Why does the developing brain undergo these critical periods in its development?
Neuroscientists do not yet fully understand the biological basis of these critical periods.
One theory is that they correspond to a period of synaptic excess in the brain: between infancy and the early grade school years, the brain actually over-produces connections–some 50 percent more than will be preserved in adulthood. During the critical period, a child’s experience–sensory, motor, emotional, and intellectual–determines which of these synapses will be preserved, through pruning of the least useful connections. In this way, each child’s brain becomes better tuned to meet the challenges of his or her particular environment.
A related theory holds that learning itself creates critical periods in a child’s brain. That is, the longer a child has been exposed to one type of experience or environment, the less likely he or she will be able to reverse the synaptic learning that has already taken place. Animal studies provide some support for this theory. For example, kittens that are deprived of all vision (as opposed to the vision in just one eye) in the first few months of life show a delayed critical period for visual experience, beginning from the time their deprivation ends. Similarly, songbirds normally learn their species-typical songs early in life, by listening to adults of the same species. However, when newly hatched birds of certain species are isolated, permitting them no song exposure during early life, their critical period for song learning is delayed, even as late as adulthood.
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Generally speaking, the central nervous system (which is composed of the brain and the spinal cord) matures in a sequence from "tail" to head.
In some ways, never. Our brains are continually re-shaping themselves to meet the demands of everyday life, even throughout adulthood.