Brain plasticity and language learning

认知神经科学的一个基本问题是大脑的语言处理系统是如何发展的，以及它是如何受到早期语言学习经历的影响的。本研究利用自然语言情境来探索与大脑可塑性和语言发展相关的问题，并将高分辨率结构和功能脑成像与行为测试相结合。我们在魁北克的不同语言学习者中提出了四个实验，重点关注与早期语言学习经验相关的语言回路的持久性和损耗。实验1考察了同时双语者（出生时两种语言均高度精通）、顺序双语者（5岁早期一种语言，5岁以后一种语言，两种语言均高度精通）和单语者（精通一种语言）语音感知的神经基础。在这里，我们假设，从出生开始接触两种语言可能会使感知空间的结构不同，因此，与晚期二语学习者，甚至与单语者观察到的神经组织不同。实验2研究了强化语言训练相关的神经变化的本质，以及这种变化如何影响语言感知。在这里，我们测试了一个假设，即对于晚期学习者来说，感知窗口将关闭，无法学习新的类别。实验3评估了在一个新的语音学习任务（印地语牙音反射）上对这些学习者群体的训练是否会为那些早期接触两种语言系统的人提供优势，而不是一个，并预测早期的感知结构可能会推广到新的语言学习情况。我们预测，两种声音系统的早期专业化将允许归纳出新的模式。最后，在实验4中，我们解决了这样一个问题：在婴儿时期突然失去一门语言后，神经回路是否仍然保持可塑性，从而使这些说话者在以后的生活中重新习得这门“第一”语言时有优势，或者这些回路是否被另一种语言经验覆盖，而没有提供任何早期优势。因为人们认为声音模式的形成过程可能发生在发育早期的最佳时期，因此在此期间接受的输入可能对以后的语言理解能力至关重要。该研究项目致力于研究人类大脑在多大程度上因语言学习而发生改变的能力；也就是说，神经模式在多大程度上是固定的，以及这些模式在多大程度上可以在以后的生活中被改变。这项工作的优势在于，我们研究了非常具体的行为变化，将大脑如何通过增强语言学习而变化联系起来，同时研究了促进个体参与者更快或更成功学习的因素。这些研究的结果应该证实，与语言感知回路相关的神经可塑性甚至可以在成人大脑中观察到，并且应该阐明大脑如何响应训练而变化。拟议的研究应有助于揭示与语言接触年龄有关的大脑发育的神经基础，因此可能导致加拿大语言教育政策的进步。

A fundamental question in cognitive neuroscience is how the brain’s language-processing system develops and how it is affected by early language learning experiences. The work proposed takes advantage of natural linguistic situations to explore issues related to brain plasticity and language development, and combines high resolution structural and functional brain-imaging with behavioural testing. We propose four experiments in different language learners of Quebec that focus on persistence and attrition of language circuits related to early language-learning experience. Experiment 1 examines the neural basis of speech perception in simultaneous bilinguals (two languages from birth and highly proficient in both), sequential bilinguals (one language early and one language after age 5 years and highly proficient in both) and monolinguals (proficient in one language). Here we hypothesize that exposure to two languages from birth may allow a different structuring of perceptual space and, hence, a different neural organization from that observed in late L2 learners and, even more so, from that of monolingual speakers. Experiment 2 examines the nature of neural change related to intensive language training and how this impacts speech perception. Here we test the hypothesis that the perceptual window will be closed to learning new categories for late learners. Experiment 3 assesses whether training of these learner groups on a novel phonetic learning task (the Hindi dental-retroflex) will provide an advantage to those with earlier exposure to two language systems as compared to one, with the prediction that early perceptual structuring may generalize to new language-learning situations. We predict that early specialization with two sound-systems will permit generalization to novel patterns. Finally, in Experiment 4, we address the question as to whether, after abrupt loss of a language in infancy, the neural circuits still remain plastic, thus yielding an advantage to these speakers in the re-acquisition of this “first” language later in life, or whether these circuits are overwritten by another language experience, without providing any early advantage. Because it is thought that that the laying down of sound patterns is a process that potentially occurs during an optimal period early in development, the input received during this time may be crucial in understanding language abilities later in life. The program of research addresses the extent to which the human brain has the capacity to change as a result of language learning; that is, the extent to which neural patterns are fixed and the extent to which these patterns can be altered later in life. This work has strength in that we examine very specific changes in behavior, to make a link with how the brain changes through enhanced language learning, while concurrently examining the factors that contribute to more rapid or more successful learning in our individual participants. The results from these studies should confirm that neuroplasticity related to speech perceptual circuits can be observed even in adult brains and should shed light on how the brain changes in response to training. The proposed studies should help reveal the neural underpinnings of brain development in relation to age of language exposure and as such could lead to advances in language-education policies in Canada.