Worldwide the majority of the human population are either bilingual or multilingual, with there being fewer monolinguals. Bilingualism is when a person is fluent in two languages and multilingualism relates to when someone is fluent in more than two languages. Monolingualism refers to people who can only speak one language fluently. Bilingual people are constantly exchanging between different sounds, concepts, grammatical structures and words when utilising two languages, and it is believed that this is a form of mental training which is consistent, permanent and rigorous (Bialystok & Viswanathan, 2009). Consequently, this is an advantage of being bilingual or multilingual and further benefits have been examined in terms of the cognitive advantages of bilingualism in delaying the onset of cognitive decline and mental disease specifically dementia.
Research has shown that bilinguals cognitive advantages include enhanced cognitive abilities, problem solving, symbol manipulation, metalinguistic awareness and improved memory (Bialystok, 1986; Blom, Küntay, Messer, Verhagen & Leseman, 2014). Additional advantages have been shown in non-verbal tasks which involve a bilingual person having to ignore misleading information in order to solve a task successfully. Bilinguals are thought to be better at these tasks due to their ability to be able to ignore one language whilst using their other language (Bialystok & Majumder, 1998; Rodriguez-Fornells, Rotte, Heinze, Nösselt & Münte 2002).
Extending this knowledge to investigate how learning a second language can enhance cognition and delay or slow down the onset of dementia reveals that being bilingual has significant effects for brain structure and brain functioning where memory is concerned (Wodniecka, Craik, Luo & Bialystok, 2010). For example, Mårtensson (2012) conducted MRI brain scans on monolinguals and bilinguals and discovered that bilingual participants had significantly larger hippocampi, an area of the brain responsible for memory (Nadel & Peterson, 2013). Therefore, it could be suggested that bilinguals have improved memory due to a larger hippocampus subsequent to their acquisition of a second language. The potential for bilingualism and multilingualism to provide long term mental health benefits suggests that second language acquisition in middle aged and older adults could slow down and delay cognitive decline which is common in older age (Hedden & Gabrieli, 2004; Nilsson, 2003; Stern, 2002). More specifically, could learning a second language delay the onset of dementia, a mental disease affecting memory and cognition?
Dementia is known to be caused be plaques and tangles which are protein based substances which accumulate in the brain and decrease the number of brain cells through destroying connections between brain cells and so fewer neurotransmitter chemicals can send signals between brain cells. Dementia also reduces neural plasticity, which is where the brain creates new neural connections and in turn neural plasticity negatively alters brain structure and function (which can also be altered positively through language acquisition) (Li, Legault & Litcofsky, 2014). Therefore, as language acquisition can encourage neural plasticity and so cognitive performance, bilingualism can act as a protective function which also enriches vasculature in the brain (Fratiglioni, Paillard-Borg & Winblad, 2004) and can delay the onset of dementia better than in monolinguals (Bialystok, Craik & Freedman, 2007; Craik, Bialystok & Freedman, 2010).
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Research conducted by Woumans et al. (2015) supported the concept that language acquisition can delay the onset of dementia. Woumans et al. investigated the effects of bilingualism on notable clinical symptoms of dementia, specifically Alzheimer’s disease (AD) utilising a European sample of individuals who had been diagnosed with probable AD. Results showed that there was a significant delay in age of manifestations of symptoms and diagnosis of AD in bilingual participants compared to monolinguals, with a delay of 4.6 years in manifestations of symptoms and 4.8 years in diagnosis. This therefore supports and gives evidence towards the notion that being bilingual can delay and slow down the onset of dementia. Further research by Bialystok, Craik and Freedman (2007) also investigated the effects of bilingualism in dementia onset. The authors controlled for factors such as cultural differences, immigration, education and employment, and found that in a sample of 184 patients with dementia that bilinguals (51%) showed symptoms of dementia on average 4.1 years later than monolinguals (49%). Whilst these results support the benefits of bilingualism for dementia, it must be noted that in the four years subsequent to diagnosis, the rate of cognitive decline was the roughly the same for all participants. Therefore, this research suggests that bilingualism can delay the onset of dementia, but it cannot reduce the rate of decline once dementia is diagnosed. In addition, however, this research was conducted using qualitative interviews with participants and their families/friends and this methodology encompasses subjectivity and required interviewees (who had no professional clinical knowledge) to estimate when symptoms of cognitive decline began, and so this study may lack validity. Furthermore, AD diagnosis was accompanied by other mental health conditions (e.g. depression and cerebrovascular disease) which could affect the age at which AD was diagnosed and so limit how much bilingualism can be attributed to the findings.
Consequently, the same authors conducted a similar follow-up study to further investigate their findings (Craik, Bialystok & Freedman, 2010). This study controlled for cognitive abilities, occupation and immigration status. The research found that the onset of symptoms was delayed by 5.1 years and diagnosis was 4.3 years later in bilinguals than in monolinguals, therefore evidencing that bilingualism can delay the onset of dementia. Nevertheless, it is important to highlight that psychological and environmental factors can affect the onset of this biologically based mental disease. For example, Paillard-Borg, Fratiglioni, Xu, Winblad & Wang, (2012) found that lifestyle and immigration status can enhance cognition through life experiences. Lifestyle factors such as physical activity, increased socialisation and cognitive stimulation have been shown to reduce cognitive decline (Lautenschlager et al., 2008; Flicker, 2009).
One further aspect to consider in respect to factors which may mediate how bilingualism affects cognitive decline includes the relation between the languages that an individual is fluent in. Related languages such as Spanish and French may produce a greater or lesser effect on cognition than un-related languages such as English and Chinese (Feldman & Shen, 1971; Hakuta & Diaz, 1985). Considering the relatedness of languages is important in the effect of bilingualism and the onset of dementia as if the relatedness of fluent languages can improve cognition then a more enhanced cognitive state may be beneficial in slowing down the onset of dementia, however this topic ought to be further investigated. Another factor which may affect the relationship between bilingualism and dementia is intelligence. Individual differences such as intelligence levels have been shown to positively impact cognition and memory abilities (Biedroń & Szczepaniak, 2012), therefore a higher level of intelligence combined with bilingualism may further delay the onset of dementia.
Although there is strong evidence for the positive effects of bilingualism in delaying the onset of dementia as previously mentioned, further alternative factors such as drugs and diet play a significant role also. Shah et al. (2009) proposed that antihypertensive medications which are used to lower blood pressure (e.g. ACE inhibitors and diuretics) may be able to reduce the progression of dementia. This concept was supported by Rouch et al. (2015) who found that in seven longitudinal studies analysing more than one million individuals that antihypertensive medication was beneficial when assessing cognitive impairment and decline. Therefore, conclusions suggested that antihypertensive medication was able to lower the risk of cognitive decline and dementia. Additionally, this was found for two types of dementia; vascular dementia and AD.
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Another factor relating to diet has also been suggested to protective against cognitive decline and dementia. In particular the consumption of caffeine (e.g. coffee and tea) (Panza et al., 2015; Santos, Costa, Santos, Vaz-Carneiro, & Lunet 2010; Arab, Khan & Lam, 2013). In addition, fruit and vegetable consumption has also been found to significantly lower the rate of cognitive decline which may lower the risk of dementia diagnosis (Loef & Walach, 2012). Loef and Walach reviewed the effects of fruit and vegetable consumption and discovered that only vegetable consumption provided significant evidence of reduced risks for cognitive decline and dementia onset. An explanation for this could be due to the high content of nutrients and vitamins in vegetables which modulate detoxifying enzymes and cholesterol synthesis, protect the immune system, and act as an antibacterial, antioxidant and neuroprotective substance (Lampe, 1999).
In conclusion, research does suggest that acquiring a second language can enhance cognition and act as a protective factor in delaying the onset of dementia, but not in preventing it. It is suggested that acquiring a second language can enhance cognition and delay the onset of dementia by increasing areas of the brain responsible for memory such as the hippocampus, encouraging neural plasticity and connectivity, and by improving vasculature in the brain. As well as bilingualism, multilingualism has been found to show similar effects. However, it is important to consider alternative explanations as many other factors such as intelligence, lifestyle, diet and medication can also delay cognitive decline and reduce the risk of dementia.
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