Let's be real. Does bilingualism really defer all types of dementia? 

It is pretty much official for many applied linguists: bilingualism appears to defer the onset of Alzheimer's and other dementias. The reason for that? That task-switching between two languages sharpens one's executive function, which later contributes to cognitive reserve in old age.

The belief in this bilingual advantage first stemmed from Bialystok and colleagues' seminal paper back in 2007. With data from 228 patients at a memory clinic in Toronto, the researchers compared the rate of cognitive decline between monolinguals and non-monolinguals. It was found that bilinguals experienced symptoms of dementia on average four years later than their monolingual counterparts. This suggested 'bilingual advantage' emerged as a victorious movement, one that used scientific evidence to empower bilingualism. It was very much a slap in the face for those entrenched in the 60s-brewed belief that bilingualism was a 'handicap' for child development.

While existing research has mainly looked at the benefits of bilingualism from the inductive perspective of cognitive psychology, neuroimaging evidence - the type that actually tracks changes in the brain as a result of bilingual activity - remains scarce at this juncture.

A group of applied linguists, however, engaged with the bilingual advantage theory with a pinch of salt, pointing to the lack of coherent statistical evidence for it. They also cautioned against an implicit scholarly bias. After all, as applied linguists, many of us are multilinguals. We, of course, would love to believe in the cognitive superpower of bilingualism, even if some experiments say otherwise.

The linguist side of me sides strongly with proponents of the bilingual advantage. The neuroscientist side of me, however, warns me to do so meticulously. While existing research has mainly looked at the benefits of bilingualism from the inductive perspective of cognitive psychology, neuroimaging evidence - the type that actually tracks changes in the brain as a result of bilingual activity - remains scarce at this juncture.

So, what does neuroscience say about the advantages of bilingualism so far? Does bilingualism really defer all types of dementia?

While there is reason to believe that the task-switching demand of daily bilingualism may lead to cognitive reserve in the frontotemporal lobes..., the same cannot easily be determined for vascular dementia, which is suspected to be caused by limited blood supply to the brain.

Representative studies in lingusitics-based neuroimaging have revealed stronger grey matter density and white matter integrity in the brain - a change that can be associated with protection against the brain's developmental loss over the years. Neuroscientists, however, still have to determine the relationship (if any) between such protection and the development of dementia. It is crucial to bear in mind that dementia itself is not an easily definable disease, but rather a complex phenomenon of impaired abilities to carry out daily cognitive activities. Although researchers generally classify dementia into a few broader types (Lewy body, frontotemporal, vascular, and Alzheimer's), there exist more than 200 kinds of dementia, each with somewhat different pathologies, affecting slightly different parts of the brain. While there is reason to believe that the task-switching demand of daily bilingualism may lead to cognitive reserve in the frontotemporal lobes (thus perhaps protecting against frontotemporal dementia), the same cannot easily be determined for vascular dementia, which is suspected to be caused by limited blood supply to the brain.

Despite the complexities of dementia pathologies and their relation to bilingualism, the scientific community should not deem it naive to hypothesise that bilingualism can, in fact, be a helpful tool to fend off dementia for as long as possible. The trickiest part of this all is that educational-psychological evidence does not exist without discrepancies, and psycholinguistic research is rarely synthesised with neuroimaging studies. We may be able to fill this gap in knowledge one day, but for now, paving the road to clarity requires long-term collaborative efforts from multiple fields of science.

References:

Bialystok, E., Craik, F. I. M., and Freedman, M. (2007). Bilingualism as a protection against the onset of symptoms of dementia. Neuropsychologia 45, 459-464. doi: 10.1016/j.neuropsychologia.2006.10.009

Paap, K. (2019). "The bilingual advantage debate: quantity and quality of the evidence." in The Handbook of the Neuroscience of Multilingualism, eds. J. Schwieter and M. Paradis (Chichester, UK: John Wiley & Sons Ltd), 701-735. doi: 10.1002/9781119387725.ch34

Pliatsikas, C., Meteyard, L., Veríssimo, J. et al. The effect of bilingualism on brain development from early childhood to young adulthood. Brain Struct Funct 225, 2131-2152 (2020). doi: 10.1007/s00429-020-02115-5