Structural and functional connectivity in the bilingual brain across the lifespan: how experience shapes neurology and cognition

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Bilingualism as an experience has the capacity to drastically change brain structure and cognition, with initial increases in grey matter, followed by increases in white matter integrity, and finally, renormalisation of subcortical and white matter structures1. Changes in functional connectivity as a result have been predicted, with some finding that bilinguals rely less on the anterior portion of the brain and that the functional networks are more widely spread across the brain, shifting reliance to more posterior areas2. The brain's major functional networks have also been found to interact with each other more readily in bilinguals, with the frontoparietal and default mode networks responding to the activation of the other2-5. Whether structural changes in the brain necessarily lead to functional changes has been debated6, as some functional connections have been found where structural connections seemingly do not exist. In bilinguals, separate structural1,7-10 and functional 2,11-14 connectivity analyses have been conducted, with findings indicating that both are affected by various factors of the language experience.Resting-state fMRI studies found increased connectivity in bilinguals between the networks involved in cognitive and language control11,15, providing a possible explanation for increased neural efficiency also found in bilinguals, meaning that they require less cognitive effort to complete tasks as monolinguals12. Another EEG study found that second-language proficiency modulates control of attention in bilinguals, indicating that any cognitive effects that bilingualism provides may exist on more of a spectrum than as a dichotomous group effect16. Resting-state EEG data has shown that bilingualism changes resting brain rhythms, and can even predict an individual's rate of language learning17,18. RS-EEG can also be used for functional connectivity analyses19,20, though these analyses are yet to be conducted on bilingual populations, with very few comparative structural versus functional connectivity analyses having been conducted amongst them either8,9.The aims of the proposed study will be threefold: Firstly, to compare structural and functional connectivity in bilinguals, and determine whether structural changes predicted by the Dynamic Restructuring Model1 correspond with functional changes. This will be achieved using diffusion tensor imaging for structural analysis, and resting-state fMRI for functional analysis, using a pre-existing dataset. Secondly, to compare functional connectivity between bi- and monolingual groups. For this, resting-state EEG measurements will be used, collected first-hand. And thirdly, to determine whether any differences in functional connectivity affect neural efficiency when completing cognitive tasks. ERP readings will be taken during a language-modified Flanker task to achieve this aim. This will also be collected first-hand from the same participants included in the RS-EEG portion of the study.