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partially constructed at birth, won’t be fully assembled for years. The
biggest construction programs aren’t finished until you are in your early
20s, with fine-tuning well into your 40s. When babies are born, their brains
have about the same number of connections as adults have. That doesn’t
last long. By the time children are 3 years old, the connections in specific
regions of their brains have doubled or even tripled. That doesn’t last long,
either. The brain soon takes thousands of tiny pruning shears and trims back
a lot of this hard work. By the time children are 8 or so, they’re back to
their adult numbers. And if kids never went through puberty, that would be
the end of the story. In fact, it is only the middle of the story. At puberty, the
whole process begins again, but with different regions in the brain. Once
again, you see frenetic neural outgrowth and furious pruning back. It isn’t
until parents begin thinking about college financial aid that children’s brains
begin to settle into their adult forms. From a connectivity point of view,
there is a great deal of activity in the terrible twos and then, during the
terrible teens, a great deal more.
Because this happens to every person at about the same time, it might
seem like cellular soldiers are obeying growth commands in lockstep
formation. But nothing approaching military precision is observed in the
messy world of brain development. And it is at this imprecise point that
brain development meets Brain Rule: Every brain is wired differently. Even
a cursory inspection of the data reveals remarkable variation in growth
patterns from one person to the next. Whether examining toddlers or
teenagers, different regions in different children develop at different rates.
There is a remarkable degree of diversity in the specific areas that grow and
prune, and with what enthusiasm they do so.
I’m reminded of this whenever I see the class pictures from my wife’s
journey through the American school system. My wife went to school with
virtually the same people for her entire K–12 experience (and actually
remained friends with most of them). Comparing the kids to each other
back then, I always shake my head in disbelief. In the first-grade picture,
the kids are all about the same age, but they don’t look it. Some kids are
short. Some are tall. Some look like mature little athletes. Some look as if
they just got out of diapers. The girls almost always appear older than the
boys. It’s even worse in the junior-high pictures. Some of the boys look as if
they haven’t developed much since third grade. Others are clearly
beginning to sprout whiskers. Some of the girls, flat-chested, look a lot like
boys. Others look developed enough to make babies. And if we could look
inside these kids’ heads, we would see that their brains are
just as unevenly
developed
as their bodies. Let’s find out why.
The Jennifer Aniston neuron
Some of the neural connections you’re born with have preset functions:
they control basic housekeeping functions like breathing, heartbeat, your
ability to know where your foot is even if you can’t see it, and so on.
Researchers call this “experience independent” wiring. The brain also holds
off connecting neurons, waiting for external experience to direct it.
“Experience expectant” wiring is related to areas such as visual acuity and
perhaps language acquisition. And, finally, we have “experience dependent”
wiring. It may best be explained by the following scene, which would be
right at home in a grade B movie.
A man is lying on a surgical table, electrodes implanted in his brain to
create a kind of GPS pinpointing electrical activity in the brain. The man
needs to have some of his neural tissue removed—resected, in surgical
parlance—because of life-threatening epilepsy, and the depth electrodes
will help surgeons determine where the seizures are starting. The man is
conscious. Suddenly, a researcher whips out a photo of Jennifer Aniston and
shows it to the patient. A neuron in the man’s head fires. The researcher lets
out a war whoop.
This experiment really happened. The neuron in question responded to
seven photographs of actress Jennifer Aniston, while it practically ignored
the 80 other images of everything else, including famous and nonfamous
people. Lead scientist Quian Quiroga said, “The first time we saw a neuron
firing to seven different pictures of Jennifer Aniston—and nothing else—
we literally jumped out of our chairs.” There is a neuron lurking in your
head that is stimulated only when Jennifer Aniston is in the room.
A Jennifer Aniston
neuron
? How could this be? Surely nothing in our
evolutionary history suggests that Jennifer Aniston is a permanent denizen
of our brain wiring. (Aniston wasn’t even born until 1969, and there are
regions in our brain whose designs are millions of years old). To make
matters worse, the researchers also found a Halle Berry–specific neuron, a
cell in a patient’s brain that wouldn’t respond to pictures of Aniston or
anything else. Just Berry. A patient also had a neuron specific to Bill
Clinton. It no doubt was helpful to have a sense of humor while doing this
kind of brain research.
Welcome to the world of experience-dependent brain wiring, where a
great deal of the brain is hardwired
not
to be hardwired. Like a beautiful,
rigorously trained ballerina, we are hardwired to be flexible. We can
immediately divide the world’s brains into those who know of Jennifer
Aniston or Halle Berry or Bill Clinton and those who don’t. The brains of
those who do are wired differently from those who don’t. This seemingly
ridiculous observation underlies a much larger concept. Our brains are so
sensitive to external inputs that their physical wiring depends upon the
culture in which they find themselves.
Even identical twins do not have identical brain wiring. Consider this
thought experiment: Suppose two adult male twins rent the Halle Berry
movie
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