Critical period

In general, a critical period is a limited time in which an event can occur, usually to result in some kind of transformation. A "critical period" in developmental psychology and developmental biology is a time in the early stages of an organism's life during which it displays a heightened sensitivity to certain environmental stimuli, and develops in particular ways due to experiences at this time. If the organism does not receive the appropriate stimulus during this "critical period", it may be difficult, or even impossible, to develop some functions later in life.

For example, the critical period for the development of a child's binocular vision is thought to be between one and three years, and further critical periods have been identified for the development of hearing and the vestibular system. There are critical periods in childhood in which imprinting can occur, such as when a greylag goose becomes attached to a parent figure within the first 36 hours after hatching. A young chaffinch must hear an adult singing before it sexually matures, or it will never properly learn the highly intricate song. These observations have led some to hypothesise a critical period for certain areas of human learning, particularly language acquisition.

Experimental research into critical periods has involved depriving animals of stimuli at different stages of development. Other studies have looked at children deprived of certain experiences due to illness (such as temporary blindness), or social isolation (such as feral children). Many of the studies investigating a critical period for language acquisition have focussed on deaf children of hearing parents.

First language acquisition

 * See also: Language acquisition

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 this phenomenon, 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 (brain lateralisation, other physical maturation, cognitive factors or other). The duration of the period also varies greatly in different accounts. Steven Pinker, in his book The Language Instinct, states that “acquisition of a normal language is guaranteed for children up to the age of six, is steadily compromised from then until shortly after puberty, and is rare thereafter” (Pinker 1994, p. 293).

The Critical Period Hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in a 1959 paper Speech and Brain Mechanisms, and was popularised by Eric Lenneberg in 1967 with Biological Foundations of Language. Lenneberg proposed brain lateralisation at puberty as the mechanism that closes down the brain's ability to acquire language, though this has since been widely disputed. Other notable proponents of the Critical Period Hypothesis include Noam Chomsky.

Second language acquisition

 * See also: Second language acquisition

The theory has often been extended to a critical period for second language acquisition, although this is much less widely accepted. Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages. David Singleton (1995) states that in learning a second language, "younger = better in the long run," but points out that there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood — long after any critical period has presumably come to a close.

While the window for learning a second language never completely closes, certain linguistic aspects appear to be more affected by the age of the learner than others. For example, adult second-language learners nearly always retain an immediately-identifiable foreign accent, including some who display perfect grammar (Oyama 1976). Some writers have suggested a younger critical age for learning phonology than for syntax. Singleton (1995) reports that there is no critical period for learning vocabulary in a second language. Robertson (2002) observed that factors other than age may be even more significant in successful second language learning, such as personal motivation, anxiety, input and output skills, settings and time commitment.

On reviewing the published material, Bialystok and Hakuta (1994) conclude that second-language learning is not necessarily subject to biological critical periods, but "on average, there is a continuous decline in ability [to learn] with age."