[4] The critical period hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in their 1959 book Speech and Brain Mechanisms,[5] and was popularized by Eric Lenneberg in 1967 with Biological Foundations of Language.
[6] 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, and that first-language acquisition relies on neuroplasticity of the brain.
A more general hypothesis holds that the critical period for language acquisition is linked to the interaction of the prolonged development of the human brain after birth and rearing in a socio-linguistic environment.
[9] Based on studies of the critical period for development of the visual system,[10] this hypothesis holds that language-specific neural networks in the brain are constructed by the functional validation of synapses that are specifically activated by exposure to a linguistic environment early in life.
[12] Some aspects of language, such as phoneme tuning, grammar processing, articulation control, and vocabulary acquisition have weak critical periods and can be significantly improved by training at any age.
The critical period hypothesis in SLA follows a "use it then lose it" approach, which dictates that as a person ages, excess neural circuitry used during L1 learning is essentially broken down.
On the other hand, a second "use it or lose it" approach dictates that if an L2 user begins to learn at an early age and continues on through their life, then their language-learning circuitry should remain active.
"[21] Some writers have argued that the critical period hypothesis does not apply to SLA, and that second-language proficiency is determined by the time and effort put into the learning process, and not the learner's age.
Research explores these ideas and hypotheses, but results are varied: some demonstrate pre-pubescent children acquire language easily, and some that older learners have the advantage, whilst others focus on existence of a CP for SLA.
Other work has challenged the biological approach; Krashen (1975) re-analysed clinical data used as evidence and concluded cerebral specialisation occurs much earlier than Lenneberg calculated.
Her case presented an ideal opportunity to test the theory that a nurturing environment could somehow make up for the total lack of language past the age of 12.
Skinner (1957) details how operant conditioning forms connections with the environment through interaction and, alongside O. Hobart Mowrer (1960), applies the ideas to language acquisition.
Input alone cannot explain language acquisition because it is degenerated by characteristic features such as stutters, and lacks corrections from which learners discover incorrect variations.
Results showed Simon learned normal and logical rules and was able to construct an organised linguistic system, despite being exposed to inconsistent input.
The parents of Singleton and Newport's (2004) patient also had linguistic abilities in line with these age-related predictions; they learned ASL after puberty and never reached complete fluency.
[30] Piaget (1926) is one psychologist reluctant to ascribe specific innate linguistic abilities to children: he considers the brain a homogeneous computational system, with language acquisition being one part of general learning.
Instead, he suggests external influences and social interaction trigger language acquisition: information collected from these sources constructs symbolic and functional schemata (thought or behaviour patterns).
Tarone, Bigelow & Hansen (2009) find significantly different results when replicating standard SLA studies with low literate L2 learners.
Specifically, learners with lower alphabetic literacy levels are significantly less likely to notice corrective feedback on form or to perform elicited imitation tasks accurately.
Empirical research has attempted to account for variables detailed by SLA theories and provide an insight into L2 learning processes, which can be applied in educational environments.
They looked at vocabulary processing and representation in Spanish-Catalan bilinguals exposed to both languages simultaneously from birth in comparison to those who had learned L2 later and were either Spanish- or Catalan-dominant.
Findings showed 'from birth bilinguals' had significantly more difficulty distinguishing Catalan words from non-words differing in specific vowels than Catalan-dominants did (measured by reaction time).
However, comparing early immersion students (average age 6.917 years) with age-matched native speakers identified common problem areas, including third person plurals and polite ‘vous’ forms.
[36] B. Harley also measured eventual attainment and found the two age groups made similar mistakes in syntax and lexical selection, often confusing French with the L1.
Some variation in grammatical performance is attributed to maturation,[36] however, all participants began immersion programs before puberty and so were too young for a strong critical period hypothesis to be directly tested.
Their results showed that Chinese-English bilinguals who had been exposed to English after puberty, learned vocabulary to a higher competence level than syntactic aspects of language.
They do, however, report that the judgment accuracies in detecting semantic anomalies were altered in subjects who were exposed to English after sixteen years of age, but were affected to a lesser degree than were grammatical aspects of language.
However, general second-language research has failed to support the critical period hypothesis in its strong form (i.e., the claim that full language acquisition is impossible beyond a certain age).
Because the simulation reveals no evolutionary advantage of acquiring more than one language, Hurford suggests that the critical period evolved simply as a result of a lack of selection pressure.
The learning mechanism in their model is based on linguistic theories of Chomsky (1980, 1993)– the language acquisition device (LAD) and the notion of universal grammar.