Explore the Effect of a Critical Period on Second Language Acquisition

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Critical Period Hypothesis:
Critical Period (CP) refers to a specific and ‘fixed’ or invariant period of time during which an organism’s neural functioning is open to effects of external experiential input. To be specific, there are three important essentials in this conception. Firstly, this developmental period is biologically determined; the onset, end and the length of the critical period are invariant, which is the consequences of some internal clock that keeps time independent of what happens during the window of time. Second, the environmental input can affect the organism during this invariant period, while the system cannot be altered by external stimulus any more beyond the critical point. Third, the lack of adequate input
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First language acquisition:
The validity of the CPH has received much attention from numerous researchers. The observation of feral children (ex. Genie, Chelsea, and Isabelle) support the CPH; these children, who underwent a degree of social isolation and experiential deprivation before the onset of puberty, cannot get a full command of language and achieve the native level (Fromkin, Krashen, Curtiss, Rigler, & Rigler, 1974; Berk, 2004).
Another compelling evidence supporting CPH is derived from comparing the acquisition of American Sign Language (ASL) among deaf children and adults. People who learn ASL in adulthood as L1 or L2 are recognizably less facile than those who have learned it in childhood. In general, people who are denied the opportunity to learn L1 during childhood are unable to become fully fluent in various aspects of a language, even if given the opportunity to do later in life (Stevens, 1999).
Contradicted evidence also put forward to against the CPH, however, Newport and Supalla suggested that there is a linear decline in both production and comprehension performance with increasing age of exposure instead of abrupt drop after a remarkable offset point by comparing the ultimate attainment of ASL among native (exposure to ASL from birth), early (between the age of 4 and 6), and late (at age 12 or later) learners. They also found that even the early

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