The Fascinating Bilingual Brain
Health care organizations, governments, and policymakers are facing social and economic challenges as the global population has been aging very rapidly the past few years. With aging comes the risk of developing health issues, like neurodegenerative disorders. In 2015, 46 million individuals worldwide were suffering from dementia. By 2050, it is expected that this number will increase to 131.5 million, mainly due to the growth of the number of older adults. So what are the determinants of healthy cognitive aging and, by consequence, how can age-associated neurodegenerative diseases like dementia be attenuated? One of the many approaches that might bring us a step closer, is the work on the relationship between bilingualism and aging.
Enhanced executive functions among bilinguals
In the past two decades studies have accumulated information that has provided comprehensive, yet inconsistent, evidence regarding the advanced cognitive control among healthy bilingual individuals. However despite the ongoing debate, those studies that have found a bilingualism effect point to it positively contributing to cognitive processes like attentional conflict, inhibitory control, and goal maintenance. These studies are not limited to older adults, but target bilingual speakers from different ages. The origin of the bilingual advantage presumably follows the experience of monitoring and switching between two or more languages on a regular basis. For example, researchers have found reduced switching costs and enhanced flexibility in mental shifting in non-verbal executive tasks for bilinguals compared to monolinguals, that form the support for the idea that managing and frequently switching between two languages affects the executive control system.
The ability to control and switch between multiple languages involves the recruitment of various components of the executive system. It follows that lifelong experience of bilingualism or multilingualism may modify the neural representation and/or brain activity of regions and circuits associated with executive control. Executive control processes are supported by a frontoparietal network in the brain. Among children and young adults, neuroimaging studies on bilingualism in connection to nonlinguistic cognitive control revealed that, compared to their monolingual peers, bilinguals show a more distributed network of brain activation, including the involvement of brain regions associated with language control. Comparable results are seen in adult bilinguals, who show similar and distinct task-related functional networks when engaged in executive control processes. The functional difference in relation to executive functioning is characterized by the additional activation of areas primarily recruited during linguistic processes (i.e. left inferior frontal gyrus (IFC), anterior cingulate cortex (ACC), left inferior parietal lobule (IPL), and left basal ganglia) by bilinguals but not monolinguals.
Brain reserve vs. cognitive reserve
Could the enhanced executive control processes play a key role in the neuroprotective effects of bilingualism on cognitive decline via preserved and stronger frontoparietal pathways? Well, it is known that even in the presence of pathological brain changes, which are expected to result in dysfunctional behavior, some individuals exhibit significantly fewer and less severe symptoms or even no clinical symptoms at all. The epidemiological evidence suggests that these so-called protective factors act as a ‘reserve’, and can explain the inter-individual variability in symptom onset and severity. There is a distinction between brain reserve and cognitive reserve, two concepts which in the past have been used interchangeably. Stern defines the former as the individual differences in the brain itself (e.g. volume, number of neurons or synapses) that allow some people to cope better than others with brain pathology. Although predominantly hard-wired, brain anatomy can be influenced by life experience via several processes including neurogenesis, up-regulating receptors that promote neural plasticity, and resistance to apoptosis. By contrast to brain reserve, cognitive reserve postulates that individual differences in mental processing allow some people to cope better than others with brain pathology. Stern further subdivides cognitive reserve in neural reserve, i.e. the heterogeneity in brain networks (e.g. efficiency, capacity, or flexibility), which might explain why some people are better capable of coping with the effects of neuropathology, and neural compensation. The latter is defined as the heterogeneity in the ability to compensate for the disrupted brain networks by utilizing brain regions and networks which, in the absence of brain pathology, are normally not associated with that particular mental process.
The potential neuroprotective effects of bilingualism
The process of aging is characterized by complex and heterogenous alterations across the brain and in cognitive performance. In a MRI-study from 2015, the authors examined global and local differences in volume and cortical thickness of regions associated with aging and dementia (i.e. temporal pole, entorhinal cortex, and hippocampus) in healthy monolingual and bilingual seniors. Monolinguals and bilinguals did not differ on measures of global gray matter volume, nor in their rate of age-related decline in relation to white or gray matter in one of the lobar regions. However, strikingly, bilinguals showed significantly greater white matter volume in the frontal lobe, and marginally greater white matter volume in the temporal lobe than monolinguals. This correlated with better executive functioning, as measured with a Stroop task. The study also exhibited that cortical thickness within the temporal lobe decreases as a function of age in monolinguals, but not in bilinguals. Collectively, these findings demonstrate that bilingualism may have neuroprotective effects, in terms of brain reserve, providing evidence for the assumption that over time the bilingual advantage of executive control might result in stronger and longer preserved pathways in the brain. At the same time, another study found evidence pointing more towards bilingualism as cognitive reserve. The authors examined white matter integrity and gray matter volume between healthy older monolinguals and bilinguals, and found that bilinguals showed more neural signs of an aging brain regions involved in memory processes compared to the monolinguals. Crucially, on a behavioral level the bilinguals did not underperform in relation to the monolinguals despite suffering greater brain atrophy. This led the authors to assume that bilinguals are flexible in using other intact (frontal) pathways, such as those involved in executive control as a way to compensate for the affected areas.
The protective effects of bilingualism upon cognitive decline have been found to reach further than healthy aging, extending to neurodegenerative diseases like Alzheimer’s Disease (AD). In one of the first studies it was revealed that (lifelong) experience of bilingualism does not lower the incidence or prevalence of dementia, but that the beneficial outcome of bilingualism is rather embedded in a delay in the onset of the symptoms. Within a sample of non-specified demented individuals, the age at onset of clinical symptoms reported by family members, was on average 4 years later than for bilingual individuals compared to monolingual individuals. The decline rates, however, did not differ between the two groups, which suggests a shift in onset only and not altered progression. These findings have led to the thought that that bilinguals deal with greater brain atrophy than monolinguals but still manage to cognitively perform on a similar level. As expected, researchers have found that bilingual AD patients showed greater atrophy in the medial temporal lobe (a well-known brain region affected by AD) than monolinguals, supporting the view that benefits of bilingualism do not directly operate by protecting against structural disruptions of AD related regions, but rather act on maintenance of cognition (via intact pathways) in the presence of brain burden. As already mentioned before, bilingualism may exert an influence on the preserved executive control pathways in the brain in both healthy and pathological aging. A recent study on metabolic connectivity in monolingual and bilingual AD patients revealed more severe left hemispheric hypometabolism among bilinguals than monolinguals (e.g. IPL, IFG, parrahippocampal gyrus, insula, putamen, and cerebellum). Interestingly, the bilinguals did show increased metabolic activity in the executive control (specifically frontoparietal networks) and default mode network compared to the monolinguals. This implies that the neuroprotective effect of bilingualism indeed involves compensatory strategies associated with the intact executive control pathways, making it possible to have a mismatch between brain damage and cognitive performance.
The precise mechanisms underlying the neuroprotective effect remain unclear. It could be that the bilingual advantage is primarily the result of neural compensation, via frontoparietal connections, when other brain regions are disrupted. At the same time, there is also evidence for better preservation of structural and functional integrity in bilinguals compared to monolinguals. It would be interesting to know whether these different protective effects of bilingualism (brain vs. cognitive reserve) are associated with individual variability in type of bilingualism language use, proficiency, or amount of switching between first and second language. Till then the bilingual brain will continue to be both fascinating and mysterious.
This blog is a shortened version of an unpublished writing assignment by Green (2016) as part of a master programme at Utrecht University.