Critical Periods

by

David P. Birdsong

• LAST REVIEWED: 12 January 2023
• LAST MODIFIED: 12 January 2023
• DOI: 10.1093/obo/9780199772810-0139

Introduction

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 drawn between “critical” and “sensitive” periods, the latter term denoting receptivity of the organism to shaping by experience (or, in certain studies, suggesting relatively mild effects). Some researchers use these terms interchangeably, while others use one but not the other. Here, “critical period” will be used as a cover term unless specific reference is being made to sensitive period.

General Overviews

Lillard and Erisir 2011 describes juvenile CPs in language, imprinting, and vision. The 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; reopening 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. Takesian and Hensch 2013 emphasizes the individual-level plasticity of the timing of CP onset, peak, and offset, which may vary according to excitatory/inhibitory circuit balance that is sensitive to drugs, sleep, trauma, and genetic perturbation (see also Werker and Hensch 2015 [cited under First-Language Acquisition (L1A)). Reh, et al. 2020 examines how plasticity is regulated at multiple timescales during development and provides examples from language processing, mental illness, and recovery from brain injury. Gabard-Durnam and McLaughlin 2020 outlines a set of current approaches to the study of sensitive periods in humans. These approaches include environmental manipulations (deprivation, enrichment, substitution), plasticity manipulations via pharmacological intervention, and computational modeling. Frankenhuis and Walasek 2020 develops an evolutionary model that accounts for sensitive periods that occur beyond the early stages of ontogeny.

• Frankenhuis, Willem E., and Nicole Walasek. 2020. Modeling the evolution of sensitive periods. Developmental Cognitive Neuroscience 41:100715.

  DOI: 10.1016/j.dcn.2019.100715

  Sensitive periods in mid-ontogeny are favored by natural selection as a function of the reliability of relevant environmental cues.

• Gabard-Durnam, Laurel, and Katie A. McLaughlin. 2020. Sensitive periods in human development: Charting a course for the future. Current Opinion in Behavioral Sciences 36:120–128.

  DOI: 10.1016/j.cobeha.2020.09.003

  Figures 1, 2 and 3 and their captions are particularly informative.

• 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.

• Reh, Rebecca K., Brian G. Dias, Charles A. Nelson III, et al. 2020. Critical period regulation across multiple timescales. Proceedings of the National Academy of Sciences of the United States of America 117.38: 23242–23251.

  DOI: 10.1073/pnas.1820836117

  A diverse group of specialists’ account of neurobiological CP mechanisms in animals and humans. Notes that “cortical plasticity is not only influenced by an animal’s life experiences but may also be modified by that of the parents. This occurs via parental behavior during the offspring’s early postnatal life, the in utero environment during gestation, or modification of the parental or fetal germ cells” (p. 23246).

• Takesian, Anne E., and Takao K. Hensch. 2013. Balancing plasticity/stability across brain development. Progress in Brain Research 207:3–34.

  DOI: 10.1016/B978-0-444-63327-9.00001-1

  Illuminates the dynamic between the intrinsic plasticity of CP and the stabilization of neural networks, which limits maladaptive proliferation of circuit rewiring past the CP.