In This Article Expand or collapse the "in this article" section Critical Periods

  • Introduction
  • General Overviews
  • Critical Periods in Language: Background Readings
  • First-Language Acquisition (L1A)
  • Bilingualism
  • First-Language Attrition
  • Sign Language
  • Foreign Language Education
  • Animal Models of Critical Periods

Linguistics Critical Periods
David P. Birdsong
  • LAST REVIEWED: 09 December 2020
  • LAST MODIFIED: 27 September 2017
  • DOI: 10.1093/obo/9780199772810-0139


A critical period is a bounded maturational span during which experiential factors interact with biological mechanisms to determine neurocognitive and behavioral outcomes. In humans, the construct of critical period (CP) is commonly applied to first-language (L1) and second-language (L2) development. Some language researchers hold that during a CP, various mechanisms are at work that result in successful language acquisition and language processing. Outside of the period, other factors and mechanisms are involved, resulting in deficits in acquisition and processing. Many researchers believe that L1 development is constrained by maturationally based CPs. However, this notion is more controversial in L2 acquisition research, where the Critical Period Hypothesis for L2 acquisition (CPH/L2A) is debated on empirical, theoretical, and methodological grounds. Bilingualism researchers study the possibility that CPs may govern the likelihood and degree of loss (attrition) of the L1 among bilinguals as they age. Studies of CPs in L1 acquisition and L2 acquisition have been conducted with learners of spoken languages, signed languages, and artificial languages. CP research is considered in educational policy, particularly in the context of foreign-language instruction. On a terminological note, a distinction is sometimes made between “critical” and “sensitive” periods, the latter term suggesting milder post-CP effects on learning outcomes than the former. Some studies use these terms interchangeably, while others use only one of them. Here, for the sake of simplicity and consistency, “critical period” will be used as a generic cover term.

General Overviews

Lillard and Erisir 2011 describes juvenile CPs in language, imprinting, and vision. Their article includes an informative table covering seven levels of neural changes in the brain in juveniles versus adults, with notes on the time course of changes and affected brain areas in animal and human models. The authors observe that changes in neural architecture triggered by early versus late experiences differ in degree more than type, and that the variety of triggering experiences is reduced with age. A second table summarizes neuroanatomical, electrophysiological, and neuroimaging techniques for observing specific types of neuroplasticity. Knudsen 2004 is exceptionally informative with respect to: prerequisites for CPs; the properties, mechanisms, and timing of plasticity; re-opening of critical periods; the roles of presence and absence of relevant stimulation; and sensitive periods versus critical periods. Knudsen points out that complex behaviors (which include language use) may be regulated by multiple CPs. Uylings 2006 looks at a range of pre- and post-natal cortical changes occurring within critical periods. Uylings considers CPs that are associated not only with normal development but also with disorders: environmentally inflicted insults occur within a finite period of vulnerability to stress, substance abuse, smoking, among others. Hensch 2005 (see also Werker and Hensch 2015 in First-language Acquisition (L1A)) includes an extensive discussion of external factors that can affect the onset and duration of CPs: one example being the administration of drugs that enhance the efficiency of neurotransmitters and plasticity in the visual circuitry of animals.

  • Hensch, Takao K. 2005. Critical period plasticity in local cortical circuits. Nature Reviews Neuroscience 6:877–888.

    DOI: 10.1038/nrn1787

    In addition to the biological mechanisms, cellular structures, and neurochemical factors involved in CPs, the author looks at external factors that alter the onset and offset of CPs.

  • Knudsen, Eric I. 2004. Sensitive periods in the development of brain and behavior. Journal of Cognitive Neuroscience 16.8: 1412–1425.

    DOI: 10.1162/0898929042304796

    Focuses on the role of experience in modifying neural circuits during periods of plasticity, leading to connectivity patterns that become stable and less energy-intensive, and making up what Knudsen calls the “stability landscape.”

  • Lillard, Angeline S., and Alev Erisir. 2011. Old dogs learning new tricks: Neuroplasticity beyond the juvenile period. Developmental Review 31:207–239.

    DOI: 10.1016/j.dr.2011.07.008

    A largely uncritical review and synthesis of well-known studies.

  • Uylings, Harry B. M. 2006. Development of the human cortex and the concept of “critical” or “sensitive” periods. Language Learning 56.1: 59–90.

    DOI: 10.1111/j.1467-9922.2006.00355.x

    Examines the interaction of genotypic and external influences in functional and structural changes.

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