Lenneberg based his theory on the under-mentioned hypotheses:
- Language development is akin to other biological developments in human growth and maturation.
- Like many inborn mechanisms, language development is also confined to a specific time or critical period.
- After critical period the brain loses plasticity due to lateralization, thereby making language acquisition incredibly difficult.
- The process of brain lateralization starts at about 2 years of age and is completed by puberty.
The notion of a critical period for language learning was first proposed by Wilder Penfield and Lamar Roberts in Speech and Brain Mechanisms (1959). Their book contained a chapter entitled “The Learning of Languages”, wherein they hypothesized about the existence of a biological clock within the brain, which sets an age limit for second language learning. Their research concluded that Children were more capable of becoming proficient in a second language than adults. Later on, Lenneberg (1967) supported their hypothesis and further studied it to observe the possible age limit for first language acquisition.
Lenneberg's research identified a link between language and the brain. He assumed that there are two hemispheres to the brain, namely the right and the left hemispheres. He also hypothesized that each domain or subject of learning is stored in separate hemispheres of the brain. Although the hemispheres are not structurally identical at birth, they are functionally identical. In this contention, initially each hemisphere is equally capable of supporting language development. For instance, if an injury occurs in the left hemisphere then it is possible for the right hemisphere to become specialized for language function. However, the right hemisphere’s capability to control language remains intact till a certain stage or a critical period, which ends after lateralization of the brain functions. Lenneberg assumed that as the brain matures, (that is, around the 2 years) the hemispheres began to become functionally specialized and each controls different areas of human activity. This process of lateralization is completed by puberty. After lateralization, a hemispheric dominance ensues: in normal persons the left hemisphere begins to control the functions of language and the right hemisphere ceases its involvement in language. Therefore, in post-critical period language acquisition becomes difficult, ultimately less successful, or even impossible.
The above graph suggests that an individual is only capable of learning a language during his/her early life and this ability comes to a standstill after maturity.
Evidence in Support of CPH
Lenneberg’s idea of Critical Period Hypothesis looks for evidence in the studies of different neurological studies such as the following:
1. Childhood AphasiaThe first evidence in support of critical period was discovered by the researches on patients with speech disorder or aphasia. Generally, damages to portions of the brain which account language cause aphasia. For majority of people these are parts of the left side of the brain. Lenneberg’s researches proved that brain damage to the left hemispheres in adults led to aphasia in 70 percent of cases, while brain damage to children had the same effect on their language, whether the damage was to the right or left hemisphere. However, children's brains were able to recover from this much faster than an adult with the same injuries. Unlike adults, in some cases, children were even able to make a full recovery of language. Even total removal of the left hemisphere did not stop children's reacquisition of language. This difference of chances for recovery of lost language functions between the children and the adults proves the existence of a critical period.
2. Down Syndrome CasesAnother evidence for CPH is the study of children with Down Syndrome. Down Syndrome is a genetic disorder, which is essentially associated with mental retardation. Children with Down Syndrome show much delays in language acquisition. According to cph their ability to learn ceases at puberty. The most notable study on Down Syndrome was conducted by Lenneberg, Nichols, and Rosenberg (1964), who in a three-year observational study of 54 individuals with Down Syndrome were able to record progress in language development only in children younger than 14 years.
3. Second Language AcquisitionThe researches of Penfield and Roberts (1959) on the existence of a critical period for second language learning also provided a strong evidence for Lenneberg’s theory. They held that older learners of a second language rarely achieve the native-like fluency that younger learners display. Since children can learn a second language faster, this must be because they are still in their critical period. Consisted with this statement it is argued that second language acquisition will be relatively fast and successful like first language only if it occurs before the age of puberty.
4. Cases of Childhood Extreme DeprivationThe critical Period Hypothesis borrows a large portion of evidence from the studies conducted upon the feral children (also known as wild children, wolf children), who were raised in social isolation with little or no human contact. Since they missed the critical period to develop language, they were unable to learn and utilize language in a fully functional manner. The under mentioned cases are the most famous ones:
Case -1This case is from the 1930s. A girl named Isabelle was found living in a darkened room with her deaf mother. When the authorities found her she was six and a half years old. Since her mother was deaf, Isabelle had no sense of language. But Isabelle was able learn language after receiving systematic training. Therefore, it is argued that she was able to learn language as she started learning before puberty, that is, within the critical period.
Case -2The second case is from the 1970s, which is by far the strongest evidence in support of CPH. It was the case of a physically and psychologically abused girl named Genie, who was rescued from Los Angles, California on November 4, 1970 by social workers. Between the ages of 20 months and about 14 years her father locked her in a dark room and deprived her of human contact or language input (Curtiss, Fromkin, Krashen, Rigler, and Rigler 1974). When Genie was found she could not walk properly, eat properly, or speak in any language. Genie could not acquire normal language even under an intensive language and psychotherapy. Initial tests suggested that Genie was not mentally ill and that her inability to learn language resulted from the abusive background. However, a subsequent testing revealed that the left side of her brain was not working. These results suggest that as the left hemisphere had never been used during the critical period, it lost its ability to learn language.
5. Deaf Children with Hearing Parents
This refers to a group of children living in normal social conditions with regular linguistic deprivation. More than 90 percent deaf children are born to hearing parents. These children are deprived of sign language since infancy and thus have great difficulty learning spoken language since they receive limited or no language input from their parents. Typically, these children do not learn sign language until they are enrolled in residential school for the deaf. Researches on deaf children proved that the older a child is when he starts learning a sign language, the harder of a time he has acquiring it. The proponents of CPH thus believe that the older children face such problem as they missed their critical period for learning language.
Case -1An interesting case in this regard was Chelsea, a deaf child in remote California town. She was mistakenly diagnosed by doctors as retarded. Her deafness was discovered when she was 31 years old. Hearing aids gave her near-normal hearing and she started to learn language. Although she managed to learn large vocabulary, her grammar was poor like Genie. It happened because she started to learn language way beyond the critical period.
Case-2Another study of linguistic deprivation is the case of E.M., a 19 years old deaf boy from Mexico. He was raised in a rural area, where he received little formal education before the age of 12 and he was also isolated from the deaf community. At the age of 15 he was fitted with hearing aids, which enabled him to hear speech spoken at a conversational level. E.M. started learning Spanish, but he failed to achieve complete mastery of any single tense.
Genie and E.M. were exposed to different languages in different contexts, but still they suffer from similar linguistic impairments. Like Genie he experienced an extended period of linguistic isolation and started learning in adolescent. But unlike Genie E.M. did not suffer from any physical and psychological abuse. E.M.’s case, therefore, suggests that Genie’s abusive background is not likely to be the only cause of her language impairments and provides converging evidence for the existence of a critical period for first language acquisition.
Counterarguments against CPH
Lenneberg's critical period hypothesis is not devoid of shortcomings. A number of researchers have tried to highlight its weak points. Up till now the following observations have been made against CPH:
- There is no conclusive evidence that lateralization necessarily starts from roughly 2 years of age and ends at puberty (about 14 years of age), as claimed by Lenneberg. In this regard the researchers have forwarded divided opinions. For Krashen (1973) it may be completed by the age of 5, whereas for Kinsbourne (1975) lateralization is completed by birth. Flynn and Manuel (1991), on the other hand, stated that it occurs long before the onset of puberty, perhaps during the first year of life. Due to dearth of proper evidence, nowadays most researchers step down from setting any starting age for critical period of language learning. Resultantly, few researchers proposed a revised version of CPH, which they termed as sensitive period hypothesis. The proponents of this hypothesis hold that there are optimal times in an individual's life when the brain is maximally sensitive to learn a language. Language can still be learnt after this period but the learning process is less efficient as the brain is less sensitive to language learning. The difference between these hypotheses is very subtle:
|Critical Period Hypothesis||Sensitive Period Hypothesis|
|The ability to acquire a language begins and ends abruptly.||The ability to learn a language begins and ends gradually.|
|Language acquisition is possible only within the specific span of age, after this time language acquisition becomes difficult or even impossible.||Language learning is easier within the specific span of age; it could still be learnt at maturity but less efficiently.|
- The case studies of linguistically deprived children also criticized for various inconsistencies. The opponents of CPH argued that the feral children could have failed to acquire language for the fact that they were retarded from infancy or that their brain was damaged due to some reason. For example, in Genie’s case it is reported that her left hemisphere was atrophied due to brain damage. Some others, however, held that the feral children’s lack of language acquisition in the later life may be due to abusive environment rather than lack of exposure to language.
- It is not true that children can master a second language better than adults. Birdsong’s (1992) experiments proved that adults too can attain native like competence in grammatical tasks and speaker performance.
- Critical period hypothesis does not clarify exactly which areas of language should be affected after critical period. Majority of researches agree that CP is most likely to affect the pronunciation and grammar.
- Another shortcoming of cp is that it does not take into account the rate of language learning in different stages of learning; rather it is primarily interested in the final state of second language acquisition. But researches proved that adult learners proceed faster than the children at the initial stages of learning.
- The cases of Down Syndrome also criticised for inconsistency. It has been observed that children with Down Syndrome learns so slowly that they passes the critical period.
- Unlike Lenneberg’s claim, children are not always capable of recovering from aphasia. It has been observed that children are more capable of recovering from traumatic aphasia than adults. But children rarely show sign of recovery for aphasia resulted from vascular disorders (Guttman 1942; Van Dongen & Loonen 1976). Again, aphasia related to convulsive disorder often fails to recover as well (Gascon, Victor, Lambroso, & Goodglass 1973; Van Dongen & Loonen 1976; Worster-Drought 1971).
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“The Critical Period Hypothesis in Second Language Acquisition: