The Impact of Bilingualism on Dementia: Integrating a Sociocultural and Biological Framework

As of 2023, there are 7164 languages spoken in the world, with 60% of the global population speaking more than one language, explained Suvarna Alladi, DM, professor neurology at the National Institute of Mental Health and Neurosciences in Bangalore, India, during a plenary session at the 2024 Alzheimer’s Association International Conference (AAIC) in Philadelphia, Pennsylvania. According to Alladi, approximately 40% of the world is monolingual, while 43% are bilingual and 17% are multilingual.¹

Alladi herself is multilingual, as she speaks English fluently, and also speaks Bengali, German, Hindi, Kannada, Tamil, and Telugu with varying levels of working proficiency. However, she noted that speaking more than one language is not exceptional in the world, and is especially not exceptional in India, where 33% of the population speaks more than one language; in India, Alladi explained that there are 122 major languages, 22 official languages, and 1599 other languages spoken.¹

According to Alladi, after decades of reflecting on language and its capacity to shape our thoughts and allow us to share those thoughts with others, she has come to better understand the impact of dementia on individuals with this neurological condition.¹

“Our capacity… to communicate and share our thoughts with others is immense. When a person develops dementia, it is this particular capacity that is lost,” Alladi said during the AAIC session. “With 7000 living languages and 60% of the world speaking more than one of those languages, it's imperative for us to talk seriously about linguistic diversity [and dementia].”¹

Alladi noted that the evolution of the genetic predisposition toward accommodating for rapid cultural changes to linguistic structures is important to recognize when reconciling the diversity of human language with what has remained a largely uniform biological basis for learning language. Throughout human history, as communities across the world have come into contact through travel, trade, and technological advancements that support broader social communication, we have seen cultural divergence, linguistic diversity, and bilingualism emerge. In recent years, social media and the internet have helped to accelerate this trend.¹

However, neuropsychological assessments, which make up the backbone of dementia diagnosis, are largely developed with a sociocultural framework that is monolingual and “Western,” and also assumes literacy, according to Alladi. However, approximately 14% of the world is illiterate. This means that developing a framework for neuropsychological testing that accounts for language diversity, cultural heterogeneity, educational variability, and complex skills is needed in order to adequately diagnose, research, and understand the disease.¹

“Models of cognition, language, and brain structure and function developed assuming monolingualism as the norm do not capture the [experience of the] majority, who are bilingual or multilingual,” Alladi said.¹

This is especially true as research has shown that people who speak more than one language may experience neurodegeneration impacting each of those languages at different rates, according to Alladi.¹ In particular, findings of a study by Ellajosyula et al showed that a person’s less proficient language is more vulnerable to neurodegeneration.² Additionally, a study by Grasso et al demonstrated that bilinguals with dementia experience significant improvements on trained communication skills, with implications regarding approach to treatment for patients with dementia who are bilingual vs monolingual.³

Grasso et al explained further that much of the dementia research conducted has investigated older adults who are non‐Hispanic and White. In their study published in 2023, they investigated bilingualism relative to monolingualism in Mexican Americans, looking at their performance on neuropsychological measures. The investigators also conducted an exploratory analysis on plasma biomarkers to contextualize their findings regarding rates of impairment in the mild cognitive impairment (MCI) patient group. Grasso et al explained that they observed a bilingual phenotype of MCI in Mexican Americans such that particular components of executive functions were relatively spared among bilingual patients. However, they also observed that executive function tasks that interact with lexical access were particularly vulnerable in unbalanced or late bilinguals. The authors explained that these findings demonstrate that clinicians should account for bilingual factors in the diagnostic decision‐making process in order to elevate the standard of care for this historically underserved and growing segment of the population.³

In her AAIC plenary presentation, Alladi discussed a study conducted by herself and colleagues in India with results published in the Archives of Clinical Neuropsychology that provide several standards to help clinicians when approaching neuropsychological testing of bilinguals¹:

• Evaluations of linguistic proficiency of bilingual speakers should use either subjective or objective measures.
• Cognitive testing should be administered in the most proficient language of the speaker.
• The examiner administering the test should be proficient in the target language used for testing.
• Language interference (ie, borrowed and language-mixed words) should be considered while interpreting the responses during testing.
• In a clinical diagnostic setting, there is no need for separate norms for monolingual and bilingual adults in the context of India.

However, it is also known that bilingualism can delay the onset of dementia, with research demonstrating that lifelong bilingualism can contribute to cognitive reserve, which acts to compensate for the accumulation of amyloid and other brain pathologies. In a study by Craik et al with results published in Neurology in 2010, the authors observed that bilingual patients were diagnosed 4.3 years later than monolingual patients, and the onset of symptoms occurred 5.1 years later than monolingual patients.^1,5 Additionally, the authors noted that the effect did not appear to be attributable to confounding factors such as education, occupational status, or immigration.⁵ According to Alladi, this study helped to support an understanding that bilingualism appears to contribute to cognitive reserve, which helps to compensate for the effects of accumulated neuropathology.^1,5

However, it is also known that bilingualism can delay the onset of dementia, with research demonstrating that lifelong bilingualism can contribute to cognitive reserve, which acts to compensate for the accumulation of amyloid and other brain pathologies. Image Credit: © Sophie - stock.adobe.com

In a paper published by Alladi et al in Neurology in 2013, they also showed similar findings, with the study cohort including 648 patients with dementia in a clinic in India.⁶ The study further assessed the impact of bilingualism on dementia in relation to education and immigration status, with their findings showing that the delay in onset of dementia occurred in patients independent of these factors.^1,6 However, Professor Emeritus James A. Mortimer, PhD, of the Department of Epidemiology and Biostatistics at the University of South Florida, claimed that the data provided in the study are not sufficient to draw this conclusion. Mortimer explained further in his response that the characteristics of the patients in the study differed considerably between the monolingual and bilingual patients, with the monolingual patients certainly having a lower education, greater illiteracy, and more rurality than the bilingual patients, which he stated are factors known to be related to lower life expectancy.¹ However, Alladi noted that the study was particularly focused on this issue, as it was the first study to report a bilingual advantage in patients who were illiterate, which the authors had noted suggests that education is not a sufficient explanation for the observed difference.^1,6 What Mortimer’s response helped to illustrate, according to Alladi, was an ongoing misunderstanding of the global bilingual experience among researchers.

Following these studies published in 2010 and 2013 in Neurology, 5 more studies were published demonstrating additional evidence of the benefit of bilingualism, independent of factors such as education, literacy, and rurality. These studies were conducted in Belgium (Woumans et al in 2015), Wales (Clare et al in 2014), Luxembourg (Perquin et al in 2015), China (Zheng et al in 2018), and Los Angeles (Mendez et al in 2020), Alladi explained.¹

However, Alladi noted there were 2 studies by Zahodne et al (2014) and Lawton et al (2015) which did not show bilingualism had a beneficial impact on dementia. These studies were conducted in Manhattan and Sacramento, with the study in Manhattan demonstrating that, although bilingualism was associated with better performance in memory and executive functioning among 1067 Spanish-speaking immigrants, bilingualism was not associated with a reduced conversion of cognitive decline. In the study in Sacramento, the cohort included 1789 Spanish-English bilinguals, the majority of whom were immigrants, with the findings showing that neither bilingualism nor education level were associated with age of dementia onset.¹

Yet, Alladi explained that sociolinguistic context matters. In particular, the degree to which neurocognitive adaptations occur depends on the nature and the degree of the bilingual experience. Overall, Alladi concluded that language, bilingualism, and brain research offer an important framework for future scientific enquiry in dementia.¹

“This framework is one that is inherently diverse, capable of integrating sociocultural and biological research, and can influence policy contextually,” Alladi said.¹

REFERENCES

1. Alladi S. Bilingualism and Dementia: Implications for Brain Health and Policy Development. 2024 Alzheimer’s Association International Conference; Philadelphia, Pennsylvania; July 28-August 1, 2024.
2. Ellajosyula R, Narayanan J, Patterson K. Striking loss of second language in bilingual patients with semantic dementia. J Neurol. 2020;267(2):551-560. doi:10.1007/s00415-019-09616-2
3. Grasso SM, Clark AL, Petersen M, O'Bryant S; Health and Aging Brain Study (HABS‐HD) Study Team. Bilingual neurocognitive resiliency, vulnerability, and Alzheimer's disease biomarker correlates in Latino older adults enrolled in the Health and Aging Brain Study - Health Disparities (HABS-HD). Alzheimers Dement (Amst). 2023;15(4):e12509. doi:10.1002/dad2.12509
4. Paplikar A, Alladi S, Varghese F, et al. Bilingualism and Its Implications for Neuropsychological Evaluation. Arch Clin Neuropsychol. 2021;36(8):1511–1522. doi:10.1093/arclin/acab012
5. Craik FI, Bialystok E, Freedman M. Delaying the onset of Alzheimer disease: bilingualism as a form of cognitive reserve. Neurology. 2010;75(19):1726-1729. doi:10.1212/WNL.0b013e3181fc2a1c
6. Alladi S, Bak TH, Duggirala V, et al. Bilingualism delays age at onset of dementia, independent of education and immigration status. Neurology. 2013;81(22):1938-1944. doi:10.1212/01.wnl.0000436620.33155.a4