In all, myelin and its associated receptors bind several important axonal growth inhibitors which help close the critical period. While the presence or absence of sensory experiences most robustly shapes brain development during the critical period, the behavioral context i. Research using this approach has highlighted the role of neuromodulation in sensory processing during the critical period.

Mechanistically, neuromodulation is increasingly being recognized for its fine-tuning of the PV cell-mediated inhibition of excitatory pyramidal neurons ' soma. The critical period hypothesis CPH states that the first few years of life constitute the time during which language develops readily and after which sometime between age 5 and puberty language acquisition is much more difficult and ultimately less successful.

Lenneberg argued for the hypothesis based on evidence that children who experience brain injury early in life develop far better language skills than adults with similar injuries. 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 Genie and the feral child Victor of Aveyron.

The children may have been cognitively disabled from infancy, or their inability to develop language may have resulted from the profound neglect and abuse they suffered. 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. 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.

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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. 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.

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.

The theory [58] has often been extended to a critical period for second language acquisition SLA , which has influenced researchers in the field on both sides of the spectrum, supportive and unsupportive of CPH, to explore. 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 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. In , Jacqueline S. Johnson and Elissa L. 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.

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. 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 mammals , neurons in the brain that process vision actually develop after birth based on signals from the eyes. A landmark experiment by David H. Hubel and Torsten Wiesel showed that cats that had one eye sewn shut from birth to three months of age monocular deprivation only fully developed vision in the open eye. They showed that columns in the primary visual cortex receiving inputs from the other eye took over the areas that would normally receive input from the deprived eye.

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In general electrophysiological analyses of axons and neurons in the lateral geniculate nucleus showed that the visual receptive field properties was comparable to adult cats. However, the layers of cortex that were deprived had less activity and fewer responses were isolated.

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The kittens had abnormally small ocular dominance columns part of the brain that processes sight connected to the closed eye, and abnormally large, wide columns connected to the open eye. Because the critical period time had elapsed, it would be impossible for the kittens to alter and develop vision in the closed eye.

This did not happen to adult cats even when one eye was sewn shut for a year because they had fully developed their vision during their critical period. Later experiments in monkeys found similar results consistent with the strong critical period.

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In a follow-up experiment, Hubel and Wiesel 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. In a similar experiment, Antonini and Stryker 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 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 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.

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Some hypothesize that being blind during childhood prevents some part of the visual system necessary for these higher-level tasks from developing properly. Expression of the protein Lynx1 has been associated with the normal end of the critical period for synaptic plasticity in the visual system. In psychology, imprinting is any type of rapid learning that occurs in a particular life stage. While this rapid learning is independent of the behavioral outcome, it also establishes it and can effect behavioral responses to different stimuli.

Konrad Lorenz is well known for his classic studies of filial imprinting in graylag geese. From to , he presented himself to a group of newly hatched gosling and took note of how he was instantly accepted, followed, and called to as if he were the one who laid them himself. As the first moving object they encountered, Lorenz studied the phenomenon in how quickly the geese were able to form such an irreversible bond. 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.

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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. Its function includes relaying sensory and motor signals to the cerebral cortex, along with the regulation of consciousness, sleep, and alertness.

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Thalamic nuclei : The ventral posterolateral nucleus receives sensory information from the body. The cortical sensory homunculus is located in the postcentral gyrus and provides a representation of the body to the brain. A cortical homunculus is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex; it is the portion of the human brain directly responsible for the movement and exchange of sensory and motor information of the body.

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There are two types of homunculus: sensory and motor. Each one shows a representation of how much of its respective cortex innervates certain body parts. The primary somesthetic cortex sensory pertains to the signals within the postcentral gyrus coming from the thalamus, and the primary motor cortex pertains to signals within the precentral gyrus coming from the premotor area of the frontal lobes. These are then transmitted from the gyri to the brain stem and spinal cord via corresponding sensory or motor nerves.

The reason for the distorted appearance of the homunculus is that the amount of cerebral tissue or cortex devoted to a given body region is proportional to how richly innervated that region is, not to its size. The homunculus is like an upside-down sensory or motor map of the contralateral side of the body. The upper extremities such as the facial body parts and hands are closer to the lateral sulcus than lower extremities such as the leg and toes.