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

双语作为一种经历，能够极大地改变大脑结构和认知，最初是灰质增加，随后是白质完整性增加，最后是皮层下和白质结构的重整1。一些人发现，双语者对大脑前部的依赖更少，而功能网络更广泛地分布在整个大脑中，从而将依赖转移到更多的后部区域。大脑的主要功能网络也被发现在双语者中更容易相互作用，额顶叶和默认模式网络对其他网络的激活做出反应。大脑的结构变化是否必然导致功能变化一直存在争议，因为在结构联系似乎不存在的地方发现了一些功能联系。在双语者中，分别进行了结构1,7-10和功能2,11-14连接分析，结果表明两者都受到语言经验的各种因素的影响。静息状态功能磁共振成像（fMRI）研究发现，双语者参与认知和语言控制的神经网络之间的连通性增加了11,15，这为双语者神经效率的提高提供了可能的解释，这意味着他们比单语者完成任务需要更少的认知努力12。另一项脑电图研究发现，第二语言熟练程度调节了双语者对注意力的控制，这表明双语所带来的任何认知影响可能存在于更多的频谱上，而不是作为一种二分类群体效应。静息状态脑电图数据显示，双语会改变静息状态下的大脑节律，甚至可以预测一个人的语言学习速度。RS-EEG也可用于功能连通性分析19,20，尽管这些分析尚未在双语人群中进行，并且在他们之间进行的结构与功能连通性比较分析很少8,9。本研究的目的有三个方面：首先，比较双语者的结构和功能连接，并确定动态重构模型1预测的结构变化是否与功能变化相对应。这将使用扩散张量成像进行结构分析，静息状态fMRI进行功能分析，使用预先存在的数据集。其次，比较双语组和单语组之间的功能连通性。为此，静息状态脑电图测量将被使用，收集第一手资料。第三，确定功能连接的差异是否会影响完成认知任务时的神经效率。为了达到这个目的，将在修改语言的Flanker任务期间进行ERP读数。这也将从参与RS-EEG部分研究的同一参与者那里收集第一手资料。

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.