Chapter 6 - Right from the start

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Right from the start	223

  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

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