|Alternative Title||Why are there critical periods for language learning？|
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语言能力是人类的特征之一.人类语言具有极其复杂的结构,可以用有限的元素生成出无数的语句,用来表达思想、情感,传承人类文明.但人类是如何获得和学习语言的问题至今未解.Science杂志在2005年提出了125个需要探索的科学问题,＂为什么存在语言学习的关键期＂（Why are there critical periods for language learning？）即是其中之一.本文重点阐述控制或调节语言学习关键期运作的因素,以及影响关键期内语言学习的重要条件,并指出了未来可能的研究方向.
The unique human ability is linguistic communication. Only human being can produce an infinite number of meaningful messages through a finite number of lexical items and use language as a tool of mutual communication and mentalactivity. It is a mystery that human infants learn language so easily and quickly. Although researchers from different areas have been devoted to explore underlying mechanism of language learning for several decades, why there are critical periods for language learning is still an open question. The classic notion of a critical period（CP） is a window, typically in early development, during which a system is open to structuring or restructuring on the basis of input from the environment. Before and after this period, environmental influences cannot affect the sensitivity or response properties of that particular system. CPs have been observed in various systems across species. Recently, it was found that even in the visual system, experience can have an impact at time points outside of the CP. The alternate term, sensitive（or optimal） period, was introduced to reflect the fact that windows do not always open and close abruptly and may never close completely. Learning is possible throughout the life span. Thus, what processes open/close（or reopen） CPs and mediate their operation is the core of answering why there are critical periods for language learning. Recent cellular and molecular findings from the development of the visual system provide insights into neural and biological mechanisms of CPs in language learning. The balance between the excitatory and inhibitory activity defines the critical period. The onset and offset of a critical period is depended upon the excitatory–inhibitory balance of neurotransmitters in the brain. Additionally, the molecular mechanism affects the critical period is different for each stage. The onset is attributed to molecular triggers that can shift neural circuits from an immature to a plasticstate. The neural circuits using the neurotransmitter gamma-aminobutyric acid（GABA） are crucial for the onset of the critical period. During the plastic period, some molecular factors enable changes in neural circuitsin response to sensory experience. Finally, the offset is defined by molecular brakes, which limit excessive circuit rewiring, prevent further structural changes, and shift the neural circuit to a stable state. A study on the speech perception development of infants whose mother had experienced depression throughout their pregnancy and had chosen to take an antidepressant（serotonin reuptake inhibitor（SRI）） showed that prenatal exposure to SRIs accelerated the entire speech perception trajectory. By 6 months of age, SRI-exposed infants had already stopped discriminating the pair-vowel contrast compared to the infants of a control group of mothers who had not experienced depression. This is direct evidence that the onset and offset of CPs in language learning can be triggered. Also, statistical learning from the distributional properties in speech and social interaction play important roles in infant initial language learning and could govern brain plasticity during critical periods. Brain and behavioral studies have indicated a very complex set of interacting brain systems in the initial acquisition of language early in infancy, many of which appear to reflect adult language processing. Thus, some interesting issues, such as how the changes of maturation and experience affect the onset and offset of a critical period, why the different aspects of language（phonology, syntax, and semantics） have different time frames in CP will continuously attract researchers＇ attentions. The combination of developmental neuroscience and molecular tools will eventually elucidate the mechanisms underlying CP in language learning.
|Keyword||语言 关键期 敏感期 语言学习 环境输入|
|Corresponding Author||李甦; 杨玉芳|
|李甦,杨玉芳. 为什么存在语言学习的关键期?[J]. 科学通报,2016,61(25):2786-2792.|
|MLA||李甦,et al."为什么存在语言学习的关键期?".科学通报 61.25(2016):2786-2792.|
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