What are critical periods of development? In pregnancy, each part of the fetus’ body forms during a specific time. This specific time is called the “critical period of development” for that body part. During this critical time of development, the body can be very sensitive to exposures. Examples of exposures may include medications, alcohol, infections, health conditions, or other substances. ( see embryology for more )
https://www.ncbi.nlm.nih.gov/books/NBK582659/
Imprinting (psychology)
<— Lorenz demonstrated how incubator-hatched geese would imprint on the first suitable moving stimulus they saw within what he called a "critical period" between 13 and 16 hours shortly after hatching. For example, the goslings would imprint on Lorenz himself (to be more specific, on his wading boots), and he is often depicted being followed by a gaggle of geese who had imprinted on him.
https://en.wikipedia.org/wiki/Imprinting_(psychology)
“ ...In developmental psychology and developmental biology, a critical period is a maturational stage in the lifespan of an organism during which the nervous system is especially sensitive to certain environmental stimuli. If, for some reason, the organism does not receive the appropriate stimulus during this "critical period" to learn a given skill or trait, it may be difficult, ultimately less successful, or even impossible, to develop some functions later in life... “
https://en.wikipedia.org/wiki/Critical_period
Children’s brains develop in spurts called critical periods. The first occurs around age 2, with a second one occurring during adolescence. At the start of these periods, the number of connections (synapses) between brain cells (neurons) doubles. Two-year-olds have twice as many synapses as adults. Because these connections between brain cells are where learning occurs, twice as many synapses enable the brain to learn faster than at any other time of life. Therefore, children’s experiences in this phase have lasting effects on their development.
This first critical period of brain development begins around age 2 and concludes around age 7. It provides a prime opportunity to lay the foundation for a holistic education for children.
Four ways to maximize this critical period include;
- encouraging a love of learning,
- focusing on breadth instead of depth,
- paying attention to emotional intelligence, and
- not treating young children’s education as merely a precursor to “real” learning.
Why Ages 2-7 Matter So Much for Brain Development
https://www.edutopia.org/article/why-ages-2-7-matter-so-much-brain-development
Animal studies have shown that there are certain windows of time during which the young are especially sensitive to their environment:
- Newborn mice must experience normal whisker sensation in the first few days of life or they will develop abnormal tactile sensitivity in the face region;
- Cats must be allowed normal visual input during the first three months or their vision will be permanently impaired; and
- Monkeys need consistent social contact during the first six months or they will end up extremely emotionally disturbed.
Many of the same critical periods appear to hold for human development, although we are less certain about their exact length. Thus,
- Babies also require normal visual input or they may suffer permanent impairment; children born with crossed or “lazy” eyes will fail to develop full acuity and depth perception if the problem is not promptly corrected.
- Language skills depend critically on verbal input (or sign language, for babies with hearing impairments) in the first few years or certain skills, particularly grammar and pronunciation, may be permanently impacted.
- The critical period for language-learning begins to close around five years of age and ends around puberty. This is why individuals who learn a new language after puberty almost always speak it with a foreign accent.
https://www.zerotothree.org/resources/1368-what-is-a-critical-period-in-brain-development
"...In psychology, preparedness is a concept developed to explain why certain associations are learned more readily than others. For example, phobias related to survival, such as snakes, spiders, and heights, are much more common and much easier to induce in the laboratory than other kinds of fears. According to Martin Seligman, this is a result of our evolutionary history. The theory states that organisms which learned to fear environmental threats faster had a survival and reproductive advantage. Consequently, the innate predisposition to fear these threats became an adaptive human trait.
The concept of preparedness has also been used to explain why taste aversions are learned so quickly and efficiently compared with other kinds of classical conditioning..."
https://en.wikipedia.org/wiki/Preparedness_(learning)
The maturation of myelination in intracortical layers coincides with critical period closure in mice, which has led to further research on the role of myelination on critical period duration.
https://www.ncbi.nlm.nih.gov/books/NBK27954/
Learning Who is Your Mother
The Behavior of Imprinting
The Critical Period
Frank Sengpiel
What is the critical period?
Also known as the sensitive period, the critical period is a time during early postnatal life when the development and maturation of functional properties of the brain, its ‘plasticity’, is strongly dependent on experience or environmental influences. The concept of a critical period therefore plays an important role in the age-old nature versus nurture debate — to what extent are our abilities determined by intrinsic factors, such as our genes, or by extrinsic factors, such as childhood experiences? To be precise, there isn’t a single critical period, but there are different critical periods for different brain functions, for example binocular vision or language acquisition.
How do we know there is a critical period?
Most evidence comes from examples where the complete absence of certain experiences early in life prevents the development of associated brain functions. Later exposure to those experiences cannot make up for the earlier loss. Human ‘feral children’, such as Victor de l’Aveyron, Kaspar Hauser and, more recently, Genie, have provided insights in particular into the critical period for acquisition of the first language. But the extent of deprivation is usually not fully known, making the interpretation of any findings difficult. In classical animal studies, sensory experiences have been withheld during various time windows in order to define critical periods for visual, auditory and somatosensory modalities. Perhaps the best-known example is that of ‘monocular deprivation’, pioneered by the Nobel laureates Hubel and Wiesel in the 1960s, in which suturing shut the lids of one eye throughout the critical period causes functional blindness in that eye, despite the fact that the retina of the deprived eye works fine after re-opening the lids. With regard to hearing, the development of an auditory space map in the midbrain has been studied extensively: it requires calibration by visual input because the input from the inner ear does not contain spatial information.
What starts the critical period?
At a functional level, it seems that the various critical periods start very shortly after the relevant sensory information first becomes available: the critical period for ‘ocular dominance’ (the relative representation of the two eyes in the primary visual cortex) begins just after eye-opening in animals such as cats or ferrets which are born with closed eyelids, or at birth in species born with open eyes, like humans. Similarly, the onset of hearing marks the start of the critical period for binaural integration in the auditory brainstem.
At a cellular level, it has remained unclear until recently what changes occur in the brain so that it develops normally in the absence of sensory input one day but goes into decline the next day if that sensory experience continues to be withheld. For example, orientation maps in the visual cortex of young ferrets raised in complete darkness are close to normal up to five weeks of age, when ferrets’ eyes open, but then disappear over the next few weeks if the animals are kept in the dark.
It now appears that a certain level of intracortical inhibition marks the onset of the critical period, at least in the visual cortex. The development of cortical inhibitory circuitry initially lags behind that of the excitatory circuitry. Of particular interest are the so-called large basket cells, which use the inhibitory transmitter γ-amino butyric acid (GABA). If their maturation is accelerated, such as in mice over-expressing the nerve growth factor BDNF, then the critical period for the effects of monocular deprivation on cortical ocular dominance starts (and ends) sooner than in normal mice. Conversely, dark- rearing delays the maturation of GABAergic transmission and the onset of the critical period and prolongs its duration. More direct evidence for the role of GABAergic neurons in the control of the critical period comes from mice deficient for an enzyme needed for the synthesis of GABA in presynaptic terminals. These mice are insensitive to monocular deprivation throughout life, but treatment with diazepam (which acts as a GABA agonist) restores cortical plasticity.
What ends the critical period?
The critical period is characterized by changes not only at the level of synaptic transmission, but increasingly by structural changes, which result in closure of the critical period. In the visual cortex, changes in the composition of the NMDA receptor, which plays a key role in synaptic strengthening and weakening, have been linked to the type of visual experience an animal has had during the critical period, for example, whether it has been reared in a normal environment or in the dark. Depending on previous experience, these receptor changes are to some extent reversible, but as the critical period comes to its end, they make it harder for further synaptic plasticity to occur.
Recent advances in live imaging methods at the microscopic level have made it possible to visualize the dynamics of dendritic spines, the presumed site of synaptic plasticity. In the course of the critical period spine turn-over and motility decrease, their number and shape becoming more stable. Spine motility is controlled by tissue plasminogen activator (tPA), which declines Magazine R743 with age but is upregulated by monocular deprivation during the critical period. It appears that tPA is a permissive factor in cortical plasticity, but it does not determine whether new synapses will be formed or existing ones eliminated — this might instead depend on local levels of pre- and postsynaptic activity.
The most significant structural changes in the cortex towards the end of the critical period are those seen in the extracellular matrix, a network of macromolecules, which becomes more and more rigid during postnatal development. A major component of the extracellular matrix are chondroitinsulfate proteoglycans: these molecules aggregate in perineuronal nets, lattice-like structures that ensheathe in particular the GABAergic large basket cells implicated in the control of the critical period, leaving just small windows at the sites of synaptic contact and inhibiting axonal sprouting. It has been shown in adult rats that enzymatic digestion of chondroitinsulfate proteoglycans makes the visual cortex susceptible again to the effects of monocular deprivation, suggesting that the maturation of the extracellular matrix plays a key role in the closure of the critical period.
Another factor that appears to contribute to the closure of the critical period is an increase in the Nogo-66 receptor for the myelin-associated growth inhibitor Nogo. It in turn activates an intracellular pathway which regulates the actin cytoskeleton and thus controls axonal growth. Mice lacking this receptor exhibit visual cortical plasticity in response to monocular deprivation well into adulthood.
Is there plasticity beyond the critical period?
Of course the critical period does not end abruptly one day, but a number of studies have now reported plasticity in the mouse visual cortex well beyond what would have been defined as the critical period. This sort of adult plasticity may or may not be based on the same molecular mechanisms as classical critical period plasticity. Also, earlier sensory experiences predispose the brain to rapidly respond again to similar experiences made later on, even in adulthood, and training is likely to enhance such adult plasticity. Probably more interesting still is the question whether one can somehow turn back the clock and put adult cortex into a plastic state equivalent to that during the critical period. This could be of great therapeutic significance if it allowed us to correct, in adulthood, things that went wrong in brain development during childhood. One such example is amblyopia, loss of visual acuity in one eye because of early ocular abnormalities, for which there is no treatment available in adulthood. A loosening of the extracellular matrix or a blockade of the Nogo-66 receptor are currently the most promising avenues of research. However, no-one yet knows whether increased cortical plasticity will have unwanted side-effects. Presumably, the relative stability of cortical circuitry attained by the end of the critical period is beneficial to the individual, at least under normal circumstances, and a loss of that stability may disrupt cortical function in unforeseen ways.
Where can I find out more?
Berardi, N., Pizzorusso, T., and Maffei, L. (2004). Extracellular matrix and visual cortical plasticity: freeing the synapse. Neuron 44, 905–908.
Hensch, T.K. (2005). Critical period plasticity in local cortical circuits. Nat. Rev. Neurosci. 6, 877–888.
Hofer, S.B., Mrsic-Flogel, T.D., Bonhoeffer, T., and Hübener, M. (2006). Prior experience enhances plasticity in adult visual cortex. Nat. Neurosci. 9, 127–132.
Kacelnik, O., Nodal, F.R., Parsons, C.H., and King, A.J. (2006). Training-induced plasticity of auditory localization in adult mammals. PLoS Biol. 4:e71.
McGee, A.W., Yang, Y., Fischer, Q.S., Daw, N.W., and Strittmatter, S.M. (2005). Experience-driven plasticity of visual cortex limited by myelin and Nogo receptor. Science 309, 2222–2226.
Philpot, B.D., Cho, K.K.A., and Bear, M.F. (2007). Obligatory role of NR2A for metaplasticity in visual cortex. Neuron 53, 495–502. School of Biosciences, Cardiff University, Cardiff CF10 3US, UK. E-mail: SengpielF@cf.ac.uk
https://www.cell.com/current-biology/pdf/S0960-9822(07)01519-9.pdf
Critical Period In Brain Development and Childhood Learning
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 changes stabilize after this window of time and the wirings become 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 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 human and animals, 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 permanently 1 .
Critical Period vs Sensitive Period
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.
Studies have found that the following functions are best developed during their critical periods.
Emotional regulation
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 children 6 . Those who remained in the orphanage suffered from deprivation of social contact or maternal care, and grew up lacking emotional regulation later in life.
The sensitive period of emotional self-regulation is therefore believed to be from birth to age 2.
Vision System
There are different critical periods for different visual functions of the visual system. They usually fall between eye-opening and puberty 7 .
For example, research results show 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 sounds as reference points 8 .
Children who started musical training between ages 4 and 6 are most likely to reach the absolute pitch.
But trainings that occur after the age of 9 rarely leads to that level of proficiency 9 in adult.
Auditory Processing
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 environments 10 .
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.
This period begins in early childhood and concludes shortly before the onset of puberty 2 .
After this window, even with a linguistically rich environment, it becomes much more difficult to acquire new language competency 3 and full mastery is unlikely.
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 of it. The time of that discontinuity reflects the close of the critical period 4.
However, we know that it is still possible for adults to learn to use a new language beyond puberty, although it’s 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 (as general speakers) seems to have more a sensitive period rather than a critical period (although this alternative definition is still controversial 5 and individual effect does vary).
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 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, 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 when the particular life experiences are 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 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, 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 in dozes of enrichment classes. What our children need are a nurturing environment and exposure to common life experiences, such as talking, playing and reading to them. Also, other underlying factors could affect the outcomes, too.
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.
References
1. Hensch TK.
Critical period plasticity in local cortical circuits. Nat Rev Neurosci.
November 2005:877-888. doi:10.1038/nrn1787
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
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
https://www.parentingforbrain.com/critical-period/
The Development of Language: A Critical Period in Humans -
Neuroscience - NCBI Bookshelf
https://www.ncbi.nlm.nih.gov/books/NBK11007/
Critical period regulation across multiple timescales | PNAS
https://www.pnas.org/content/117/38/23242
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