Identifying factors that influence critical periods
Bombarded with sensory information, the developing brain works to sort relevant from irrelevant details as it begins to make sense of the world. It does so by forming new connections (or strengthening existing ones) between some cells and pruning the connections between others — a process termed synaptic plasticity. Increased plasticity is a hallmark of critical periods. As the brain ages, however, it becomes less “plastic,” or easily modifiable, marking the closure of critical periods. While this decrease in plasticity is a key step toward the maturation of communication networks in the nervous system, researchers realized it might be possible to reopen critical period windows to improve sensory development in people who experienced early sensory deprivation.

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The human brain begins forming very early in prenatal life (just three weeks after conception), but in many ways, brain development is a lifelong project. That is because the same events that shape the brain during development are also responsible for storing information—new skills and memories—throughout life. The major difference between brain development in a child versus learning an adult is a matter of degree: the brain is far more impressionable (neuroscientists use the term plastic) in early life than in maturity. This plasticity has both a positive and a negative side. On the positive side, it means that young children's brains are more open to learning and enriching influences. On the negative side, it also means that young children's brains are more vulnerable to developmental problems should their environment prove especially impoverished or un-nurturing.

What is a "critical period" in brain development?
Pruning or selection of active neural circuits takes place throughout life, but is far more common in early childhood. Animal studies have shown that there are certain windows of time during which the young are especially sensitive to their environment: newborn mice must experience normal whisker sensation in the first few days of life or they will develop abnormal tactile sensitivity in the face region; cats must be allowed normal visual input during the first three months or their vision will be permanently impaired; and monkeys need consistent social contact during the first six months or they will end up extremely emotionally disturbed. Many of the same critical periods appear to hold for human development, although we are less certain about their exact length. Thus, babies also require normal visual input or they may suffer permanent impairment; children born with crossed or "lazy" eyes will fail to develop full acuity and depth perception if the problem is not promptly corrected. Language skills depend critically on verbal input (or sign language, for babies with hearing impairments) in the first few years or certain skills, particularly grammar and pronunciation, may be permanently impacted. The critical period for language-learning begins to close around five years of age and ends around puberty. This is why individuals who learn a new language after puberty almost always speak it with a foreign accent.

Which plays a more important role in brain development, nature (genes) or nurture (environment)?
Genes and environment interact at every step of brain development, but they play very different roles. Generally speaking, genes are responsible for the basic wiring plan—for forming all of the cells (neurons) and general connections between different brain regions--while experience is responsible for fine-tuning those connections, helping each child adapt to the particular environment (geographical, cultural, family, school, peer-group) to which he belongs. An analogy that is often used is wiring a phone network: genes would specify the number of phones and the major trunk lines that connect one relay station to the next. Experience would specify the finer branches of this network-the connections between the relay station and each person's home or office.
For example, each of us is born with the potential to learn language. Our brains are programmed to recognize human speech, to discriminate subtle differences between individual speech sounds, to put words and meaning together, and to pick up the grammatical rules for ordering words in sentences. However, the particular language each child masters, the size of his vocabulary, and the exact dialect and accent with which he speaks are determined by the social environment in which he is raised--that is, the thousands of hours he has spent (beginning even before birth) listening and speaking to others. Genetic potential is necessary, but DNA alone cannot teach a child to talk.

What role do parents play in a baby's brain development?
Parents are another important part of the developmental equation. Infants prefer human stimuli--your face, voice, touch, and even smell--over everything else. They innately orient to people's faces and would rather listen to a speech or singing than any other kind of sound.
Just as newborn babies are born with a set of very useful instincts for surviving and orienting to their new environment, parents are equally programmed to love and respond to our babies' cues. Most adults (and children) find infants irresistible, and instinctively want to nurture and protect them. It is certainly no accident that the affection most parents feel towards their babies and the kind of attention we most want to shower them with—touching, holding, comforting, rocking, singing and talking to—provide precisely the best kind of stimulation for their growing brains. Because brain development is so heavily dependent on early experience, most babies will receive the right kind of nurturing from their earliest days, through our loving urges and parenting instincts.
In spite of all the recent hype about "making your baby smarter," scientists have not discovered any special tricks for enhancing the natural wiring phase in children's brain development. Normal, loving, responsive caregiving seems to provide babies with the ideal environment for encouraging their own exploration, which is always the best route to learning.
The one form of stimulation that has been proven to make a difference is language: infants and children who are conversed with, read to, and otherwise engaged in lots of verbal interaction show somewhat more advanced linguistic skills than children who are not as verbally engaged by their caregivers. Because language is fundamental to most of the rest of cognitive development, this simple action—talking and