Part 1

The critical period hypothesis, introduced by Lenneberg (1969), says that there is a period in which one’s first language is acquired more naturally and accurately, and this period has a certain onset and offset. Lenneberg claims that this period occurs between ages 2 to 13 within which one is able to acquire the syntax, grammar, and the sophisticated system of language - and after that date one is no longer able to do so in a normative developmental fashion. If children miss their critical period, they will miss language acquisition (Lenneberg, 1969). Lenneberg adopts the nativist approach to language acquisition where there is only one acquisition strategy that is present from birth, and this inherent capacity must develop alongside input from a language-rich environment - and Lenneberg adds that this must occur during the critical period.

Lenneberg shows that a critical period for first language acquisition, no matter the language, is universal. In one study of 3-month old babies of deaf parents, the babies made human sounds independent of the amount, frequency, or timing of the sounds made by the parents (Lenneberg, 1969, pp 636-7).

Being born with the capacity for language is not enough to fully acquire it. Children can take advantage of their natural capacity for language only if the environment provides a minimum of language stimulation within the critical period of first language acquisition (Lenneberg, 1969, p 637). Children with congenital hearing loss have the capacity for first language acquisition, but fail to develop it for lack of exposure within the critical period. Lenneberg showed this by bringing a study where congenitally deaf children did not grow up in an environment with sign language or speech until school age, at which point they were taught their first language as a second language rather than by immersion. These individuals had trouble with grammar and syntax for the rest of their lives (Lenneberg, 1969, pp 638, 640). In contrast, the children of deaf parents who were surrounded by a rich language environment from the rest of their community had no developmental difficulties because they learned gestured language within their critical period (Lenneberg, 1969). Further, children who grew up in an environment where they received language input from birth and then lost their hearing at age 3 to 4 have a more successful first language acquisition process when entering schools for the deaf compared to congenitally deaf children, despite living without language for two to three years before they had entered school (Lenneberg, 1969, p 640).

Hoshi and Miyazato (2016) accepts Lenneberg’s hypothesis that there is a critical period for first language acquisition from 2-13 years of age, but points out that Lenneberg only talked about linguistic input and not linguistic output in reference to a critical period. In fact, Lenneberg (1969) claims that linguistic output, or the development of articulator motor skills, is not subject to a critical period. So in principle, output, or externalization of a first language, could happen after the critical period.

Hoshi and Miyazato (2016) tests whether externalization of a first language may happen after the critical period in children with early aphasia if linguistic input was available within the critical period. Children with aphasia have a deficit in the neural system for language comprehension (language input) and for articulatory motor skills of speech production (language output) (Hoshi & MIyazato, 2016; Lenneberg, 1969). If childhood aphasia is alleviated or disappears before the end of the critical period of first language acquisition, such as before 13 years of age - and during that time the child is surrounded by a rich language environment to secure linguistic input - Hoshi and Miyazato (2016) concludes that first language acquisition, including language output, remains possible.

Linguists and scientists who accept Lenneberg’s hypothesis may debate on the age at which the critical period for first language acquisition ends. Lenneberg (1969) introduced the hypothesis saying that the critical age is from 2 to 13 years of age (Lenneberg, 1969). Friedmann and Rusou (2015) accepts Lenneberg’s critical period hypothesis but finds that the critical age ends at 1 years old for the acquisition of a first language, specifically for the acquisition of syntax, due to brain maturation and loss of plasticity.

Children who miss this critical period for acquiring a first language later show severe syntactic impairments. For example, congenitally deaf children who were raised without sign language (usually because they were born to hearing families who do not sign) and who did not receive hearing devices before 8 months old show syntactic impairments for the rest of their lives. Syntactically, they have a hard time understanding and producing relative clauses and wh-questions later on in their lives (Friedmann & Rusou, 2015). In contrast, congenitally deaf children who acquire sign language since birth from deaf parents experience normal language development (Friedmann & Rusou, 2015). Similarly, children who lose their hearing after their first year of life, as well as children whose hearing loss is only from one ear, show normal syntactic abilities (Friedmann & Rusou, 2015). Therefore, the age of identification of hearing loss and the age at which hearing intervention begins are important predictors of normal language development: hearing intervention must begin by 8 months to insure language input within the critical first year period of a child’s life (Friedmann & Rusou, 2015).

Other children who miss this critical period for acquiring a first language include those who experience a thiamine deficiency during their first year (Friedmann & Rusou, 2015). Even if these children receive thiamine several weeks or months after their first year of life, it is too late. They are unable to understand or produce sentences with normal syntax for the rest of their lives (as cited in Friedmann & Rusou, 2015).

Part 2

Not all research agrees with Lenneberg (1969)’s critical period hypothesis. Balari and Lorenzo (2015) debate whether there is a critical age for first language acquisition, and introduce the idea of a “language gradient.” Balari and Lorenzo (2015) explain that classical concepts of the critical period are normative and refer to a predetermined innate age where the opportunity window for acquiring a first language is closed (Balari & Lorenzo, 2015). The study laments that the term “normal” when referring to “normal” language development tends to be compared with a false point of reference. The study beseeches that the developmental perspective must not take into consideration normative behaviors, but rather cognitive varieties (Balari & Lorenzo, 2015). The idea of language as a gradient is that language may display more than one normal form of neurological architecture and associated behaviors (Balari & Lorenzo, 2015). For example, familial sinistrality plays a role in the speed and retrieval of declarative lexical information (Balari & Lorenzo, 2015). Other ways that the range of variability in first language acquisition may point toward different styles include mental disabilities which impact language capacity, such as Down's Syndrome, and developmental conditions that alter the ability to deal with names as compared to verbs, such as Potocki-Lupski Syndrome (Balari & Lorenzo, 2015).

Friedmann and Rusou (2015) who agree with Lenneberg’s critical period hypothesis (albeit place an alternate age at its onset and offset) offer studies that show that the function for ending the critical period is brain maturation which finishes at a certain age whether there is language input or not. According to Balari and Lorenzo (2015), this classical concept of the critical period focuses on potentialities left behind rather than on new states and recruited skills to come. Language development involves a continual dynamic negotiation of predominant skills among the abilities successfully developed (Balari & Lorenzo, 2015). Instead of supporting a critical period, Balari and Lorenzo (2015) talks about a sensitive period for first language acquisition, although the study terms it “development as usual.” In this view of language as a gradient, the development of one’s first language requires that different components of the brain participate at different points and at different amounts to create a gradual hybridization. This begins with a baby’s phonetic abilities and continues through the development of further language skills. Language skills develop in a continuous chain where earlier abilities are entrenched and provide scaffolds for later language abilities (Balari & Lorenzo, 2015)..

This may seem similar to the classical critical period of first language acquisition where there is a predefined development window, but Balari and Lorenzo (2015) explain that language development is variable with the example that while lower levels of neural stabilization of language skills cause a slower acquisition of further skills, they favor the development of a second language as seen in bilingual infants, as long as the environment includes foreign language input. Therefore, first language acquisition develops dynamically into variable conclusions and at different rates, and is not a similar process in each individual (Balari & Lorenzo, 2015).

Normal first language acquisition is based on the average adult brain, but this norm, according to Balari and Lorenzo (2015), is artificial. Rather, there is a gradient of linguistic conditions where language is not compartmentalized to a certain location in brain development, but is part of an interactive system of bodily capacities subject to internal and external developmental influences (Balari & Lorenzo, 2015). While there are sensitive periods in language development such that certain processes must be entrenched by a certain time to allow for other language skills to build on them, development is also dynamic in each individual and depends on their genes and the order and timing of varying environmental inputs.

References

Balari, S. & Lorenzo, G. (2015). Should it stay or should it go? A critical reflection on the critical period for language. Biolinguistics, 9, 8-42.

Friedmann, N. & Rusou, D. (2015). Critical period for first language: The crucial role of language input during the first year of life. Current Opinion in Neurobiology, 35, 27-34. doi:10.1016/j.conb.2015.06.003

Hoshi, K. & Miyazato, K. (2016). Architecture of human language from the perspective of a case of childhood Aphasia - Landau-Kleffner Syndrome. Biolinguists, 10, 136-196.

Lenneberg, E. H. (1969). On explaining language. Science, 164(3880), 635-643.