The impact of bilingualism on brain reserve and metabolic connectivity in Alzheimer's dementia

Abstract

Cognitive reserve (CR) prevents cognitive decline and delays neurodegeneration. Recent epidemiological evidence suggests that lifelong bilingualism may act as CR delaying the onset of dementia by ∼4.5 y. Much controversy surrounds the issue of bilingualism and its putative neuroprotective effects. We studied brain metabolism, a direct index of synaptic function and density, and neural connectivity to shed light on the effects of bilingualism in vivo in Alzheimer's dementia (AD). Eighty-five patients with probable AD and matched for disease duration (45 German-Italian bilingual speakers and 40 monolingual speakers) were included. Notably, bilingual individuals were on average 5 y older than their monolingual peers. In agreement with our predictions and with models of CR, cerebral hypometabolism was more severe in the group of bilingual individuals with AD. The metabolic connectivity analyses crucially supported the neuroprotective effect of bilingualism by showing an increased connectivity in the executive control and the default mode networks in the bilingual, compared with the monolingual, AD patients. Furthermore, the degree of lifelong bilingualism (i.e., high, moderate, or low use) was significantly correlated to functional modulations in crucial neural networks, suggesting both neural reserve and compensatory mechanisms. These findings indicate that lifelong bilingualism acts as a powerful CR proxy in dementia and exerts neuroprotective effects against neurodegeneration. Delaying the onset of dementia is a top priority of modern societies, and the present in vivo neurobiological evidence should stimulate social programs and interventions to support bilingual or multilingual education and the maintenance of the second language among senior citizens.bilingualism | Alzheimer's dementia | fluorine-18-fluorodeoxyglucose PET | brain reserve | brain metabolic connectivity

On Bilingual Aptitude for Learning New Languages: The Roles of Linguistic and Nonlinguistic Individual Differences

“…This goes against common perceptions of older adults as poor learners but parallels Rogers et al.’s () finding of no differences in aptitude between young adults and adults aged 30 to 70 years. Additionally, it complements evidence for domain‐general cognitive and neurological reserve in aging bilinguals (Abutalebi & Green, ; Gold, Johnson, & Powell, ; Perani et al., ) and preserved skills for processing known languages in older adults (Wingfield & Grossman, ). Future studies should further explore the possibility for potential maintenance of language aptitude in older age.…”

Effects of bilingualism on white matter integrity in older adults

“…For example, proponents of the cognitive reserve perspective (e.g., Craik et al, 2010; Perani et al, 2017; Schweizer et al, 2012) usually include Alzheimer’s disease patients in their studies whereas proponents of the neural reserve perspective typically recruit healthy older adults without disease progression (e.g., Abutalebi et al, 2015b; Li et al, 2017; Li et al, 2014; Olsen et al, 2015). Therefore, the theories largely describe different populations.…”

“…For example, Schweizer et al (2012) showed that bilingual patients with Alzheimer’s disease (AD) showed more brain atrophy in regions associated with the disease than monolinguals, despite equivalence on cognitive outcomes. More recently, Perani et al (2017) examined monolingual and bilingual Alzheimer’s disease patients that were matched for disease duration using positron emission tomography (PET). They showed that bilinguals were not only five years older than monolingual patients but also showed greater brain hypometabolism, which is a physiological index of the severity of Alzheimer’s disease.…”

Vulnerability of multiple large‐scale brain networks in dementia with Lewy bodies

“…Building on the core principle that brain regions whose metabolism is correlated at rest are functionally interconnected (Horwitz, Duara, & Rapoport, ), we applied a voxelwise seed‐based interregional correlation analysis (Lee et al, ). This method was previously validated for [18F]FDG–PET data (Horwitz et al, ; Lee et al, ) and allows to derive resting‐state metabolic networks starting from proper seed regions (Ballarini et al, ; Iaccarino et al, ; Perani et al, ; Sala et al, ; Tomasi et al, ; see also Sala & Perani, for a recent review).…”