The impact of bilingualism on cognitive aging and dementia
Many past studies argue that bilinguals demonstrate a cognitive advantage when compared with their monolingual counterparts. Research studies with children seem to have dealt with the cognitive effect of bilingualism more directly. Bilingual benefits are reported across such diverse domains as problem solving, creativity, and perceptual disembedding (Bialystok et al 2004). However, several studies have also reported negative bilingual effects. To resolve this disparity, it is important to take into account the cognitive processes linked to in different tasks used to evaluate the impact of bilingualism. Generally, tasks revealing bilingual benefit are typified by differences in representational and control processes. One of the main challenges facing researchers conducting research to compare older adults and young children with respect to bilingualism is identifying a task suitable across the different age groups. In this case, the Simon task has found widespread application in bilingual studies owing to its ability to meet set of assessing the performance benefits of bilingual individuals.
The Simon task hinges on stimulus-response compatibility and examines the level to which “the prepotent association to irrelevant spatial information affects participants’ response to task-relevant nonspatial information” (Bialystok et al. 2004, p. 291).
Research shows that bilingual children outperform their monolingual counterparts on different control tasks. Similar inhibitory benefits of bilingualism have also been reported in younger adults, albeit at an inconsistent rate, with various studies comparing the inhibitory benefits of bilingualism in older adults and children also suggesting a likely larger bilingual benefit on inhibition tasks in older adults. Bilingual children are reported to do better than their monolingual counterparts on different inhibitory tasks, with a number of studies comparing older adults with younger adults suggesting that the former group possess larger bilingual advantages on inhibition tasks (de Bruin et al., 2015). However, other studies have failed to notice a behavioural bilingual advantage in older adults, younger adults, and children. Bilinguals have also been shown to switch between non-verbal tasks when compared with monolinguals in groups of older adults, younger adults, and children.
Immigration is an important confounding variable for comparing monolingual and bilingual groups on their ability to perform inhibitory control tasks. Immigration is linked to improved cognitive functioning and health probably due to the change in lifestyle and flexibility required of immigrants (Fuller-Thompson & Kuh, 2014). This might explain why certain studies report positive effects between migration and bilingualism, while such positive relationship is lacking among the non-immigrants. However, more recent studies (for example, de Bruin et al., 2015) could not establish this kind of ‘bilingual advantage’ on inhibition tasks.
Numerous studies have compared difference in immigration status on bilingualism. This difference may be linked to a number of factors, such as the ability of immigrants to adapt to new circumstances, thus resulting in enhanced cognitive flexibility variation in immigrant status has also been shown to result in differences in ethic, cultural, educational, genetic background, and lifestyle, with several studies indicating that immigrants, holding bilingualism constant, reveal less cognitive decline and enhanced cognitive control in comparison with non-immigrants. The ‘health immigrant effect’ has also been implicated in the observed differences in immigration status and bilingualism (Fuller-Thomson & Kuh, 2014), with studies suggesting that healthy individuals are more likely to migrate, thus enhancing their bilingual capability. This is a clear indication that immigrant status is a significant, but usually ignored, background variable that could partly explain the benefits of bilingualism. Positive effects of bilingualism have also been reported in dementia patients and non-immigrant healthy adults as well. However, no positive cognitive effect of bilingualism has been reported by non-immigrants samples involving younger adults and children, even though studies with healthy older adults, ERP data, and dementia patients revealed some differences.
Kirk et al (2014) compared Gaelic-English bilinguals to monolinguals, bidialectals, monodialectals, and Asian-English bilinguals. The five different language groups demonstrated comparable Simon costs. The Gaelic-English bilinguals were living in an isolated rural environment, while the control groups resided in an urban environment. The study findings revealed confounding effects of environment and lifestyle on bilingualism. However, the bilingual group does not have to always constitute the immigrants. For example, the study by Clare et al (2014) consisted of monolinguals, majority of whom had migrated to Wales from various parts of the country. On the other hand, the bilingual group was largely made up of autochthonous population. This study did not show any evidence that the bilingualism had a delaying effect on the onset of Alzheimer’s disease.
While several studies (for example, Schweizer, Craik & Bialystok 2013) have evidently expressed a strong hypothetical justification for why we ought to link bilingualism with delayed onset of Alzheimer’s disease, a number of recent prospective studies indicate that immigration status could partially explain the protective effect of bilingualism reported by various retrospective review studies. Prospective cohort design studies have so far provided the strongest indication of a likely association between a delay in onset of dementia and bilingualism. Fuller-Thompson & Kuh (2014) associate the slower rates of cognitive decline reported among immigrants to lower prevalence of chronic health conditions linked to dementia, including high cholesterol, diabetes, and high blood pressure. Fuller-Thompson & Kuh (2014) further indicate that immigrants also possess a higher IQ (intelligent quotient) in comparison with non-immigrants who come from the same age group. In this case, IQ is recognized as a highly protective factor against dementia and the associated cognitive decline.
In their study, Fuller-Thompson and Kuh (2014) sought to determine the confounding the role of healthy migrant effect in confounding the relationships between delayed onset of Alzheimer’s disease and bilingualism. As a means of separating immigration from bilingualism, the researchers took into account evidence from prospective cohort studies at birth, in the United Kingdom and New Zealand. The study also used data from the NSHD (National Survey of Health and Development) to assess if immigrants from the UK to Australia and Canada have higher average cognitive ability at childhood, in comparison with non-immigrants. To assess childhood IQ, the researchers relied on tests developed by the National Foundation for Educational Research. This enabled them to measure nonverbal and verbal ability, such as word reading ability, picture intelligence, vocabulary, and comprehension. Study findings revealed a higher mean IQ among emigrants, compared to non-immigrants.
Elsewhere, de Bruin, Bak and Sala (2015) assessed the impact of bilingualism on executive control in a non-migrant sample of older adults by comparing inactive and active monolinguals and bilinguals matched on socio-0economic status, IQ, lifestyle, age, education, and gender. With respect to the Simon arrow task, the researchers could not establish bilingualism on either the Simon effect or overall RTs. There was also no significant difference between inactive and active monolinguals and bilinguals in the task-switching paradigm.
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