Critical period

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. 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' would not acquire their first language fluently. 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. 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' would not acquire their first language fluently. Some researchers differentiate between 'strong critical periods' and 'weak critical periods' (a.k.a. '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 strong critical periods include monocular deprivation, filial imprinting, monaural occlusion, and Prefrontal Synthesis acquisition. These traits cannot be acquired after the end of the critical period. 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. Critical periods of plasticity occur in the prenatal brain and continue throughout childhood until adolescence and are very limited during adulthood. Two major factors influence the opening of critical periods: cellular events (i.e. changes in molecular landscape) and sensory experience (i.e. hearing sound, visual input, etc). Both need to coincide for the critical period to open properly. At the cellular level, critical periods are characterized by maturation of the inhibitory circuits. More precisely, factors such as brain-derived neurotrophic factor (BDNF) and orthodenticle homeobox 2 (Otx2) contribute to the maturation of a major class of inhibitory neurons: parvalbumin-positive interneurons (PV cells). Prior to the onset of the critical period, modulation of this circuit is hampered by early factors such as polysialic acid (PSA). PSA acts, in part, by preventing Otx2 interaction with PV cells. Soon after the opening of the critical period, PSA levels decrease, allowing PV cell maturation by activating inhibitory GABAa receptors that facilitate inhibitory circuit remodeling. Artificially removing PSA, or experimentally manipulating inhibitory transmission can result in early opening of the critical period . While the timing of these molecular events seems to be partially explained by clock genes, experience is crucial as sensory deprivation experiments have been shown to interfere with the proper timing of critical periods .