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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 certain associated functions later in life. Functions that are indispensable to an organism's survival, such as vision, are particularly likely to develop during critical periods. "Critical period" also relates to the ability to acquire one's first language. Researchers found that people who passed the "critical period" without having developed communication skills would not acquire their first language fluently.
Some researchers differentiate between 'strong critical periods' and 'weak critical periods' (also known as 'sensitive' periods)—defining 'weak critical periods' / 'sensitive periods' as more extended periods, after which learning is still possible. Other researchers consider these the same phenomenon.
For example, the critical period for the development of a human child's binocular vision is thought to be between three and eight months, with sensitivity to damage extending up to at least three years of age. Further critical periods have been identified for the development of hearing and the vestibular system.
Examples of weak critical periods include phoneme tuning, grammar processing, articulation control, vocabulary acquisition, music training, auditory processing, sport training, and many other traits that can be significantly improved by training at any age.
Additionally, PNNs are negatively charged, which is theorized to create a cation-rich environment around cells, potentially leading to an increased firing rate of inhibitory neurons, thereby allowing for increased inhibition after the formation of PNNs and helping to close the critical period. The role of PNNs in critical period closure is further supported by the finding that fast-spiking parvalbulmin-positive interneurons are often surrounded by PNNs.
Perineuronal nets have also been found to contain chemorepulsive factors, such as semaphorin3A, which restrict axon growth necessary for plasticity during critical periods. In all, these data suggest a role for PNNs in the maturation of CNS inhibition, the prevention of plastic axonal growth, and subsequently, critical period closure.
Myelin is known to bind many different axonal growth inhibitors that prevent plasticity seen in critical periods. The Nogo receptor is expressed in myelin and binds to the axonal growth inhibitors Nogo and Myelin-associated glycoprotein (MAG) (among others), preventing axon growth in mature, myelinated neurons. Instead of affecting the timing of the critical period, mutations of the Nogo receptor prolong the critical period temporarily. A mutation of the Nogo receptor in mice was found to extend the critical period for monocular dominance from around 20–32 days to 45 or 120 days, suggesting a likely role of the myelin Nogo receptor in critical period closure.
Additionally, the effects of myelination are temporally limited, since myelination itself may have its own critical period and timing. Research has shown that social isolation of mice leads to reduced myelin thickness and poor working memory, but only during a juvenile critical period. In primates, isolation is correlated with abnormal changes in white matter potentially due to decreased myelination.
In all, myelin and its associated receptors bind several important axonal growth inhibitors which help close the critical period. The timing of this myelination, however, is dependent on the brain region and external factors such as the social environment.
Mechanistically, neuromodulation is increasingly being recognized for its fine-tuning of the PV cell-mediated inhibition of excitatory Pyramidal cell' soma. Central to the neuromodulatory regulation of PV cell activity is the existence of distinct subsets of inhibitory neurons, which are responsive to activation by neuromodulators and which inhibit PV cells. Within these cells, some also inhibit specific pyramidal cell dendrites. By inhibiting PV cells activity, the neuromodulator-sensitive inhibitory cells such as those expressing vasoactive intestinal peptide (VIP) or Somatostatin lift the inhibition of the pyramidal neurons; in other words, the activity of VIP and SST-expressing cells result in the disinhibition of pyramidal neurons. Then, by inhibiting only certain dendritic branches of these now dis-inhibited pyramidal neurons, the neuromodulation-activated cells allow select sensory inputs to excite the pyramidal neurons and be represented in the brain circuitry. Thus, in a landscape of global inhibition by maturing inhibitory signaling, neuromodulation allows windows of dis-inhibition, temporally and spatially, that allow behaviorally important sensory inputs the opportunity to influence the brain.
Maria Montessori was one of the earlier educators who brought attention to this phenomenon and called it "sensitive periods", which is one of the pillars of her philosophy of education.
The two most famous cases of children who failed to acquire language after the critical period are the Victor of Aveyron and Genie. However, the tragic circumstances of these cases and the moral and ethical impermissibility of replicating them make it difficult to draw conclusions about them. The children may have been cognitively disabled from infancy, or their inability to develop language may have resulted from the profound neglect and child abuse they suffered.
Many subsequent researchers have further developed the CPH, most notably Elissa Newport and Rachel Mayberry. Studies conducted by these researchers demonstrated that profoundly deaf individuals who are not exposed to a sign language as children never achieve full proficiency, even after 30 years of daily use. While the effect is most profound for individuals who receive no sign language input until after the age of 12, even those deaf people who began learning a sign language at age 5 were significantly less fluent than native deaf signers (whose exposure to a sign language began at birth). Early language exposure also affects the ability to learn a second language later in life: profoundly deaf individuals with early language exposure achieve comparable levels of proficiency in a second language to hearing individuals with early language exposure. In contrast, deaf individuals without early language exposure perform far worse.
Other evidence comes from neuropsychology where it is known that adults well beyond the critical period are more likely to suffer permanent language impairment from brain damage than are children, believed to be due to youthful resiliency of neural reorganization.
Steven Pinker discusses the CPH in his book, The Language Instinct. According to Pinker, language must be viewed as a concept rather than a specific language because the sounds, grammar, meaning, vocabulary, and social norms play an important role in the acquisition of language.Johnson, Eric. "First-Language Acquisition." Encyclopedia of Bilingual Education. Ed. Josué M. González. Vol. 1. Thousand Oaks, CA: SAGE Publications, 2008. 299–304. Gale Virtual Reference Library. Web. 22 Oct. 2014. Physiological changes in the brain are also conceivable causes for the terminus of the critical period for language acquisition. As language acquisition is crucial during this phase, similarly infant–parent attachment is crucial for social development of the infant. An infant learns to trust and feel safe with the parent, but there are cases in which the infant might be staying at an orphanage where it does not receive the same attachment with their caregiver. Research shows that infants who were unable to develop this attachment had major difficulty in keeping close relationships, and had maladaptive behaviors with adopted parents.
The discussion of language critical period suffers from the lack of a commonly accepted definition of language. Some aspects of language, such as phoneme tuning, grammar processing, articulation control, and vocabulary acquisition can be significantly improved by training at any age and therefore have weak critical periods. Other aspects of language, such as prefrontal synthesis, have strong critical periods and cannot be acquired after the end of the critical period. Consequently, when language is discussed in general, without dissection into components, arguments can be constructed both in favor and against the strong critical period of L1 acquisition.
Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages. This is generally accepted as evidence supporting the CPH. Incorporating the idea, "younger equals better" by Penfield, David Singleton (1995) states that in learning a second language there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood—long after any critical period has presumably come to a close. The critical period hypothesis holds that first language acquisition must occur before cerebral lateralization completes, at about the age of puberty. One prediction of this hypothesis is that second language acquisition is relatively fast, successful, and qualitatively similar to first language only if it occurs before the age of puberty. To grasp a better understanding of SLA, it is essential to consider linguistic, cognitive, and social factors rather than age alone, as they are all essential to the learner's language acquisition.
Over the years, many experimenters have tried to find evidence in support of or against the critical periods for second language acquisition. Many have found evidence that young children acquire language more easily than adults, but there are also special cases of adults acquiring a second language with native-like proficiency. Thus it has been difficult for researchers to separate correlation from Causality.
In 1989, Jacqueline S. Johnson and Elissa L. Newport found support for the claim that second languages are more easily acquired before puberty, or more specifically before the age of seven. They tested second language learners of English language who arrived in the United States at various ages ranging from three to thirty-nine, and found that there was a decline in grammatical correctness after the age of seven. Johnson and Newport attributed this claim to a decline in language learning ability with age. Opponents of the critical period argue that the difference in language ability found by Johnson and Newport could be due to the different types of input that children and adults receive; children received reduced input while adults receive more complicated structures.
Additional evidence against a strict critical period is also found in the work of Pallier et al. (2003) who found that children adopted to France from Korea were able to become native-like in their performance of French language even after the critical period for phonology. Their experiment may represent a special case where subjects must lose their first language in order to more perfectly acquire their second.
There is also some debate as to how one can judge the native-like quality of the speech participants produce and what exactly it means to be a near-native speaker of a second language. White et al. found that it is possible for non-native speakers of a language to become native-like in some aspects, but those aspects are influenced by their first language.
Recently, a connectionist model has been developed to explain the changes that take place in second language learning assuming that sensitive period affects lexical learning and syntactic learning parts of the system differently, which sheds further light on how first and second language acquisition changes over the course of learners development.
In a follow-up experiment, Hubel and Wiesel (1963) explored the cortical responses present in kittens after binocular deprivation; they found it difficult to find any active cells in the cortex, and the responses they did get were either slow-moving or fast-fatiguing. Furthermore, the cells that did respond selected for edges and bars with distinct orientation preferences. Nevertheless, these kittens developed normal binocularity. Hubel and Wiesel first explained the mechanism, known as orientation selectivity, in the mammalian visual cortex. Orientation tuning, a model that originated with their model, is a concept in which receptive fields of neurons in the LGN excite a cortical simple cell and are arranged in rows. This model was important because it was able to describe a strong critical period for the proper development of normal ocular dominance columns in the lateral geniculate nucleus, and thus able to explain the effects of monocular deprivation during this critical period. The critical period for cats is about three months and for monkeys, about six months. Experiment Module: Effects of Visual Deprivation During the Critical Period for Development of Vision. McGill University, The Brain from Top to Bottom
In a similar experiment, Antonini and Stryker (1993) examined the anatomical changes that can be observed after monocular deprivation. They compared geniculocortical axonal arbors in monocularly deprived animals in the long term (4 weeks) to short term (6–7 days) during the critical period established by Hubel and Wiesel (1993). They found that in the long term, monocular deprivation causes reduced branching at the end of neurons, while the amount of afferents allocated to the nondeprived eye increased. Even in the short term, Antonini and Stryker (1993) found that geniculocortical neurons were similarly affected. This supports the aforementioned concept of a critical period for proper neural development for vision in the cortex.
Studies of people whose sight has been restored after a long blindness (whether from birth or a later point in life) reveal that they cannot necessarily recognize objects and faces (as opposed to color, motion, and simple geometric shapes). Some hypothesize that being blind during childhood prevents some part of the visual system necessary for these higher-level tasks from developing properly. Man with restored sight provides new insight into how vision develops The general belief that a critical period lasts until age 5 or 6 was challenged by a 2007 study that found that older patients could improve these abilities with years of exposure. Out Of Darkness, Sight: Rare Cases Of Restored Vision Reveal How The Brain Learns To See
Expression of the protein Lynx1 has been associated with the normal end of the critical period for synaptic plasticity in the visual system.
Lorenz also discovered a long-lasting effect of his studies, and that was a shift in the species' sexual imprinting as a result from imprinting upon a foster mother of a second species. For certain species, when raised by a second one, they develop and retain imprinted preferences and approach the second species they were raised by rather than choose their own, if given a choice.
Imprinting serves as the distinguishing factor between one's own mother and other mother figures. The mother and the infant both identify with each other, this is a strong bonding moment for humans. It provides a sort of model or guide to adult behaviors in addition to other factors such as nurture, protection in infancy, guidance, and nourishment. The imprinting process, Lorenz also found, brought about a sense of familiarity for the young animals. When such a strong bond is formed at such an early stage, it creates a sense of security and comfort for the subject and actually encourages the imprinting behavior.
Pheromones play a key role in the imprinting process, they trigger a biochemical response in the recipient, leading to a confirmed identification in the other individual. If direct contact between mother and infant is not maintained during the critical imprinting period, then the mother goose may reject the infant because she is unfamiliar with her newborn's scent. If that does happen, then the infant's life would be in jeopardy unless it were claimed by a substitute mother, possibly leading to awkward social behavior in later life. In relation to humans, a newborn during the critical period identifies with its mother's and other peoples' scents since its scent is one of the most developed senses at that stage in life. The newborn uses this pheromone identification to seek the people it identifies with, when in times of distress, hunger, and discomfort as a survival skill. Inferences could be made for newborns based upon Lorenz's studies. When imprinting on their mothers, newborns look to them for nourishment, a sense of security, and comfort. Human newborns are among the most helpless known with orangutang newborns ranking second. Newborns of these species have a very limited array of innate survival abilities. Their most important and functional ability is to form bonds with close individuals who are able to keep them alive. Imprinting is a crucial factor of the critical period because it facilitates the newborn's abilities to form bonds with other individuals, from infancy to adulthood.
First reports on critical periods came from deaf children and animals that received a cochlear implant to restore hearing. Approximately at the same time, both an electroencephalographic study by Sharma, Dorman and Spahr and an in-vivo investigation of the cortical plasticity in deaf cats by Kral and colleagues demonstrated that the adaptation to the cochlear implant is subject to an early, developmental sensitive period. The closure of sensitive periods likely involves a multitude of processes that in their combination make it difficult to reopen these behaviorally. The understanding of the mechanisms behind critical periods has consequences for medical therapy of hearing loss. M. Merzenich and colleagues showed that during an early critical period, noise exposure can affect the frequency organization of the auditory cortex.
Recent studies have examined the possibility of a critical period for thalamocortical connectivity in the auditory system. For example, Zhou and Merzenich (2008) studied the effects of noise on development in the primary auditory cortex in rats. In their study, rats were exposed to pulsed noise during the critical period and the effect on cortical processing was measured. Rats that were exposed to pulsed noise during the critical period had cortical neurons that were less able to respond to repeated stimuli; the early auditory environment interrupted normal structural organization during development.
In a related study, Barkat, Polley and Hensch (2011) looked at how exposure to different sound frequencies influences the development of the tonotopic map in the primary auditory cortex and the ventral medical geniculate body. In this experiment, mice were reared either in normal environments or in the presence of 7 kHz tones during early postnatal days. They found that mice that were exposed to an abnormal auditory environment during a critical period P11-P15 had an atypical tonotopic map in the primary auditory cortex. These studies support the notion that exposure to certain sounds within the critical period can influence the development of tonotopic maps and the response properties of neurons. Critical periods are important for the development of the brain for the function from a pattern of connectivity. In general, the early auditory environment influences the structural development and response specificity of the primary auditory cortex.
The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the retina during head movement. It produces an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. Studies in fish and amphibians revealed a sensitivity in their VOR. They launched into space flight for 9–10, some with developing VORs and others with already developed reflexes. The fish with developing reflexes developed an upward bend in their tails. The altered gravity resulted in a shift of orientation. Those who were already matured with the reflex were insensitive to the microgravity exposure.
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