Find out what a critical period is in brain development and how parents can use this knowledge to promote optimal brain growth in our children.
Table of Contents
A critical period is a phase during which the brain cell connections are more plastic and receptive to the influence of a certain kind of experience. These connections, called synapses, can form or strengthen more easily during this period. Synaptic connections usually stabilize after this window of time and the wiring becomes harder to change.
Critical Period Hypothesis
In developmental psychology, the critical period hypothesis states that during this critical period of brain development, a new skill or trait is usually formed given a specific experience is presented. If the critical experience is not available during this time, it becomes much harder, less successful or even impossible to acquire the skill or trait after the window of opportunity closes.
This is proven true in sensory systems, such as vision and hearing.
For example, if one eye (but not both) is covered right after birth, the deprived eye will lose visual acuity permanently, even if the covered period is brief postnatal. This is because covering an eye during the critical period can alter the physical structure of the brain permanently1.
Related: Brain Development In Children
Critical Period For Language Acquisition
When applied to language acquisition, the Critical Period Hypothesis states that there is a critical period during which individuals are more easily able to acquire new languages with native-like proficiency.
This period begins in early childhood and concludes shortly before the onset of puberty2.
After this window, even with a linguistically rich environment, it becomes much more difficult to acquire new language competency3.
The hypothesis was first popularized by Eric Lenneberg, a linguist and neurologist, in a landmark book Biological Foundations of Language in 1967.
According to this theory, learning a new language is constrained by a critical period. There is a distinct discontinuity in outcomes between learning within the critical period and learning outside of it. The time of that discontinuity reflects the close of the critical period4.
A sensitive period is similar to a critical period in which a region of the brain is relatively more plastic and more sensitive to the influence of experience in forming new synapses. New synapses can still form for an extended period of time outside of the optimal period despite being more difficult.
Some scientists refer to this as a weak critical period.
Sensitive Period vs Critical Period
It is still possible for grownups to learn to use a new language beyond puberty, although it’s harder and may take longer compared to young children.
Therefore, learning perfect phonology and grammar in a second language has a critical period, but learning the usage of it seems to have more a sensitive period rather than a critical period (although this alternative definition is still controversial5).
Why Is Critical Period Important
Critical periods are important because many crucial functions of our body are established during those periods, and some only during those periods.
Scientists have found that the following functions are best developed during their critical or sensitive periods:
In a study in a Romanian orphanage, only orphans who were adopted by foster families before the age of 2 developed emotional regulation skills comparable to those of the never institutionalized children6.
The sensitive period of emotional regulation is therefore believed to be from birth to age 2.
Related: The Science Of Emotional Regulation
There are different critical periods for different visual functions of the visual system.
They usually fall between eye-opening and puberty7.
For example, visual acuity usually develops from birth to around age 5 and the period between ages 3 and 5 shows the most growth.
On the other hand, stereopsis, the perception of depth, has a critical period that ends at age 2.
Sensitivity to damage in visual development also has its own critical period.
For instance, amblyopia, the condition where one of the eyes has reduced vision because the eye and brain are not working together properly, can result between several months old and 7 or 8 years old.
As mentioned, the critical period for new language acquisition is believed to be before puberty.
After puberty, it is very hard, if not impossible, to attain proficiency in accent and grammar of the new language.
Absolute pitch in music listening
Absolute pitch is the ability to identify and produce the pitch of a musical sound without an external reference point8.
Children who started musical training between ages 4 and 6 are most likely to reach the absolute pitch.
But training that begins after the age of 9 rarely leads to that level of proficiency9.
For children who are born with congenital deafness, the absence of auditory input from birth can affect the normal growth of a functional auditory system, severely affecting their ability to learn to speak.
Scientists have found that when cochlear implants are installed to bypass the non-functional inner ears in these children before age 3.5, they can most likely learn to speak successfully, especially if they are also exposed to language-rich environments10.
Final Thoughts On Critical Periods
It may feel overwhelming that there are so many different critical periods in the brain development journey.
As parents, we should make sure our children are not deprived of critical experiences, especially during critical periods.
But some of the development is actually experience-expectant, meaning the stimuli required for development are practically guaranteed to be available in daily life, e.g. the capacity for language, vision, and hearing. Parents rarely have to make an effort to introduce those common experiences.
Abilities that depend on the presence of specific experience to develop are called experience-dependent development, e.g. emotional regulation, a second language, and the perfect pitch.
Among the different experience-dependent abilities, emotional regulation is hands down the most essential one parents should work on.
So the most important thing for parents to do is to provide a nurturing environment for your child and help your child build resilience.
- 1.Hensch TK. Critical period plasticity in local cortical circuits. Nat Rev Neurosci. November 2005:877-888. doi:10.1038/nrn1787
- 2.Hakuta K, Bialystok E, Wiley E. Critical Evidence. Psychol Sci. January 2003:31-38. doi:10.1111/1467-9280.01415
- 3.Friederici AD, Steinhauer K, Pfeifer E. Brain signatures of artificial language processing: Evidence challenging the critical period hypothesis. Proceedings of the National Academy of Sciences. January 2002:529-534. doi:10.1073/pnas.012611199
- 4.Birdsong D. Second Language Acquisition and the Critical Period Hypothesis. Routledge; 1999.
- 5.Birdsong D, Molis M. On the Evidence for Maturational Constraints in Second-Language Acquisition. Journal of Memory and Language. February 2001:235-249. doi:10.1006/jmla.2000.2750
- 6.McLaughlin KA, Sheridan MA, Tibu F, Fox NA, Zeanah CH, Nelson CA III. Causal effects of the early caregiving environment on development of stress response systems in children. Proc Natl Acad Sci USA. April 2015:5637-5642. doi:10.1073/pnas.1423363112
- 7.Daw NW. Critical Periods and Amblyopia. Arch Ophthalmol. April 1998:502. doi:10.1001/archopht.116.4.502
- 8.Levitin DJ, Rogers SE. Absolute pitch: perception, coding, and controversies. Trends in Cognitive Sciences. January 2005:26-33. doi:10.1016/j.tics.2004.11.007
- 9.Gervain J, Vines BW, Chen LM, et al. Valproate reopens critical-period learning of absolute pitch. Front Syst Neurosci. 2013. doi:10.3389/fnsys.2013.00102
- 10.Kral A, Sharma A. Developmental neuroplasticity after cochlear implantation. Trends in Neurosciences. February 2012:111-122. doi:10.1016/j.tins.2011.09.004