Inconsistency of findings due to low power: A structural MRI study of bilingualism

A Comparison of White Matter Brain Differences in Monolingual and Highly Proficient Multilingual Speakers

Language processing relies on the communication between brain regions that is achieved through several white matter tracts, part of the dorsal, ventral, and medial pathways involved in language processing and control (Coggins et al., 2004; Friederici & Gierhan, 2013; Hickok & Poeppel, 2007; Luk et al., 2011). While changes in white matter tract morphology have been reported as a function of second language learning in bilinguals, little is known about changes that may be present in multilanguage users. Here we investigate white matter morphometry in a group of highly proficient multilinguals, (individuals with proficiency in four or more languages), compared to a group of monolinguals. White matter morphometry was quantified using a fixel-based analysis (Raffelt et al., 2015; Raffelt et al., 2017; Tournier et al., 2007). Higher fiber cross-section and lower fiber density values were observed for the multilinguals, in the dorsal pathways (superior longitudinal fasciculus and arcuate fasciculus) and the ventral pathway, including the inferior fronto-occipital fasciculus, inferior longitudinal fasciculus, and the uncinate fasciculus. Segments of the corpus callosum, the fornix, and the cortico-spinal tract showed decreases in all three morphometry measures for multilinguals. The findings suggest differential efficiencies in neural communication between domain-specific language regions and domain-general cognitive processes underlying multilingual language use. We discuss the results in relation to the bilingual Anterior to Posterior and Subcortical Shift (BAPSS) hypothesis (Grundy et al., 2017) and the Dynamic Restructuring Model (Pliatsikas, 2020).

Does Diglossia Impact Brain Structure? Data from Swiss German Early Diglossic Speakers

(1) Background: Bilingualism has been reported to shape the brain by inducing cortical changes in cortical and subcortical language and executive networks. Similar yet different to bilingualism, diglossia is common in Switzerland, where the German-speaking population switches between an everyday spoken Swiss German (CH-GER) dialect and the standard German (stGER) used for reading and writing. However, no data are available for diglossia, defined as the use of different varieties or dialects of the same language, regarding brain structure. The aim of our study is to investigate if the presence of this type of diglossia has an impact on the brain structure, similar to the effects seen in bilingualism. (2) Methods: T1-weighted anatomical MRI scans of participants were used to compare the grey matter density and grey matter volume of 22 early diglossic CH-GER-speaking and 20 non-diglossic French-speaking right-handed university students, matched for age, linguistics and academic background. The images were processed with Statistical Parametric Mapping SPM12 and analyzed via voxel- and surface-based morphometry. (3) Results: A Bayesian ANCOVA on the whole brain revealed no differences between the groups. Also, for the five regions of interest (i.e., planum temporale, caudate nucleus, ACC, DLPFC and left interior parietal lobule), no differences in the cortical volume or thickness were found using the same statistical approach. (4) Conclusion: The results of this study may suggest that early diglossia does not shape the brain structure in the same manner as bilingualism.

Replicable brain-phenotype associations require large-scale neuroimaging data

Numerous neuroimaging studies have investigated the neural basis of interindividual differences but the replicability of brain-phenotype associations remains largely unknown. We used the UK Biobank neuroimaging dataset (N = 37,447) to examine associations with six variables related to physical and mental health: age, body mass index, intelligence, memory, neuroticism and alcohol consumption, and assessed the improvement of replicability for brain-phenotype associations with increasing sampling sizes. Age may require only 300 individuals to provide highly replicable associations but other phenotypes required 1,500 to 3,900 individuals. The required sample size showed a negative power law relation with the estimated effect size. When only comparing the upper and lower quarters, the minimally required sample sizes for imaging decreased by 15-75%. Our findings demonstrate that large-scale neuroimaging data are required for replicable brain-phenotype associations, that this can be mitigated by preselection of individuals and that small-scale studies may have reported false positive findings.

Cortical Thickness in bilingual and monolingual children: Relationships to language use and language skill

There is a growing body of evidence based on adult neuroimaging that suggests that the brain adapts to bilingual experiences to support language proficiency. The Adolescent Brain Cognitive Development (ABCD) Study is a useful source of data for evaluating this claim during childhood, as it involves data from a large sample of American children. Using the baseline ABCD Study data collected at ages nine and ten, the goal of this study was to identify differences in cortical thickness between bilinguals and monolinguals and to evaluate how variability in English vocabulary and English use within bilinguals might explain these group differences. We identified bilingual participants as children who spoke a non-English language and were exposed to the non-English language at home. We then identified a matched sample of English monolingual participants based on age, sex, pubertal status, parent education, household income, non-verbal IQ, and handedness. Bilinguals had thinner cortex than monolinguals in widespread cortical regions. Within bilinguals, more English use was associated with greater frontal and parietal cortical thickness; greater English vocabulary was associated with greater frontal and temporal cortical thickness. These findings replicate and extend previous research with bilingual children and highlight unexplained cortical thickness differences between bilinguals and monolinguals.

A role for the cerebellum in the control of verbal interference: Comparison of bilingual and monolingual adults

We evaluate brain structure sensitivity to verbal interference in a sentence interpretation task, building on previously reported evidence that those with better control of verbal interference show higher grey matter density in the posterior paravermis of the right cerebellum. We compare brain structure sensitivity to verbal interference control across two groups, English monolingual (N = 41) and multilingual (N = 46) adults. Using voxel-based morphometry, our primary goal was to identify and explore differences in regional patterns of grey matter sensitivity to performance on the sentence interpretation task, controlling for group variability in age, nonverbal reasoning and vocabulary knowledge. There was no group difference in performance but there was a significant group effect in grey matter sensitivity to task performance in our region of interest: stronger sensitivity in the paravermis in bilinguals compared to monolinguals in accuracy performance in the high (relative to low) verbal interference condition. This effect was observed when the linguistic interference was presented in an unfamiliar language (Greek) but not when presented in the familiar language (English). Our findings suggest that multilanguage acquisition mediates regional involvement within the language network, conferring enhanced functional plasticity within structures (including the paravermis) in the service of control of linguistic interference.

The relationship between bilingual experience and gyrification in adulthood: A cross-sectional surface-based morphometry study

Neuroimaging evidence suggests that bilingualism may act as a source of neural plasticity. However, prior work has mostly focused on bilingualism-induced alterations in gray matter volume and white matter tract microstructure, with additional effects related to other neurostructural indices that might have remained undetected. The degree of cortical folding or gyrification is a morphometric parameter which provides information about changes on the brain's surface during development, aging and disease. We used Surface-based Morphometry (SBM) to investigate the contribution of bilingual experience to gyrification from early adulthood to old age in a sample of bilinguals and monolingual controls. Despite widespread cortical folding reductions for all participants with increasing age, preserved gyrification exclusive to bilinguals was detected in the right cingulate and entorhinal cortices, regions vulnerable with normal and pathological brain aging. Our results provide novel insights on experience-related cortical reshaping and bilingualism-induced cortical plasticity in adulthood.

Age of acquisition impacts the brain differently depending on neuroanatomical metric

Although researchers generally agree that a certain set of brain areas underlie bilingual language processing, there is discrepancy regarding what effect timing of language acquisition has on these regions. We aimed to investigate the neuroanatomical correlates of age of acquisition (AoA), which has been examined previously, but with inconsistent results, likely influenced by methodological differences across studies. We analyzed gray matter density, volume, and thickness using whole-brain linear models in 334 bilinguals and monolinguals. Neuroanatomical correlates of AoA differed depending on gray matter metric. Relative to early bilinguals, late bilinguals had thicker cortex in language processing and cognitive control regions, and greater density in multiple frontal areas and the right middle temporal and supramarginal gyri. Early bilinguals had greater volume than late bilinguals in the left middle temporal gyrus. Overall, volume was the least sensitive to AoA-related differences. Multiple regions not classically implicated in dual-language processing were also found, which highlights the important role of whole-brain analyses in neuroscience. This is the first study to investigate AoA and gray matter thickness, volume, and density all in the same sample. We conclude that cognitive models of bilingualism should consider the roles of development and neuroanatomical metric in driving our understanding of bilingual and monolingual language organization.