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The early-development timetable is
set in part by the sequence of myelination. A thin spiral of sheathing around axons is
present at birth, but the full insulation is then laid down around the body, and the
brain, in sequence. Overall, in the body that starts at the top and works down. That's why
you can make sounds before you learn to walk - the long axons transmitting messages to
your toes and calf-muscles take longer to coat than the axons to your tongue and larynx.
In the brain, full myelination starts at the back and
moves to the front. That's why you learn to see before you learn to talk and reason: your
optical nerve-center is at the back of the brain, your speech-center is further forward,
and your reasoning-center is at the front. The process is completed in the center of the
brain - what some scientists call the "association cortex": the part you use to
sort incoming information and blend it with data already in your storage files.
When axons are fully covered by their myelin
sheath, they can transmit messages around the body up to 12 times faster than they could
before. In fact, the speed of transmission around the body can vary from one mile an hour
to 150 miles.8
Just as the fetus grows in spurts, so does the new
infant brain. And the timing of those bursts can be vital.
Close one eye of a two-year-old for as little as a
week, for instance, and you will almost certainly damage its ability to see. This is
because the growing brain is laying down its main visual pathways from the eyes to the
vision-center at the rear of the brain. The two separate pathways are competing for
dominance. Shut one eye for any length of time and the other one will lay down the
dominant pathway. Close one eye for a week when you're 20 and it won't matter, because by
then your basic pathways have been laid down.
Says Stanford University human biology professor Robert
Ornstein: "The critical period during which the two eyes establish their zones of
dominance seems to be about the first six years in humans, six months in monkeys, and
perhaps three months in cats. It is a very sensitive period. If one eye of a kitten is
kept closed for only one day, it will have poor vision in that eye as an adult.
"There is a very important practical lesson from
this basic work on the visual brain. Do not ever keep one eye of a human infant
closed for an extended period of time. Keeping both eyes closed is better; after all,
infants sleep a good bit of the time."9
Contents Page Preface
Introduction
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