Critical Periods in Brain Development: Neural Plasticity in Early Life
by Tyler Wong
Infants have the ability to distinguish phonemes from any language; however, they lose this ability as they grow older (“What Is a ‘Critical Period’”). This phenomenon reflects a concept known as critical periods, a specific window in a child's life when the brain is at its most flexible. During this period, the brain has the ability to consume numerous amounts of information rapidly. Compared to adults, children have higher brain flexibility, which allows them to develop long-term skills and behaviors (Durman, Nelson, 2021). This suggests that critical periods are essential windows in a child's life when the brain is at its most flexible, allowing the brain to adapt to environmental input, which is evident in both animal and human studies.
In a Nobel prize-winning study, researchers chose cats as their subjects as they shared similar brain and ocular structures to humans. Their study opened a gateway to understanding and exploring critical periods. The paper, “Kitten Monocular Deprivation” published in 1960 by David Hubel and Torsten Wiesel investigated monocular deprivation between kittens and adult cats and how it affected them. The experiment began by suturing the eye of a cat, simulating blindness in one eye. After an 8-week period of monocular deprivation, vision in that eye was restored. Adult cats' vision recovered to nearly full function, whereas kittens acquired permanent vision loss from deprived eyes, though both eyes were healthy (Hubel, Wiesel). The study demonstrated that neural circuits controlling vision adapted optimally during critical periods, highlighting the plasticity of the developing brain.
Building on these animal studies, Nelson and Gabard-Durnam (2020) review highlights the effects of early adversity and the long-term effects on brain development. Rather than conducting an experiment, the authors synthesize findings from other experiments, arguing that Critical periods are windows of heightened plasticity when environmental input strongly shapes neurological circuits. When adversity replaced expected environmental inputs during these periods, the brain responded by developing atypical outcomes. For example, in their review, they reference a study in which adversity led to abnormal development, including lower brain volume and unusual social behaviors (Nelson, Gabard-Durman). Their review explains how, when the brain is in a critical period, neural circuits adapt to fit the environment both physically and socially. Together, these reviews and studies show that critical periods are a sensitive time for brain development. During critical periods, the brain is the most flexible, allowing rapid acquisition of environmental information and formation of neurological connections that underlie long-term skills, behavior, and development.
Citation: Gabard-Durnam, Laurel J., and Charles A. Nelson III. “Early Adversity and Critical Periods: Neurodevelopmental Consequences of Violating the Expectable Environment.” Trends in Neurosciences, vol. 43, no. 3, 2020, pp. 133–43. Elsevier, https://doi.org/10.1016/j.tins.2020.01.002.
Kumpik, Daniel P., and Andrew J. King. “A Review of the Effects of Unilateral Hearing Loss on Spatial Hearing.” Hearing Research, vol. 372, 2019, pp. 17–28. Elsevier, https://doi.org/10.1016/j.heares.2018.08.003.
Mitchell, Donald E., et al. “Recovery of Visual Functions in Amblyopic Animals Following Brief Exposure to Total Darkness.” The Journal of Physiology, vol. 594, no. 1, 2016, pp. 149–67. Wiley, https://doi.org/10.1113/JP270981.
“Why Do Children Learn More Quickly than Adults? New Study Offers Clues.” Brown University, 15 Nov. 2022, www.brown.edu/news/2022-11-15/children-learning. Accessed 24 Aug. 2025.
“What Is a ‘Critical Period’ in Brain Development?” Zero to Three, 25 Feb. 2025, www.zerotothree.org/resource/what-is-a-critical-period-in-brain-development/?utm. Accessed 24 Aug. 2025.
