What are Critical Periods
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 of time. Synaptic connections usually mature and stabilize after this time period, making the wirings harder to change at a later age. Lacking certain experiences in the early years of life can have a profound effect on developing neural connections.
Critical Period Hypothesis
According to the Critical Period Hypothesis, during the critical period, new skills or traits can be formed given the proper life experience. If the necessary 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 in humans and animals, such as the development of the visual system.
In animal studies, 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 sensory deprivation and the lack of visual input during that time can cause structural changes in the brain1.
Critical vs Sensitive Period
The main difference between a critical period and a sensitive period is that the critical period is a limited time frame during which certain development can occur, while the sensitive period is a window of time where development is more easily achieved.
A sensitive period is similar to a critical period in which neural circuits are relatively more plastic and more sensitive to the influence of experience. However, new synapses can still form for an extended period of time outside of the sensitive period despite being harder.
Some scientists refer to these as weak critical periods.
Why Is Critical Period Important
Critical periods are important because many crucial functions of our body are established during a specific time, and some only during those periods.
Research findings have found that the following functions are best developed during their critical periods.
Self regulation in children is the ability to monitor and modulate emotions. Learning to self-regulate is a key milestone in a child’s early development. It can significantly impact a child’s relationships, academic performance, mental health, and well-being in the long term.
In human studies conducted 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. Social impairments and the lack of emotional regulation skills were evident.
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. They usually fall between birth and puberty7.
Early research shows that visual acuity develops from birth to around age 5 and the period between ages 3 and 5 shows the most growth.
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 of age and 7 to 8 years.
Absolute Pitch in Music Listening
Absolute pitch is the ability to identify and produce the perfect pitch of a musical sound without external sounds as reference points8.
Children who started musical training between ages 4 and 6 are most likely to reach the absolute pitch.
But training that occurs 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 severely affect their ability to learn to speak.
Scientists have found that when cochlear implants are installed to bypass the non-functional inner ears in deaf 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 learning, 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.
The critical period of language development begins in early childhood and concludes shortly before the onset of puberty2.
After that period of development, it becomes much more difficult to acquire new language competency3 and full mastery, even in a linguistically rich environment.
The original 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, the process of 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 it. The time of that discontinuity reflects the end of the critical period4.
However, it is still possible for adults to learn to use a new language beyond puberty. It is just harder and may take longer compared to young children.
Thus, learning perfect phonology and grammar in a second language has a critical period, but learning a foreign language (as general speakers) seems to have more of a sensitive period rather than a critical period (although this alternative definition is still controversial5 and individual results do vary).
What Parents Should Know
It may feel overwhelming that there are so many different critical periods at different ages in brain development.
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 is true.
The critical period is a controversial science concept because it implies there is a hard cutoff. If the skill is not developed during that time, the opportunity to develop this function will be gone forever.
But some of those skills are actually experience-expectant rather than experience-dependent. Experience-expectant means the stimuli from life experiences are expected in normal development. For instance, language, vision, and hearing are expected experiences practically guaranteed to be available to human infants. Parents rarely have to make an effort to introduce such sensory 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. Emotional regulation, speech & language, and absolute pitch are experience-dependent examples.
But the good news is many experience-dependent traits have sensitive periods rather than critical periods. Even when early exposure is missing during the optimal time, the skills can still develop. It might just be harder or take longer.
Among the experience-dependent abilities, emotional regulation plays an important role in a child’s growth and future well-being. So the most important thing for parents to do is to provide a nurturing environment for their children and help them build resilience.
Final Thoughts On Critical Period
As parents, it’s better if we 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 for dozes of enrichment classes. What our children need is a nurturing environment and exposure to 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.
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- 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