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

作为一种体验，双语能力能够极大地改变大脑结构和认知，最初灰质增加，随后白质完整性增加，最后皮质下和白质结构重新正常化1。因此，功能连接的变化已经被预测到了，一些人发现，双语者对大脑前部的依赖较少，功能网络更广泛地分布在大脑中，将依赖转移到更后面的区域2。在双语者中，大脑的主要功能网络也被发现更容易相互作用，额顶和默认模式网络对其他2-5的激活做出反应。大脑的结构变化是否一定会导致功能变化一直存在争议，因为已经发现了一些结构连接似乎并不存在的功能连接。在双语者中，分别进行了结构上的1，7-10和功能上的2，11-14的连接性分析，结果表明两者都受到语言体验的各种因素的影响。最近的fMRI研究发现，双语者参与认知和语言控制的网络11，15之间的连接性增强，这为双语者也发现了神经效率的提高提供了可能的解释，这意味着他们作为单一语言者完成任务所需的认知努力更少12。另一项脑电研究发现，第二语言熟练程度会调节双语者的注意力控制，这表明，双语提供的任何认知效应可能更多地存在于一种频谱上，而不是作为一种二分群体效应16。静态脑电数据表明，双语会改变静止的大脑节奏，甚至可以预测一个人的语言学习速度17，18。RS-EEG也可以用于功能连通性分析19，20，尽管这些分析还没有在双语人群中进行，很少有人对结构和功能连通性进行比较分析8，9。本研究的目的有三个：第一，比较双语者的结构和功能连通性，并确定动态重组模型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，并使用预先存在的数据集使用静息状态功能磁共振成像进行功能分析。其次，比较双语组和单语组的功能连接性。为此，将使用静态脑电测量，并收集第一手资料。第三，确定在完成认知任务时，功能连通性的任何差异是否会影响神经效率。为了实现这一目标，将在一项语言修改的侧翼任务中获取事件相关电位读数。这也将从包括在研究的RS-EEG部分的相同参与者那里第一手收集。

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.