Role of Experience in Development

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The role of sensory experience during sensitive periods has been documented in numerous animal studies. Experience in part controls the selection of axons, dendrites, synapses, and neurons that will form the functional neural circuits. For example, during the sensitive period for ocular dominance, visual deprivation induced by monocular eyelid suture results in shrinkage of the ocular dominance columns serving the closed eye. Outside the sensitive period, visual deprivation has little effect on the pattern of ocular dominance. There is good agreement in the literature that experience affects the organization of local circuits rather than major pathways because the main topographical and columnar organization of the cortex has already been achieved by the time most sensitive periods have been reported to occur. Experience has also been implicated in the onset of sensitive periods. In cats, rearing in the dark results in delayed onset of the sensitive period for ocular dominance formation. Similar observations of the role of experience have been made in the auditory systems of songbirds and humans. For example, the maturation of an early auditory evoked response displays an extended time course of development after cochlear implantation in congenitally deaf children. In this case, the number of years of auditory experience, rather than chronological age per se, was predictive of the maturational time course, even in individuals implanted during adulthood. Whereas this result indicates that this early auditory response retains its capacity to mature throughout life, most systems typically exhibit limits on the period of time when experience leads to normal maturation. Kittens raised with both eyes sutured at the age of 1.5-16 months show significantly reduced contrast sensitivity, human babies with bilateral cataracts show little recovery if operated on after 2 years of age, and children exposed to a natural language for the first time around puberty fail to master that language.

Evidence indicates that some sensitive periods are mediated, at least in part, by changes in the sensitivity of the N-methyl-d-aspartate (NMDA) receptors; in particular, these receptors require stronger input for learning to proceed as a sensitive period unfolds. This change in sensitivity occurs over time but also appears to be modulated by experience. For example, the visual cortex of older, binocularly deprived cats exhibits the same large NMDA component in their response to light as younger kittens

Figure 1 (a) Mean synaptic density in synapses/100 mm3 in visual, auditory, and prefrontal cortices at various ages during human development. The data suggest regional differences in synaptogenesis [adapted from Huttenlocher and Dabholkar (1997)]. (b) In contrast, the mean synaptic density in synapses/100 mm2 of neuropil was found to be comparable across visual, somatosensory, motor, and prefrontal cortices during development in monkeys [adapted from Rakic, Bourgeois (1986)].

Figure 1 (a) Mean synaptic density in synapses/100 mm3 in visual, auditory, and prefrontal cortices at various ages during human development. The data suggest regional differences in synaptogenesis [adapted from Huttenlocher and Dabholkar (1997)]. (b) In contrast, the mean synaptic density in synapses/100 mm2 of neuropil was found to be comparable across visual, somatosensory, motor, and prefrontal cortices during development in monkeys [adapted from Rakic, Bourgeois (1986)].

that have had a similar amount of exposure to light, indicating that visual experience affects this change. Thus, the maturational status of the brain is affected by both chronological age and experience, illustrating the inextricable roles of nature and nurture during the course of development.

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