We have seen how a baby's genes provide the raw material for the development of her brain, but the actual organization of the brain is yet to be completed. In order for this to happen, the brain must get input from the environment to know how best to use its resources: genes have set the potential, but it is the environment that determines how much of this potential will be realized.
Much of what is considered learning and development occurs in the network of synapses, those little spaces between neurons. It is this network that determines how, and how well, information will be transmitted throughout the brain and it is the experiences that a baby has, particularly in the first years of life, that determine the way in which synapses are connected.
Prepared to Learn
Peter Huttenlocher, a neurologist from the University of Chicago, has conducted extensive studies in the area of how the number of synaptic connections rises and falls between infancy and adulthood. Using exceptionally high-resolution microscopes, Huttenlocher painstakingly counted the number of synaptic connections in the human brain at different ages. The findings are quite remarkable:
o At birth, babies have approximately 50 trillion synaptic connections - one-tenth as many as adults.
o By the age of three, babies have approximately twice as many synaptic connections as an adult - an estimated 1,000 trillion!
o By the age of 14, children again have as many connections as adults.
Why would an infant develop more synaptic connections than she will eventually need? The answer seems to be that an overabundance of the synaptic connections enables a baby to deal with any number of environments she might be born into. As a result, the brain can learn to speak Russian or Farsi, eat with chopsticks or a fork, track animals in the South American jungle or navigate the streets of New York City. So why do the number of synapses go down?
Experience: Food for Thought
In the earliest stages, each new experience your baby has adds new synaptic connections, resulting in the dramatic increase in synaptic density. As she experiences more, her brain fine-tunes itself through a process of synaptic selection. It turns out that there is a competition between synapses. Winners are determined by experience. The more times a synapse is used, the more likely it will become permanent. Each time your baby hears your voice, for example, certain synapses process this information. If she hears your voice many times, the synaptic connections are strengthened and they will survive. The synapses that are not used frequently wither and die - a process scientists refer to as synaptogenesis; through synaptogenesis, the brain becomes highly efficient at processing information.
Too Many Synapses?
You might wonder what the effect would be if the number of synapses was not pruned down. Wouldn't it make for even a smarter baby? The answer is no. In fact, the evidence seems to say that a failure to do away with extraneous synaptic connections is related to many developmental disorders, and may even be one of the causes of schizophrenia.
Sensitive Periods
There is evidence that at certain points during the early years of intense growing and learning, a child is more primed to learn certain types of information than others. Scientists have referred to these prime times as Sensitive Periods or Windows of Opportunity and parents have understandably reacted to them with great interest.
In describing how these Sensitive Periods have evolved, William T. Greenough, developmental psychologist from the University of Illinois at Urbana-Campaign,
And his colleagues have described certain types of learning as being experience-expectant. These types of learning are based upon experiences that all members of a species would expect to have. Language is a perfect example:
o In the normal course of development, almost all children are exposed to language in the first year of life.
o Because language is a normal experience, structures in the brain have been primed to expect to hear it.
o When these structures "hear" what they expect, they become more efficient at processing it.
o If language exposure does not come during the period when it is expected, the brain's future capacity to learn language will be hindered. That is why it is so important to expose your baby to lots of verbal communication during the first years of life.
It should be noted that just because something might be easier to learn during a specific time frame does not mean it is impossible to learn at a later point in time.
Clearly, people can, and do, learn second languages as adults, but it is much more difficult than learning as a child, and certain nuances of the language will never be learned as well.
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