACE) and the Centre for Clinical Brain Sciences (CCBS).

Around 50 years ago, when I was growing up in Cracow (Poland) as a son of a Polish-speaking father and German-speaking mother, my parents decided, after a careful consideration, to prevent me from learning German, fearing that being bilingual could lead to negative consequences for my mental development. There were neither practical nor political reasons for this decision: my father was fluent in German and his father had studied in Vienna, as was usual for educated citizens of the Austro-Hungarian empire. Their decision was also not based on ignorance: as doctors, they had consulted what was the dominant academic view of the time. Psychologists, speech and language therapists as well as teachers were convinced that bilingualism diminishes children’s intelligence, confuses them and may even cause schizophrenia. It was also by no means a view confined to the former Soviet Block: I have met many people from all over the world growing up in the same time, whose parents made the same decision and this for very similar reasons.

What my parents could not have known was that around this time in far away Montreal scientists were just beginning to question these assumptions. Canadian researchers Peal and Lambert set out to examine whether bilingual children performed worse than monolingual ones on a range of cognitive tests. Contrary to the expectations, bilingual children were not worse than their monolingual counterparts: in fact, they were better. Over the following decades, it became increasingly clear that bilingualism can be associated with cognitive benefits and this across the whole lifespan: in children, in young adults, and also in cognitive ageing. In 2007, a seminal study from Ellen Bialystok’s research group in Toronto showed that people who spoke more than one language developed dementia 4 years later than those who spoke only one. Together with my colleague Suvarna Alladi from Nizam’s Medical Institute in Hyderabad (India) we decided to replicate these findings in a genetically, culturally and linguistically completely different population. Our results showed exactly the same delay of 4-5 years, a finding replicated meanwhile by another research group in Ghent (Belgium).

Importantly, the benefits of bilingualism are not confined to those who learned a second language early on in their childhood. Using the extensive data from the Lothian Birth Cohort 1937 study in Edinburgh I was able to demonstrate that people who learned a second language after the age of 11 years performed better on many cognitive tests in the age of 73 years than could be predicted from their childhood intelligence. Meanwhile, many more strands of evidence point in the same direction. We found an improved performance on attentional tests in students learning foreign languages and this from the age of 18 to 78 years. Other groups conducted neuroimaging studies confirming that bilingualism can influence grey matter thickness and connectivity between different brain areas.

Not all studies find a difference between bilinguals and monolinguals; in those which do, the results could often be influenced by a wide range of other factors such as ethnicity, education, lifestyle, socio-economic status or immigration. However, recent studies went a long way to minimise the impact of these so-called “confounding variables”. Furthermore, it is worth noting that if any differences are detected at all, they go almost invariably in the direction of a bilingual advantage.

In terms of a possible theoretical explanation, the idea of a positive effect of bilingualism on brain functions reflects wider tendencies in contemporary neuroscience. Until relatively recently, the brain used to be conceived as a static organ, in which any kind of damage will lead invariably to lasting and irreparable deficits. However, advances in clinical and basic sciences have questioned this assumption. The current view postulates that the brain is trying to adapt to new circumstances and to compensate for potentially damaging influences through a mechanism referred to as “neuronal plasticity”. Physical and mental activity throughout lifetime can lead to the development of “cognitive reserve”: a better ability of the brain to cope with negative influences. Speaking several languages is believed to stimulate different cognitive functions and to contribute in this way to cognitive reserve.

However, if this hypothesis is correct we should find a positive effect of bilingualism not only in delaying the onset of dementia but also in helping people who suffered brain damage to recover from it. This is exactly the point addressed by the study just published on-line in the journal “Stroke”. We have examined the cognitive outcome of 608 patients who suffered an acute stroke. If the differences between bilinguals and monolinguals were due to a generally healthier lifestyle (e.g. healthier nutrition, more physical activity etc) in bilinguals, we should expect that the stroke in bilingual patients should occur later than in the monolingual ones. In contrast, if the difference is due to a better “cognitive reserve” in bilinguals, the event of stroke will occur at the same age, but the recovery from it should be faster. This is exactly what we found: the age at stroke was the same in both groups but the bilinguals showed a complete cognitive recovery more than twice as often as monolinguals. These results have far-reaching consequences not only because of their theoretical implications: stroke is the second most common cause of cognitive decline after dementia and anything that can be done to improve its outcome will be of great practical relevance.