Information

Bilingualism and cognitive decline (or dementia risk)

Bilingualism and cognitive decline (or dementia risk)


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

There's apparently a fairly contested research area of bilingualism being neuroprotective against cognitive decline in old age, such as dementia.

What are the main points for and against this hypothesis?


The impact of bilingualism on cognitive ageing and dementia

  • Received: 05 Jan 2015
  • Accepted: 15 Sep 2015
  • Version of Record published : 25 Jan 2016

Abstract

Within the current debates on cognitive reserve, cognitive ageing and dementia increasingly showing a positive effect of mental, social and physical activities on health in older age, bilingualism remains one of the most controversial issues. Some reasons for it might be social or even ideological. However, one of the most important genuine problems facing bilingualism research is the high number of potential confounding variables. Bilingual communities often differ from monolingual ones in a range of genetic and environmental variables. In addition, within the same population, bilingual individuals could be different from the outset from those who remain monolingual. We discuss the most common confounding variables in the study of bilingualism, aging and dementia, such as group heterogeneity, migration, social factors, differences in general intelligence and the related issue of reverse causality. We describe different ways in which they can be minimised by the choice of the studied populations and the collected data. In this way, the emerging picture of the interaction between bilingualism and cognitive aging becomes more complex, but also more convincing.


Obesity, cognitive functioning and dementia: back to the future

The conditions of chronic obesity and overweight status are risk factors for lower cognitive performance, cognitive decline, cognitive deficit, and dementia. But lower cognitive performance early in life itself may be a risk factor for an increase in body weight over time. With this in mind, we review important papers in the literature that advance our knowledge of relations between weight and cognitive functioning, with an emphasis on papers that illustrate methodological and theoretical issues of importance. We describe the evolution in research on weight and cognition with respect to two major features: (a) the move backward in time from the diagnosis of dementia to the pre-clinical period of dementia in order to better identify risk factors and (b) the evolution of studies from an earlier emphasis on obesity-related cardiovascular risk factors as major mediators of relations between obesity and cognition to a more recent emphasis on metabolic variables, lifestyle variables, genotype, and other mechanisms that explain relations among weight change, obesity, and cognition. We conclude that: 1) a complete understanding of the causal links between weight and cognitive functioning requires a lifespan perspective 2) practically speaking, lifespan research may need to amalgamate and integrate research at different segments of the lifespan until such time that we can include the entire life cycle within a single study of weight and cognition and 3) we need more studies that examine reciprocal relations between weight and cognition, especially early in life.


Bilingualism and Cognitive Decline: A Story of Pride and Prejudice

Affiliations: [ a ] Department of Experimental Psychology, Ghent University, Ghent, Belgium | [ b ] Department of Neurology, Brussels University Hospital, Vrije Universiteit Brussel, Brussels, Belgium | [ c ] Department of Neurology, Ghent University Hospital, Ghent University, Ghent, Belgium

Correspondence: [*] Correspondence to: Evy Woumans, Department of Experimental Psychology, Ghent University, Henri Dunantlaan 2, B9000, Ghent, Belgium. Tel.: +32 474 54 83 46 E-mail: [email protected] .

Abstract: In a recent review, Mukadam, Sommerlad, and Livingston (2017) argue that bilingualism offers no protection against cognitive decline. The authors examined the results of 13 studies (five prospective, eight retrospective) in which monolinguals and bilinguals were compared for cognitive decline and o nset of dementia symptoms. Analysis of four of the five prospective studies resulted in the conclusion that there was no difference between monolinguals and bilinguals, whereas seven of the eight retrospective studies actually showed bilingualism to result in a four-to-five year delay of symptom onset. The authors decided to ignore the results from the retrospective studies in favor of those from the prospective studies, reasoning that the former may be confounded by participants’ cultural background and education levels. In this commentary, we argue that most of these studies actually controlled for these two variables and still found a positive effect of bilingualism. Furthermore, we argue that the meta-analysis of the prospective studies is not complete, lacking the results of two crucial reports. We conclude that the literature offers substantial evidence for a bilingual effect on the development of cognitive decline and dementia.

Keywords: Cognitive decline, dementia, multilingualism, prospective cohort studies, retrospective studies

Journal: Journal of Alzheimer's Disease, vol. 60, no. 4, pp. 1237-1239, 2017


Improvements in Learning

Being bilingual can have tangible practical benefits. The improvements in cognitive and sensory processing driven by bilingual experience may help a bilingual person to better process information in the environment, leading to a clearer signal for learning. This kind of improved attention to detail may help explain why bilingual adults learn a third language better than monolingual adults learn a second language. 22 The bilingual language-learning advantage may be rooted in the ability to focus on information about the new language while reducing interference from the languages they already know. 23 This ability would allow bilingual people to more easily access newly learned words, leading to larger gains in vocabulary than those experienced by monolingual people who aren’t as skilled at inhibiting competing information.

Furthermore, the benefits associated with bilingual experience seem to start quite early—researchers have shown bilingualism to positively influence attention and conflict management in infants as young as seven months. In one study, researchers taught babies growing up in monolingual or bilingual homes that when they heard a tinkling sound, a puppet appeared on one side of a screen. Halfway through the study, the puppet began appearing on the opposite side of the screen. In order to get a reward, the infants had to adjust the rule they’d learned only the bilingual babies were able to successfully learn the new rule. 24 This suggests that even for very young children, navigating a multilingual environment imparts advantages that transfer beyond language.


Does Bilingualism Protect Against Dementia?

The evidence for a protective effect of bilingualism on the incidence of dementia is considerable. Numerous studies have examined dementia incidence in hospital records and concluded that bilingualism exerts a protective effect. The first such study by Bialystok et al. (2007) revealed that lifelong bilinguals showed a delay in the onset of symptoms of dementia by 4 years compared to monolinguals. Similarly, Craik et al. (2010) reported that bilingual patients had been diagnosed with Alzheimer’s disease 4.3 years later and had reported the onset of symptoms 5.1 years later than the monolingual patients. Additionally, Woumans et al. (2015) found that bilingual patients had been diagnosed with Alzheimer’s disease 4.8 years later and presented symptoms 4.6 years later than monolingual patients. Similarly, speakers of two or more languages had a delayed onset of Alzheimer’s disease by up to 5 years and a protective effect was significant when speaking at least two to four languages (Freedman et al., 2014). Looking at specific dementia subtypes, bilingualism delayed the age at onset in the behavioral but not in the aphasic variants of Frontotemporal Dementia (Alladi et al., 2017), a finding consistent with the observation that bilingualism has positive effects on behavioral syndromes but not on language disorders. Indeed the effects of bilingualism on language functions are not always beneficial (e.g., smaller vocabulary size in a single language, slower lexical processing, reduced verbal fluency etc.). Further, a similar study by Alladi et al. (2016) comparing monolingual and bilingual stroke patients found that bilinguals had a significantly lower frequency of post-stroke dementia and mild cognitive impairment but the same frequency of post-stroke aphasia. Moreover, Atkinson (2016) reviewed nine papers and concluded that frequent use of two languages over a lifetime may be protective against dementia, and that inconsistencies arise due to study design or definitions of bilingualism. This evidence supports the protective effect of bilingualism against the symptoms of dementia (Bialystok et al., 2016), as well as the later onset of symptoms of mild cognitive impairment compared to monolinguals (Bialystok et al., 2014). Bilingual individuals diagnosed with single-domain amnesic mild cognitive impairment demonstrated a later age of diagnosis than did monolinguals (Ossher et al., 2013). Cerebral hypometabolism was more severe in the left hemisphere in bilinguals with Alzheimer’s dementia compared to monolinguals, but nevertheless bilinguals outperformed monolinguals on memory tasks, suggesting that bilinguals are better able to compensate for the loss of brain structure and function (Perani et al., 2017). Furthermore, exposure to foreign language instruction during childhood and adolescence has been associated with lower risk of developing mild cognitive impairment in old age (Wilson et al., 2015). Bilingualism has been associated with delayed onset of dementia and is also observed in illiterate patients (Alladi et al., 2013). Taken together, this body of work suggests that bilingual experience delays the onset of neurodegenerative disease.

However, an increasing number of studies have failed to detect a bilingual advantage in dementia incidence. A cohort design with non-immigrant samples found no significant differences in the onset of dementia between mono- and bilingual subjects (Lawton et al., 2015). No significant association was found between non-native English speakers and the incidence of dementia or Alzheimer’s disease (Sanders et al., 2012). In that study, non-native English speakers with at least 16 years of education had a fourfold increased risk for dementia compared to those with less education, which is an unusual finding and inconsistent with past literature on the protective effect of education. Yeung et al. (2014) found no association between dementia diagnoses for bilinguals (English as a second language and bilingual English) and monolinguals. Zahodne et al. (2014) reported that adult learners of English had better memory and executive function than monolinguals, but that bilingualism was not associated with cognitive decline or dementia. Fuller-Thomson (2015) has claimed that the support for a bilingual advantage in dementia onset is questionable, and has attributed the current state of the literature to the file drawer problem, a bias against publishing non-significant findings from small studies with low to medium statistical power, a selection bias due to use of patients from a memory clinic, potential recall bias in caregivers’ reporting of age of onset of dementia and confounding by immigration status. Indeed, Clare et al. (2016) did not observe any advantage for delay in Alzheimer’s onset in Welsh-English bilinguals over English monolinguals (but see Bak, 2016 for a discussion of how this finding is conflated by the unusual situation of monolingual migration). A recent meta-analysis concluded that bilingualism offers no protection against cognitive decline (Mukadam et al., 2017), and that retrospective studies supporting the bilingual protective effect against dementia are marred by methodological confounds. Note, however, that this meta-analysis has already been criticized as misleading and incomplete (Woumans et al., 2017). In sum, these studies have led to questions regarding the robustness (or in some cases the validity) of the bilingual dementia advantage.

In order to resolve the debate, attempts have been made to understand the role of any potential mediating factors and experimental confounds. Gollan et al. (2011) claim that higher degrees of bilingualism are associated with increasingly later age of diagnosis and symptom onset, but this may be obscured by interactions between education and bilingualism, and a failure to obtain objective measures of bilingualism. Bak and Alladi (2014) highlight that although there exists support that bilingualism has a positive effect on cognition throughout the lifespan, common misconceptions concerning the nature of bilingualism persist, including that bilingualism is an unusual phenomenon, the holistic nature of bilingualism and its effects on cognition and bilingual diversity. Further, Fuller-Thomson’s (2015) and Lawton et al.’s (2015) assertions that monolinguals and bilinguals do not differ in the onset of dementia have been criticized as overly simplistic. Bak and Alladi (2016) point out that it is necessary to study the effects of bilingualism separately from those of immigration and education, and to use data from both community-based approaches and memory clinics. Bak (2016) further highlights the importance of addressing confounding variables in bilingualism, aging and dementia research which include heterogeneity, migration, social factors, differences in general intelligence and the related issue of reverse causality.

The above literature review has demonstrated that bilingualism yields executive functioning advantages, and these may contribute to building cognitive reserve, which may ultimately delay the onset of dementia. The exact mechanisms are not agreed upon, and there exists counterevidence that limits the generalisability of these claims. A possible fruitful avenue is the recent suggestion that sustained activation of noradrenergic signaling pathways associated with bilingualism could provide a possible mechanism linking current and previous results supporting a delayed onset of dementia in bilinguals (Bak and Robertson, 2017). The following sections of this article are devoted to proposing additional possible explanations and mechanisms that may provide parsimonious explanations for the seemingly conflicting findings currently in the literature.


Can bilingualism protect the brain even with early stages of dementia?

A study by York University psychology researchers provides new evidence that bilingualism can delay symptoms of dementia.

Alzheimer's disease is the most common form of dementia, making up 60 to 70 per cent of dementia cases. Of all activities with neuroplastic benefits, language use is the most sustained, consuming the largest proportion of time within a day. It also activates regions across the entire brain. Ellen Bialystok, Distinguished Research Professor in York's Department of Psychology, Faculty of Health, and her team tested the theory that bilingualism can increase cognitive reserve and thus delay the age of onset of Alzheimer's disease symptoms in elderly patients.

Their study is believed to be the first to investigate conversion times from mild cognitive impairment to Alzheimer's disease in monolingual and bilingual patients. Although bilingualism delays the onset of symptoms, Bialystok says, once diagnosed, the decline to full-blown Alzheimer's disease is much faster in bilingual people than in monolingual people because the disease is actually more severe.

"Imagine sandbags holding back the floodgates of a river. At some point the river is going to win," says Bialystok. "The cognitive reserve is holding back the flood and at the point that they were when they were diagnosed with mild cognitive impairment they already had substantial pathology but there was no evidence of it because they were able to function because of the cognitive reserve. When they can no longer do this, the floodgates get completely washed out, so they crash faster."

In the five-year study, researchers followed 158 patients who had been diagnosed with mild cognitive impairment. For the study, they classified bilingual people as having high cognitive reserve and monolingual people as having low cognitive reserve.

Patients were matched on age, education, and cognitive level at the time of diagnosis of mild cognitive impairment. The researchers followed their six-month interval appointments at a hospital memory clinic to see the point at which diagnoses changed from mild cognitive impairment to Alzheimer's disease. The conversion time for bilinguals, 1.8 years after initial diagnosis, was significantly faster than it was for monolinguals, who took 2.6 years to convert to Alzheimer's disease. This difference suggests that bilingual patients had more neuropathology at the time they were diagnosed with mild cognitive impairment than the monolinguals, even though they presented with the same level of cognitive function.

These results contribute to the growing body of evidence showing that bilinguals are more resilient in dealing with neurodegeneration than monolinguals. They operate at a higher level of functioning because of the cognitive reserve, which means that many of these individuals will be independent longer, Bialystok says. This study adds new evidence by showing that the decline is more rapid once a clinical threshold has been crossed, presumably because there is more disease already in the brain.

"Given that there is no effective treatment for Alzheimer's or dementia, the very best you can hope for is keeping these people functioning so that they live independently so that they don't lose connection with family and friends. That's huge."


Bilingualism and cognitive decline (or dementia risk) - Psychology

Cognitive reserve is the most promising avenue to maintaining cognitive health in older age and averting some of the more devastating consequences of cognitive decline and dementia.

Cognitive reserve is characterized by a dissociation between cognitive level and brain structure in that deterioration of brain structure may not impact cognitive function.

Bilingualism appears to contribute to cognitive reserve in that it is associated with (i) better cognitive performance than would be predicted by brain structure in older adults, (ii) later evidence of symptoms of dementia compared with monolinguals, (iii) greater pathology for comparable levels of dementia as found for monolinguals, and (iv) more rapid decline of cognitive function in more advanced stages of dementia.

More support should be given to societal attitudes to foreign language learning and bilingualism.

Cognitive reserve is characterized by a dissociation between cognitive level and brain structure, thereby reducing the impact of deteriorating brain structure on cognitive function. Cognitive reserve is therefore a promising approach to maintaining cognitive function and protecting against symptoms of dementia. The present paper evaluates evidence supporting the claim that bilingualism contributes to cognitive reserve. Four types of evidence are presented: (i) brain and cognitive function in healthy aging, (ii) age of onset of symptoms of dementia, (iii) relation between clinical level and neuropathology for patients, and (iv) rate of cognitive decline in later stages of dementia. In all cases, bilinguals revealed patterns that were consistent with the interpretation of protection from cognitive reserve when compared with monolinguals.


Results

Table 1 provides background information for all included studies.

Each of the analyses were conducted in R using the metafor package (Viechtbauer, 2010) using the Hunter–Schmidt method of pooling variance. Forest plots were generated using the “meta” package (Schwarzer, 2007). We conducted five analyses: The first analysis examined the impact of incidence versus age-of-onset studies the second analysis was a sensitivity analysis and included only a single contribution per study the third analysis examined prospective studies only. Two final analyses used meta-regression to examine how education and SES interacted with bilingualism to affect Alzheimer’s age (combined incidence and onset).

Incidence versus age of onset

In this initial approach, effects from 21 studies were converted into d' (positive values indicate bilinguals were older) and entered separately for incidence and age of onset measures. Three studies reported both incidence and age of onset and were thus entered twice.

This initial analysis revealed a moderate overall effect of bilingualism, standardized mean difference (SMD) = 0.32, CI [0.22, 0.42] (see Fig. 1). The effect was stronger for studies examining age of onset, SMD = 0.40, CI [0.29, 0.51], than those which examined incidence, SMD = 0.10, CI [−0.08, 0.28], and the test for subgroup differences was significant. The confidence intervals for the incidence studies included zero and therefore did not reach statistical significance. A trim-and-fill analysis was then used to test for publication bias and recompute a corrected effect size (see Fig. 1b). The trim-and-fill procedure identified six “missing” studies (indicated by the hollow points in the funnel plot), and computed effect sizes for hypothetical studies that would normalize the distribution of effect sizes. Recomputing the meta-analysis led to an overall SMD of 0.22, CI [0.11, 0.33], which was still significant, Z = 3.98, p < .0001, suggesting that even after accounting for publication bias, bilinguals are older on average when they encounter Alzheimer’s disease than are monolinguals.

Effect of bilingualism in incidental and age of onset studies on Alzheimer’s. a A forest plot with subgroupings by study type. b A trim-and-fill funnel plot for the same data

Sensitivity analysis

A sensitivity analysis was conducted to limit each study to a single contribution, and thus the three studies with more than one effect (e.g., incidental and age-of-onset) were averaged prior to analysis (see Fig. 2). The sensitivity analysis yielded an overall SMD of 0.35, CI [0.24, 0.47], which was comparable to the uncorrected initial model. Trim-and-fill analysis also yielded a similar result, SMD = 0.25, CI [ 0.13, 0.37], suggesting that the initial analysis was not badly affected by the inclusion of separate effects for the three studies in question. Thus, although the first analysis did not reveal that incidence rates were reliably lower for bilinguals than for monolinguals, combining the effect sizes for incidence rates and age of onset in studies that report both (i.e. in prospective studies) leads to an overall reliable effect size. This suggests that even prospective studies may provide a protective effect of bilingualism on Alzheimer’s disease.

Sensitivity analysis results. a A forest plot of the results. b A trim-and-fill funnel plot for the same data

Prospective Studies

We next restricted the analysis to prospective studies only to examine how this type of design affected reported effect sizes for incidence and age of onset. Only six studies met this criterion, including the three which had both incidental and age-of-onset measures (see Fig. 3). Here, effect sizes were more moderate, SMD = 0.16, CI [0.04, 0.028], but were still reliably different from zero. Importantly, the test for subgroup differences was not significant, χ 2 = 1.00, p = .32, suggesting that bilingualism was indiscriminately associated with fewer incidents of Alzheimer’s disease and later age of onset of Alzheimer’s symptoms. The trim-and-fill analysis did not reveal any evidence of publication bias.

Prospective studies. a The forest plot results. b The trim-and-fill funnel plot

Sensitivity analysis for prospective studies

Once again, a sensitivity analysis was conducted to limit each prospective study to a single contribution, and again the three studies with two contributions were averaged prior to analysis (see Fig. 4). The sensitivity analysis yielded an overall SMD of 0.14, CI [0.00, 0.28], which, again, was similar to the initial model. Figure 5.

Prospective studies sensitivity analysis

Metaregression analyses between the age of onset/incidence and (a) education and (b) socioeconomic status (SES). All values are effect sizes (e.g., the difference in average education between monolinguals and bilinguals). Studies are weighted by their contribution, and this is represented by the size of each point on the plot

Meta-regression: Effects of education and socioeconomic status

For studies reporting education and SES or a close proxy such as level of occupational attainment, separate meta-analytic regressions were fit with age of onset/incidence between groups as the predicted outcome, and the difference between bilinguals and monolinguals on education and SES measures converted to d’ as the predictor. Once again, higher values indicate that bilinguals had higher scores. Both findings are represented in Fig. 4. Briefly, the meta-analytic effect of education differences between bilinguals and monolinguals on Alzheimer’s age was 0.013, CI [−0.15, 0.18], while the meta-analytic effect of SES differences between bilinguals and monolinguals on Alzheimer’s age was −0.1016, CI [−0.58, 0.38]. In short, while it is possible that each of these effects is predictive of Alzheimer’s on their own, differences between bilinguals and monolinguals on SES or education were not predictive of differences in age of onset or incidence of Alzheimer’s.

Effect of lab

Finally, given the recent trend among meta-analyses in the field of bilingualism attempting to disentangle potential laboratory biases from outcome measures, we also conducted a subgroup meta-analysis for age of onset by laboratory. We did not conduct a subgroup analysis for incidence studies as no lab produced more than one study of this type, and the results would not differ from those presented earlier.

We coded each lab numerically and ran a subgroup analysis as described above using “Lab” as the grouping variable (see Fig. 6). We grouped studies from labs reporting only a single result into an “Others” category. Figure 6a shows that there was a significant effect of subgroup, χ 2 = 14.6, p = .002 however, this was driven entirely by Lab 3, which provided lower estimates of age of onset than average, and once they were removed (see Fig. 6b), there were no longer any significant differences between research groups, χ 2 = 2.48, p = .29. This suggests that on the whole, the effect sizes being reported across research groups for age of onset is consistent.

Analysis of age of onset by lab group. a All the lab groups. b The subset of homogeneous groups


Bilingual brains are more resilient to dementia cause by Alzheimer’s disease

Lifelong bilinguals have increased connectivity in certain areas of the brain that help to protect them from dementia, according to a study published on Monday 30 January.

People who speak more than one language develop dementia symptoms an average of five years later and are able to cope with a greater level of brain dysfunction than monolinguals living in the same geographic area.

Researchers scanned the brains of 85 people in Northern Italy who were all at a similar stage of dementia due to probable Alzheimer’s disease. Forty five of them were German-Italian bilingual speakers and 40 were monolingual German or Italian speakers. Researchers used FDG-PET brain scans that detect glucose uptake to reveal how active different parts of the brain are and how well they are functionally connected to other brain regions.

On average, the bilinguals in the study were five years older than the monolinguals, despite being at the same stage of Alzheimer’s disease. Their brains showed reduced metabolism in key brain areas which implies a greater levels of dysfunction despite all study participants having a similar degree of impairment due to dementia. Compared to monolinguals, bilinguals showed increased functional connections between areas of the brain involved in executive control and the extent to which they use their second language was significantly correlated to activity in key neural networks.

Dr Clare Walton, Research Manager at Alzheimer’s Society, said:

'This elegant study provides new evidence that people who are fluent in more than one language have some protection against dementia. Brain scans showed that lifelong bilinguals have stronger connections between certain brain areas compared to those who only speak one language – this appears to allow their brains to cope better with damage before they start to show outward signs of dementia.

'In terms of lifestyle and risk of dementia, this type of study provides a vital piece of the puzzle – it doesn’t just tell us that bilingualism is linked to reduced risk of dementia, it begins to tell us why. As societies become more multicultural, this study indicates that the benefits of bilingualism could extend to helping future generations reduce their risk of the condition.'


Bilingualism and cognitive decline (or dementia risk) - Psychology

Objective: Clinic-based studies suggest that dementia is diagnosed at older ages in bilinguals compared with monolinguals. The current study sought to test this hypothesis in a large, prospective, community-based study of initially nondemented Hispanic immigrants living in a Spanish-speaking enclave of northern Manhattan. Method: Participants included 1,067 participants in the Washington/Hamilton Heights Inwood Columbia Aging Project (WHICAP) who were tested in Spanish and followed at 18–24 month intervals for up to 23 years. Spanish-English bilingualism was estimated via both self-report and an objective measure of English reading level. Multilevel models for change estimated the independent effects of bilingualism on cognitive decline in 4 domains: episodic memory, language, executive function, and speed. Over the course of the study, 282 participants developed dementia. Cox regression was used to estimate the independent effect of bilingualism on dementia conversion. Covariates included country of origin, gender, education, time spent in the United States, recruitment cohort, and age at enrollment. Results: Independent of the covariates, bilingualism was associated with better memory and executive function at baseline. However, bilingualism was not independently associated with rates of cognitive decline or dementia conversion. Results were similar whether bilingualism was measured via self-report or an objective test of reading level. Conclusions: This study does not support a protective effect of bilingualism on age-related cognitive decline or the development of dementia. In this sample of Hispanic immigrants, bilingualism is related to higher initial scores on cognitive tests and higher educational attainment and may not represent a unique source of cognitive reserve


Watch the video: Bilingualism and Cognition: How bilingualism wires your mind (May 2022).