Being bilingual may prevent cognitive decline and delay the onset of Alzheimer's disease. Scientists have now identified some of the neural mechanisms that may be responsible for this phenomenon.
In recent years, the cognitive reserve theory has been widely discussed by scientists in the field of Alzheimer's research, to account for discrepancies between degrees of brain damage and clinical manifestations in some people.
The idea is that some environmental factors and cognitive activities can lead to individual differences in cognitive processes and neural connections. This may allow some people to cope better than others with brain damage.
"Observational studies have shown that spending a greater number of years in education or having more complex job is associated with a delay in cognitive decline and in the emergence of dementia symptoms in some people. The cognitive reserve theory comes out of these observations to make sense of the data and to explain why some people are more resilient to brain damage", Dr Clare Walton, Research Manager at Alzheimer's Society, told IBTimes UK.
Some scientists think that bilingualism may provide a cognitive reserve, and that people who speak more than one language are better protected against cognitive decline. Studies have been published in which bilingual people were shown to develop Alzheimer's disease at an older age than monolinguals.
However, this only indicated a correlation and these findings were sometimes met with scepticism, especially in the absence of more in-depth research into potential neural mechanisms.
The new study published in PNAS is one of the first to investigate this. "It goes further than we have gone before, by looking at mechanisms and trying to put all the pieces of the puzzle together, to understand why bilingualism is linked to reduced risk of dementia", Walton said.
Brain damage and symptoms
The team, from San Raffaele Hospital (Italy), used PET imaging to look at brain metabolism – brain cells' activity – and neural connectivity, in 85 patients with Alzheimer's disease. Out of these 85 people, 45 were bilinguals (Italian-German) and 40 monolinguals. This is quite a large sample of people for such a precise, in vivo brain imaging study.
The bilingual participants were five years older on average and the scientists discovered that their brains showed more signs of damage – they displayed even less brain cell activity than their monolingual peers. However, they had more functional connections between key areas of their brains, which is consistent with them using and switching between two languages.
When the researchers looked at the participants' clinical symptoms, they found that they were similar in both groups. Bilingual patients didn't have more severe disease manifestation, despite more reduced brain metabolism. This suggests that speaking two languages might have delayed cognitive decline, and helped them cope with dementia more efficiently compared with monolinguals.
Scientists showed this effect was greater for people if they had been exposed their whole life to the two languages and used them constantly.
"Bilingualism is a global phenomenon, and that half of the world is actually bilingual, it is highly unlikely that half of the world is protected against dementia. Crucially, what actually protects the brain may be specific types of bilingualism. We have elsewhere suggested that only those bilinguals with lifelong exposure to and use of both languages will have the maximum benefits", the authors say.
The study does not conclude on whether taking on a second language at midlife can provide protection, but the scientists say that programs that promote long-term bilingual or multilingual education and support the maintenance of the second language among older adults may be useful.