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The critical period hypothesis is the subject of a long-standing debate in linguistics and language acquisition over the extent to which the ability to acquire language is biologically linked to age. The hypothesis claims that there is an ideal time window to acquire language in a linguistically rich environment, after which further language acquisition becomes much more difficult and effortful. The critical period hypothesis states that the first few years of life is the crucial time in which an individual can acquire a first language if presented with adequate stimuli. If language input doesn't occur until after this time, the individual will never achieve a full command of language—especially grammatical systems. The evidence for such a period is limited, and support stems largely from theoretical arguments and analogies to other critical periods in biology such as visual development, but nonetheless is widely accepted. The nature of such a critical period, however, has been one of the most fiercely debated issues in psycholinguistics and cognitive science in general for decades. Some writers have suggested a "sensitive" or "optimal" period rather than a critical one; others dispute the causes (physical maturation, cognitive factors). The duration of the period also varies greatly in different accounts. In second-language acquisition, the strongest evidence for the critical period hypothesis is in the study of accent, where most older learners do not reach a native-like level. However, under certain conditions, native-like accent has been observed, suggesting that accent is affected by multiple factors, such as identity and motivation, rather than a critical period biological constraint. The assumption that there is a critical period is closely related to early immersion like a production. ==History== The critical period hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in their 1959 book ''Speech and Brain Mechanisms'', and was popularized by Eric Lenneberg in 1967 with ''Biological Foundations of Language.''〔.〕 Lenneberg's critical period hypothesis states that there are maturational constraints on the time a first language can be acquired. First language acquisition relies on neuroplasticity. If language acquisition does not occur by puberty, some aspects of language can be learned but full mastery cannot be achieved.〔.〕 Support for the critical period theory stems largely from theoretical arguments and analogies to other critical periods in biology such as visual development. Strictly speaking, the experimentally verified critical period relates to a time span during which ''damage'' to the development of the visual system can occur, for example if animals are deprived of the necessary binocular input for developing stereopsis. It has however been considered "likely",〔"The high degree of susceptibility to abnormal binocular visual experience during early infancy suggests that early infancy represents an activity-dependent stage of binocular maturation. Because infantile ET () interferes with binocular sensory maturation during this stage, it is likely that only prompt eye realignment can support the development of stereopsis." Quoted from: 〕 and has in many cases been flatly presented as fact, that experimental evidence would point to a comparable critical period also for ''recovery'' of such development and ''treatment''; however this is a hypothesis. Recently, doubts have arisen concerning the validity of this critical period hypothesis with regard to visual development, in particular since the time it became known that neuroscientist Susan R. Barry and others have achieved stereopsis as adults, long after the supposed critical period for acquiring this skill.〔Susan R. Barry: Thwarted at every turn, Guest Editorial, ''Optometry - Journal of the American Optometric Association'', vol. 81, no. 1 , pp. 2-3, January 2010, 〕 Recently, it has been suggested that if a critical period does exist, it may be due at least partially to the delayed development of the prefrontal cortex in human children.〔.〕 Researchers have suggested that delayed development of the prefrontal cortex and an associated delay in the development of cognitive control may facilitate convention learning, allowing young children to learn language far more easily than cognitively mature adults and older children. This pattern of prefrontal development is unique to humans among similar mammalian (and primate) species, and may explain why humans—and not chimpanzees—are so adept at learning language.〔"(The Advantages of Being Helpless. )" ''Scientific American Mind''. 9 Feb 2010.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Critical period hypothesis」の詳細全文を読む スポンサード リンク
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