The apparent specialization of the left hemisphere for language is usually described in terms of lateral dominance or lateralization. Lateralization process begins in early childhood. It coincides with the period during which language acquisition takes place. During childhood, there is a period when the human brain is most ready to receive input and learn a particular language. This is known as the critical period. The general view is that the critical period for first language acquisition lasts from birth until puberty. This process of development is called Maturation. The idea of a critical period for development of particular processes is not unique to humans. Songbirds display hemispheric specialization in that only one hemisphere controls singing.

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There are three accounts of how lateralization emerges (Bates Roe, 2001; Tomas, 2003). The equipotentiality hypothesis states that the two hemispheres are similar at birth with respect to language, each able in principle to acquire the processes responsible for language, with the left hemisphere maturing to become specialized for language functions. The irreversible determinism hypothesis states that the left side is specialized for language at birth and the right hemisphere only takes over language functions if the left is damaged over a wide area (Rasmussen &Milner, 1975; Woods &Carey, 1979). Irreversible determinism says that language has an affinity for the left hemisphere because of innate anatomical organization, and will not abandon it unless an entire center is destroyed. The critical difference between the equipotentiality and irreversible determinism hypotheses is that in the former, either hemisphere can become specialized for language, but in the latter, the left hemisphere becomes specialized for language unless there is a very good reason otherwise. The emergentist account brings together these two extremes, saying that the two hemispheres of the brain are characterized at birth by innate biases in types of information processing that are not specific to language processing, such that the left hemisphere is better suited to being dominant, although both hemispheres play a role acquiring language (Lidzha & Krageloh-Mann, 2005).
The Critical Period Hypothesis is the best-known version of the equipotentiality hypothesis. Lenneberg (1967) argued that a birth the left and right hemispheres of the brain are equipotential. There is no cerebral asymmetry at birth; instead lateralization occurs as a result of maturation. The process of lateralization develops rapidly between the ages of 2 and 5 years, and then slows down, being complete by puberty. The completion of lateralization means the end of the critical period.
There are many theories about Critical Period Hypothesis, some of them confirm the existence of a critical period in acquiring a language and others bring this existence into question or make a distiction between the presence of a critical period in FLA and SLA.
The idea of a Critical Period Hypothesis comes from the nativists, lead by Lenneberg and Chomsky, whose explanation is that there is a critical period because the brain is pre-programmed to acquire language early in development. Bever (1981) argued that it is a normal property of growth, arising from a loss of plasticity as brain cells and processes become more specialized and more independent.
The Critical Period Hypothesis of Lenneberg (1967) comprises two related ideas, The first idea is that certain biological events related to language development can only happen in an early critical period. In particular, hemispheric specialization takes place during the critical period, and during this time children possesses a degree of flexibility that is lost when the critical period is finished. The second component of the Critical Period Hypothesis is that certain linguistic events must happen to the child during this period for development to proceed normally. Proponents of this theory argue that language is acquired most efficiently during the critical period.
The most important idea of Critical Period Hypothesis is that unless children receive linguistic input during the critical period, they will be unable to acquire language normally. One of the most famous of these cases was the Wild Boy of Aveyron, a child found in isolated woods in south of France in 1800. Despite attempts by an educationalist named Dr Itard to socialize the boy, given the name Victor, and to teach him language, he never learned more than two words.
It is less easy to apply this argument to the unfortunate child known as “Genie”. Genie was a child who was apparently normal at birth, but suffered severe linguistic deprivation. From the age of 20 months until she was 13 years and 9 months, when she was found, she had been isolated in a small room. Not surprisingly, Genie’s linguistic abilities were virtually non-existent.
Critical period in SLA
Theories in favour of the existence of a critical period in SLA
According to the nativist theory, once the critical period is over, usually postulated to be sometime during puberty, it is assumed that a person who begins to learn a L2 will be unable to achieve the native-like competence and performance in it.
The basic assumption of a biologically determined critical period is that some essential capacities of younger children are not available to adult learners. One such capacity is the learner’s access to Universal Grammar, that is, the innate system of linguistic categories, mechanisms and constraints shared by all human languages (Chomsky, 1995).
Mark Patkowski hypothesized that only those who had begun learning their second language before the age of fifteen could ever achieve full, native-like mastery of that language. These results gave added support to the Critical Period Hypothesis for second language acquisition.
Theories against the existence of a critical period in SLA
There are two reasons for rejecting a strong version of the Critical Period Hypothesis. Children can acquire some language outside of the critical period, and lateralization does not occur wholly within it.
A critical period appears to be involved in early phonological development and the development of syntax. The weakened version is often called a sensitive period hypothesis. There is a sensitive period for language acquisition, but it seems confined to complex aspects of syntactic processing. (Bialystok&Hakuta, 1994). Locke (1997), argues that a sensitive period arises because of the interplay of developing specialized neural systems, early perceptual experience, and discontinuities in linguistic development. Lack of appropriate activation during development acts like physical damage to some areas of the brain. The distinction between the Critical Period Hypothesis and the sensitive period hypothesis is whether acquisition is “possible only within the definite span of age” or “easier within the period”. Seliger’s proposal (1978), is that there may be multiple critical or sensitive periods for different aspects of language.
The maturational explanation is that certain advantages are lost as the child’s cognitive and neurological system matures. In particular, what might first appear to be a limitation of the immature cognitive system might turn out to be an advantage for the child learning language.
The results of experimental studies have two important implications for adult second language learning. One is that children’s acquisition of a foreign language is different from that of adults’. The other is that acquisition of pronunciation and grammar is also different because it involves a problem of physiologic aging process. Adults can learn the grammar of a new language more easily and rapidly than children but that they retain foreign accents.
Theories that consider the existence of a critical period in FLA but not in SLA
It is widely believed that the ability to acquire language declines with increasing age. Today it is generally agreed that a critical period does exist for first language acquisition but the hypothesis is not as uniformly accepted as applicable to SLA.
When considering separately the time required for L2 learning and the ultimate success achieved in the L2, some researchers suggested a compromise conclusion that older is faster but younger is better. At initial stages of L2 acquisition, older learners were at an advantage in rate of acquisition but only in limited aspects.
In a recent critical review of the Critical Period Hypothesis literature, Marinova observed that, despite general perceptions that older learners are slower L2 learners, the research has long revealed that, in fact, older learners are faster in process of L2 acquisition, especially at the initial stages.
Theoretically, if the critical period for L2 acquisition exists, and older learners are strictly at a disadvantage due to age and some biological or maturation constraints, then all late L2 learners should be performing well below the younger learners. However, many studies, whether supporting of challenging the Critical Period Hypothesis, have shown that younger learners tend to perform fairly similarly to one another, while generally older learners show greater variation in their L2 performances.
The effects of the L2 learning process and the type of L2 learning environment have been studied more formally on a larger scale. It has been argued that if adults are able to learn an L2 implicitly in more natural settings, similar to the way children learn language, then they may achieve similar levels of performance at a faster rate (Neufeld).
The Critical Period Hypothesis has traditionally been used to explain why second language acquisition is difficult for older children and adults. Johnson and Newport (1989) examined the way in which the critical period hypothesis might account for second language acquisition. They distinguished two hypotheses, both of which assume that humans have a superior capacity for learning language early in life. According to the maturational state hypothesis, this capacity disappears or declines as maturation progresses, regardless of other factors. The exercise hypothesis further states that unless this capacity is exercised early, it is lost. Both hypotheses predict that children will be better than adults in acquiring the first language. The exercise hypothesis predicts that as long as a child has acquired a first language during childhood, the ability to acquire other languages will remain intact and can be used at any age. The maturational hypothesis predicts that children will be superior at second language learning, because the capacity to acquire language dismisses with age.
Are children in fact better than adults at learning language? The evidence is not clear-cut as is usually thought. Snow (1983) concluded that contrary to popular opinion, adults are in fact no worse than young children at learning a second language, and indeed might even be better. Children spend much more time than adults learning the language.
Snow and Hoefnagel-Hohle (1978) compared English children with English adults in their first year of living in the Netherlands learning to speak Dutch. The young children 3-4 years old, performed worst of all. In addition, a great deal of the advantage for young children usually attributed to the critical period may be explicable in terms of differences in the type and amount of information available to learners. There is also a great deal of variation: some adults are capable of near-native performance on a second language, whereas some children are less successful. They proposed that there is a change in maturational state, from plasticity to a steady state, at about age 16. The younger a person is, the better they seem to acquire a second language.
There is evidence for a critical period for some aspects of syntactic development and, even more strongly, for phonological development. However, rather than any dramatic discontinuity, decline seems to be gradual. Second language acquisition is not a perfect test of the hypothesis, however, because the speakers have usually acquired at least some of a first language.
Lenneberg supplied some evidence to support the CPH and he found that injuries to the right side caused more language problems in children than in adults. He also provided evidence to show that whereas children rapidly recovered total language control after such operations, and adults did not so, but instead continued to display permanent linguistic impairment. However, this evidence doesn’t demonstrate that is easier to acquire a language before puberty. In fact he assumed that LA was easy for children. The CPH is an inadequate account of the role played in SLA, because this assumption was only partially correct. Only where pronunciation is concerned is an early start an advantage, and even then only in terms of success, not rate of acquisition. Developmental changes in the brain, it is argued, affect the nature of language acquisition, and language learning that occurs after the end of the critical period may not be based on the innate biological structures believed to contribute to first language acquisition or second language acquisition in early childhood. Rather, older learners may depend on more general learning abilities.
In educational settings, learners who begin learning a second language at primary school level do not always achieve greater proficiency in the long run than those who begin in adolescence.
The Critical Period Hypothesis is a particularly relevant case in point. This is the claim that there is, indeed, an optimal period for language acquisition, ending at puberty. However, in its original formulation (Lenneberg 1967), evidence for its existence was based on the relearning of impaired L1 skills, rather than the learning of a second language under normal circumstances.
Conclusion
As well as there is an agreement that corroborates the Critical Period Hypothesis set up by the nativists during the L1 acquisition, there is not such agreement when considering L2 acquisition. Contrary to what was thought about the impossibility to acquire an L2 after the end of the critical period, there is some evidence that show learning an L2 after puberty is also achievable. The theories that support this idea say that an adult or an adolescent learner will be able to acquire a native-like mastery in the L2 as a younger learner will do.
Since the study of human brain is still very limited, some theorists contradict the non-presence of a critical period in SLA. For this reason, although it seems to be a prevalent theory about this aspect, it will be difficult to arrive to a general consensus.