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
What is Critical Period
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 life experience. These connections, called synapses, can form or strengthen more easily during this period. Synaptic connections usually mature and stabilize after this window of time and the wiring becomes harder to change.
Critical Period Hypothesis
According to the Critical Period Hypothesis, during the critical period, a new skill or trait can be formed given the proper life experience. If the 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 pathways of the brain permanently1.
A sensitive period is similar to a critical period in which 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 this optimal period despite being more difficult.
Some scientists refer to this as a weak critical period.
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 periods.
Emotional self regulation is the ability to monitor and modulate emotions. Learning to self-regulate is a key milestone in a child’s development. It can significantly impact a child’s relationships, academic performance, mental health and long-term well-being.
In a study in a Romanian orphanage, only orphans who were adopted by foster families before the age of 2 were able to develop emotional regulation skills comparable to those of the never institutionalized children6. Those who remained in the orphanage suffered from deprivation of social contact or maternal care, and grew up lacking emotional regulation.
The sensitive period of emotional self regulation is therefore believed to be from birth to age 2.
There are different critical periods for different visual functions of the visual system. They usually fall between eye-opening and puberty7.
For example, research shows that 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 2 years of age.
Susceptibility 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 of age.
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.
Critical Period For Language Acquisition
When applied to language acquisition, the Critical Period Hypothesis states that there is a critical time during which individuals are more capable of acquiring 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.
However, we know that 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).
What Parents Should Know
It may feel overwhelming that there are so many different critical periods in the brain development journey.
Parents who have “missed” some of the critical periods are worried that their children are now destined to fail. Those who have “met” the critical periods successfully are glad that their children are now set for life and their jobs are done.
The truth is that neither of these are true.
Critical period is a controversial concept because it implies there is a hard cutoff. If the skill is not developed during that time, it will be gone forever.
But some of those skills are actually experience-expectant rather than experience-dependent, meaning the stimuli required for development are expected. The expected experiences are practically guaranteed to be available in everyday 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 experiences are experience-dependent. Parents need to provide the appropriate early life experiences for these skills to develop. Some examples are emotional regulation, a second language, and the absolute pitch.
But the good news is many experience-dependent traits have sensitive periods rather than critical periods. Even though life experiences are missing during the optimal time, the skills can still develop. It may just be harder or take longer.
Among the experience-dependent abilities, emotional regulation is hands down the most essential one to a child’s growth and future well-being. So the most important thing for parents to do is to provide a nurturing environment for your child and help your child build resilience.
Final Thoughts On Critical Period
As parents, we should make sure our children are not deprived of critical experiences, especially during critical periods. However, it doesn’t mean we should buy the latest “mozart for babies” DVD or sign our toddlers up in dozes of enrichment classes. What our children need are a nurturing environment and common life experiences, such as talking, playing and reading to them.
There is also no need to sweat over missing the optimal times, because it’s never too late to start providing good life experiences to our kids.
- 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