Bilingualism across the lifespan: Neuroanatomical correlates

Abstract

Recently, an increasing number of studies addressing the neuroanatomical bases of bilingualism have appeared (Garcia-Penton et al., 2016). However, the results are variable and in sorne cases conflicting,and consequen y it is still a matter of debate how brain changes due to bilingual experience.The present study will try to shed sorne light on the field by adding fresh new evidence testing children and elderly high proficient early Spanish-Basque bilinguals, two very typologically different languages . The proposed work will use large-scale brain-mapping techniques to explore the relationship between structure and function, as a more holistic and realistic approach to understanding comprehensively the neural bases of bilingualism. This integrational perspectiva will also promote convergent evidence about the specialization and integration of the neural networks in bilingualism. As such, this work will study the organisation of brain networks,either due to slow changes in brain areas and their wiring (namely, the structural plasticity), or due to fast modulation of their interactions (namely, functional plasticity).This thesis will employ Functional Magnetic Resonance lmaging (fMRI) during resting-state in combination with Diffusion-Weighted Magnetic Resonance lmaging (DW-MRI) to determine functional and structural connectivity, respectively. Both techniques will make it possible to model the large-scale structural/functional connectivity maps by means of a high­ dimensional parcellation of the grey matter (GM) in the brain instead of limiting analysis to specific regions of interest, as done in previous studies. A 30 high resolution whole-head anatomical sean (T1-MRI) will be used in order to generate GM parcellations employed in the connectivity analysis, but also to identify regional differential structural patterns associated with bilingualism, using voxel-based and surface-based analyses of the GM. Network­ based statistics (Zalesky et al., 2010) and graph theoretical approaches (Latora & Marchiori, 2001; Rubinov and Spoms, 201O) will be employed to investigate differences between groups in connectiv ity pattems, by isolating sets of regions interconnected differently between groups, and in topological properties of the networks, by measuring global/local efficiency. The main findings of this research on bilingualism across different groups of age (childhood and elderly) suggested that structural brain plasticity related to bilingualism was so small, unstable, subtle and transient that it was very difficult to detect even in lifelong bilinguals. A fact that is consisten! with the curren! ambiguous picture in bilingualism studies (Garcia-Pentón et al.,2016; see also others, Baum & Titone,2014; Costa,& Sebastián-Gallés , 2014; Li, Legault, & Litcofsky, 2014; Paap et al., 2015; de Bruin et al., 2015a). However, this study suggested that even when the brain did not display focal brain differences (i.e. did not show any specialization) it could still show differences at the global level. Specifically,the evidence draws attention that lifelong bilingualism could pinpoint a gain toward a better neural reserve in aging due to the whole-network graph-efficiency observed in elderly lifelono bilinouals.