Dear Moms & Dads,
Di bawah ini saya kirim artikel mengenai perkembangan otak
anak. Isinya sama dengan brosur Basics About Babies' Brain
Development, hanya lebih detail.
Semoga bermanfaat.
Regards,
Erik
NB. E-mail saya ke Mbak Evi Eryani mental terus, nanti
saya coba lagi, ya, Mbak! Alamat yang betul itu
[EMAIL PROTECTED]
atau
[EMAIL PROTECTED]
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FERTILE MINDS
BY J. MADELEINE NASH
TIME MAGAZINE, FEBRUARY 3, 1997 VOL. 149 NO. 5
FROM BIRTH, A BABY'S BRAIN CELLS PROLIFERATE WILDLY, MAKING
CONNECTIONS THAT MAY SHAPE A LIFETIME OF EXPERIENCE. THE
FIRST THREE YEARS ARE CRITICAL
Rat-a-tat-tat. rat-a-tat-tat. Rat-a-tat-tat. If scientists
could eavesdrop on the brain of a human embryo 10, maybe 12
weeks after conception, they would hear an astonishing
racket. Inside the womb, long before light first strikes
the retina of the eye or the earliest dreamy images flicker
through the cortex, nerve cells in the developing brain
crackle with purposeful activity. Like teenagers with
telephones, cells in one neighborhood of the brain are
calling friends in another, and these cells are calling
their friends, and they keep calling one another over and
over again, "almost," says neurobiologist Carla Shatz of
the University of California, Berkeley, "as if they were
autodialing."
But these neurons--as the long, wiry cells that carry
electrical messages through the nervous system and the
brain are called--are not transmitting signals in
scattershot fashion. That would produce a featureless
static, the sort of noise picked up by a radio tuned
between stations. On the contrary, evidence is growing that
the staccato bursts of electricity that form those
distinctive rat-a-tat-tats arise from coordinated waves of
neural activity, and that those pulsing waves, like
currents shifting sand on the ocean floor, actually change
the shape of the brain, carving mental circuits into
patterns that over time will enable the newborn infant to
perceive a father's voice, a mother's touch, a shiny mobile
twirling over the crib.
Of all the discoveries that have poured out of neuroscience
labs in recent years, the finding that the electrical
activity of brain cells changes the physical structure of
the brain is perhaps the most breathtaking. For the
rhythmic firing of neurons is no longer assumed to be a by-
product of building the brain but essential to the process,
and it begins, scientists have established, well before
birth. A brain is not a computer. Nature does not cobble it
together, then turn it on. No, the brain begins working
long before it is finished. And the same processes that
wire the brain before birth, neuroscientists are finding,
also drive the explosion of learning that occurs
immediately afterward.
At birth a baby's brain contains 100 billion neurons,
roughly as many nerve cells as there are stars in the Milky
Way. Also in place are a trillion glial cells, named after
the Greek word for glue, which form a kind of honeycomb
that protects and nourishes the neurons. But while the
brain contains virtually all the nerve cells it will ever
have, the pattern of wiring between them has yet to
stabilize. Up to this point, says Shatz, "what the brain
has done is lay out circuits that are its best guess about
what's required for vision, for language, for whatever."
And now it is up to neural activity--no longer spontaneous,
but driven by a flood of sensory experiences--to take this
rough blueprint and progressively refine it.
During the first years of life, the brain undergoes a
series of extraordinary changes. Starting shortly after
birth, a baby's brain, in a display of biological
exuberance, produces trillions more connections between
neurons than it can possibly use. Then, through a process
that resembles Darwinian competition, the brain eliminates
connections, or synapses, that are seldom or never used.
The excess synapses in a child's brain undergo a draconian
pruning, starting around the age of 10 or earlier, leaving
behind a mind whose patterns of emotion and thought are,
for better or worse, unique.
Deprived of a stimulating environment, a child's brain
suffers. Researchers at Baylor College of Medicine, for
example, have found that children who don't play much or
are rarely touched develop brains 20% to 30% smaller than
normal for their age. Laboratory animals provide another
provocative parallel. Not only do young rats reared in toy-
strewn cages exhibit more complex behavior than rats
confined to sterile, uninteresting boxes, researchers at
the University of Illinois at Urbana-Champaign have found,
but the brains of these rats contain as many as 25% more
synapses per neuron. Rich experiences, in other words,
really do produce rich brains.
The new insights into brain development are more than just
interesting science. They have profound implications for
parents and policymakers. In an age when mothers and
fathers are increasingly pressed for time--and may already
be feeling guilty about how many hours they spend away from
their children--the results coming out of the labs are
likely to increase concerns about leaving very young
children in the care of others. For the data underscore the
importance of hands-on parenting, of finding the time to
cuddle a baby, talk with a toddler and provide infants with
stimulating experiences.
The new insights have begun to infuse new passion into the
political debate over early education and day care. There
is an urgent need, say child-development experts, for
preschool programs designed to boost the brain power of
youngsters born into impoverished rural and inner-city
households. Without such programs, they warn, the current
drive to curtail welfare costs by pushing mothers with
infants and toddlers into the work force may well backfire.
"There is a time scale to brain development, and the most
important year is the first," notes Frank Newman, president
of the Education Commission of the States. By the age of
three, a child who is neglected or abused bears marks that,
if not indelible, are exceedingly difficult to erase.
But the new research offers hope as well. Scientists have
found that the brain during the first years of life is so
malleable that very young children who suffer strokes or
injuries that wipe out an entire hemisphere can still
mature into highly functional adults. Moreover, it is
becoming increasingly clear that well-designed preschool
programs can help many children overcome glaring deficits
in their home environment. With appropriate therapy, say
researchers, even serious disorders like dyslexia may be
treatable. While inherited problems may place certain
children at greater risk than others, says Dr. Harry
Chugani, a pediatric neurologist at Wayne State University
in Detroit, that is no excuse for ignoring the environment'
s power to remodel the brain. "We may not do much to change
what happens before birth, but we can change what happens
after a baby is born," he observes.
Strong evidence that activity changes the brain began
accumulating in the 1970s. But only recently have
researchers had tools powerful enough to reveal the precise
mechanisms by which those changes are brought about. Neural
activity triggers a biochemical cascade that reaches all
the way to the nucleus of cells and the coils of DNA that
encode specific genes. In fact, two of the genes affected
by neural activity in embryonic fruit flies, neurobiologist
Corey Goodman and his colleagues at Berkeley reported late
last year, are identical to those that other studies have
linked to learning and memory. How thrilling, exclaims
Goodman, how intellectually satisfying that the snippets of
DNA that embryos use to build their brains are the very
same ones that will later allow adult organisms to process
and store new information.
As researchers explore the once hidden links between brain
activity and brain structure, they are beginning to
construct a sturdy bridge over the chasm that previously
separated genes from the environment. Experts now agree
that a baby does not come into the world as a genetically
preprogrammed automaton or a blank slate at the mercy of
the environment, but arrives as something much more
interesting. For this reason the debate that engaged
countless generations of philosophers--whether nature or
nurture calls the shots--no longer interests most
scientists. They are much too busy chronicling the myriad
ways in which genes and the environment interact. "It's not
a competition," says Dr. Stanley Greenspan, a psychiatrist
at George Washington University. "It's a dance."
THE IMPORTANCE OF GENES
That dance begins at around the third week of gestation,
when a thin layer of cells in the developing embryo
performs an origami-like trick, folding inward to give rise
to a fluid-filled cylinder known as the neural tube. As
cells in the neural tube proliferate at the astonishing
rate of 250,000 a minute, the brain and spinal cord
assemble themselves in a series of tightly choreographed
steps. Nature is the dominant partner during this phase of
development, but nurture plays a vital supportive role.
Changes in the environment of the womb--whether caused by
maternal malnutrition, drug abuse or a viral infection--can
wreck the clockwork precision of the neural assembly line.
Some forms of epilepsy, mental retardation, autism and
schizophrenia appear to be the results of developmental
processes gone awry.
But what awes scientists who study the brain, what still
stuns them, is not that things occasionally go wrong in the
developing brain but that so much of the time they go right.
This is all the more remarkable, says Berkeley's Shatz, as
the central nervous system of an embryo is not a miniature
of the adult system but more like a tadpole that gives rise
to a frog. Among other things, the cells produced in the
neural tube must migrate to distant locations and
accurately lay down the connections that link one part of
the brain to another. In addition, the embryonic brain must
construct a variety of temporary structures, including the
neural tube, that will, like a tadpole's tail, eventually
disappear.
What biochemical magic underlies this incredible
metamorphosis? The instructions programmed into the genes,
of course. Scientists have recently discovered, for
instance, that a gene nicknamed "sonic hedgehog" (after the
popular video game Sonic the Hedgehog) determines the fate
of neurons in the spinal cord and the brain. Like a strong
scent carried by the wind, the protein encoded by the
hedgehog gene (so called because in its absence, fruit-fly
embryos sprout a coat of prickles) diffuses outward from
the cells that produce it, becoming fainter and fainter.
Columbia University neurobiologist Thomas Jessell has found
that it takes middling concentrations of this potent
morphing factor to produce a motor neuron and lower
concentrations to make an interneuron (a cell that relays
signals to other neurons, instead of to muscle fibers, as
motor neurons do).
Scientists are also beginning to identify some of the genes
that guide neurons in their long migrations. Consider the
problem faced by neurons destined to become part of the
cerebral cortex. Because they arise relatively late in the
development of the mammalian brain, billions of these cells
must push and shove their way through dense colonies
established by earlier migrants. "It's as if the entire
population of the East Coast decided to move en masse to
the West Coast," marvels Yale University neuroscientist Dr.
Pasko Rakic, and marched through Cleveland, Chicago and
Denver to get there.
But of all the problems the growing nervous system must
solve, the most daunting is posed by the wiring itself.
After birth, when the number of connections explodes, each
of the brain's billions of neurons will forge links to
thousands of others. First they must spin out a web of
wirelike fibers known as axons (which transmit signals) and
dendrites (which receive them). The objective is to form a
synapse, the gap-like structure over which the axon of one
neuron beams a signal to the dendrites of another. Before
this can happen, axons and dendrites must almost touch. And
while the short, bushy dendrites don't have to travel very
far, axons--the heavy-duty cables of the nervous system--
must traverse distances that are the microscopic equivalent
of miles.
What guides an axon on its incredible voyage is a "growth
cone," a creepy, crawly sprout that looks something like an
amoeba. Scientists have known about growth cones since the
turn of the century. What they didn't know until recently
was that growth cones come equipped with the molecular
equivalent of sonar and radar. Just as instruments in a
submarine or airplane scan the environment for signals, so
molecules arrayed on the surface of growth cones search
their surroundings for the presence of certain proteins.
Some of these proteins, it turns out, are attractants that
pull the growth cones toward them, while others are
repellents that push them away.
THE FIRST STIRRINGS
Up to this point, genes have controlled the unfolding of
the brain. As soon as axons make their first connections,
however, the nerves begin to fire, and what they do starts
to matter more and more. In essence, say scientists, the
developing nervous system has strung the equivalent of
telephone trunk lines between the right neighborhoods in
the right cities. Now it has to sort out which wires belong
to which house, a problem that cannot be solved by genes
alone for reasons that boil down to simple arithmetic.
Eventually, Berkeley's Goodman estimates, a human brain
must forge quadrillions of connections. But there are only
100,000 genes in human DNA. Even though half these genes--
some 50,000--appear to be dedicated to constructing and
maintaining the nervous system, he observes, that's not
enough to specify more than a tiny fraction of the
connections required by a fully functioning brain.
In adult mammals, for example, the axons that connect the
brain's visual system arrange themselves in striking layers
and columns that reflect the division between the left eye
and the right. But these axons start out as scrambled as a
bowl of spaghetti, according to Michael Stryker, chairman
of the physiology department at the University of
California at San Francisco. What sorts out the mess,
scientists have established, is neural activity. In a
series of experiments viewed as classics by scientists in
the field, Berkeley's Shatz chemically blocked neural
activity in embryonic cats. The result? The axons that
connect neurons in the retina of the eye to the brain never
formed the left eye-right eye geometry needed to support
vision.
But no recent finding has intrigued researchers more than
the results reported in October by Corey Goodman and his
Berkeley colleagues. In studying a deceptively simple
problem--how axons from motor neurons in the fly's central
nerve cord establish connections with muscle cells in its
limbs--the Berkeley researchers made an unexpected
discovery. They knew there was a gene that keeps bundles of
axons together as they race toward their muscle-cell
targets. What they discovered was that the electrical
activity produced by neurons inhibited this gene,
dramatically increasing the number of connections the axons
made. Even more intriguing, the signals amplified the
activity of a second gene--a gene called CREB.
The discovery of the CREB amplifier, more than any other,
links the developmental processes that occur before birth
to those that continue long after. For the twin processes
of memory and learning in adult animals, Columbia
University neurophysiologist Eric Kandel has shown, rely on
the CREB molecule. When Kandel blocked the activity of CREB
in giant snails, their brains changed in ways that
suggested that they could still learn but could remember
what they learned for only a short period of time. Without
CREB, it seems, snails--and by extension, more developed
animals like humans--can form no long-term memories. And
without long-term memories, it is hard to imagine that
infant brains could ever master more than rudimentary
skills. "Nurture is important," says Kandel. "But nurture
works through nature."
EXPERIENCE KICKS IN
When a baby is born, it can see and hear and smell and
respond to touch, but only dimly. The brain stem, a
primitive region that controls vital functions like
heartbeat and breathing, has completed its wiring.
Elsewhere the connections between neurons are wispy and
weak. But over the first few months of life, the brain's
higher centers explode with new synapses. And as dendrites
and axons swell with buds and branches like trees in spring,
metabolism soars. By the age of two, a child's brain
contains twice as many synapses and consumes twice as much
energy as the brain of a normal adult.
University of Chicago pediatric neurologist Dr. Peter
Huttenlocher has chronicled this extraordinary epoch in
brain development by autopsying the brains of infants and
young children who have died unexpectedly. The number of
synapses in one layer of the visual cortex, Huttenlocher
reports, rises from around 2,500 per neuron at birth to as
many as 18,000 about six months later. Other regions of the
cortex score similarly spectacular increases but on
slightly different schedules. And while these microscopic
connections between nerve fibers continue to form
throughout life, they reach their highest average densities
(15,000 synapses per neuron) at around the age of two and
remain at that level until the age of 10 or 11.
This profusion of connections lends the growing brain
exceptional flexibility and resilience. Consider the case
of 13-year-old Brandi Binder, who developed such severe
epilepsy that surgeons at UCLA had to remove the entire
right side of her cortex when she was six. Binder lost
virtually all the control she had established over muscles
on the left side of her body, the side controlled by the
right side of the brain. Yet today, after years of therapy
ranging from leg lifts to math and music drills, Binder is
an A student at the Holmes Middle School in Colorado
Springs, Colorado. She loves music, math and art--skills
usually associated with the right half of the brain. And
while Binder's recuperation is not 100%--for example, she
has never regained the use of her left arm--it comes close.
Says UCLA pediatric neurologist Dr. Donald Shields: "If
there's a way to compensate, the developing brain will find
it."
What wires a child's brain, say neuroscientists--or rewires
it after physical trauma--is repeated experience. Each time
a baby tries to touch a tantalizing object or gazes
intently at a face or listens to a lullaby, tiny bursts of
electricity shoot through the brain, knitting neurons into
circuits as well defined as those etched onto silicon chips.
The results are those behavioral mileposts that never cease
to delight and awe parents. Around the age of two months,
for example, the motor-control centers of the brain develop
to the point that infants can suddenly reach out and grab a
nearby object. Around the age of four months, the cortex
begins to refine the connections needed for depth
perception and binocular vision. And around the age of 12
months, the speech centers of the brain are poised to
produce what is perhaps the most magical moment of
childhood: the first word that marks the flowering of
language.
When the brain does not receive the right information--or
shuts it out--the result can be devastating. Some children
who display early signs of autism, for example, retreat
from the world because they are hypersensitive to sensory
stimulation, others because their senses are underactive
and provide them with too little information. To be
effective, then, says George Washington University's
Greenspan, treatment must target the underlying condition,
protecting some children from disorienting noises and
lights, providing others with attention-grabbing
stimulation. But when parents and therapists collaborate in
an intensive effort to reach these abnormal brains, writes
Greenspan in a new book, The Growth of the Mind (Addison-
Wesley, 1997), three-year-olds who begin the descent into
the autistic's limited universe can sometimes be snatched
back.
Indeed, parents are the brain's first and most important
teachers. Among other things, they appear to help babies
learn by adopting the rhythmic, high-pitched speaking style
known as Parentese. When speaking to babies, Stanford
University psychologist Anne Fernald has found, mothers and
fathers from many cultures change their speech patterns in
the same peculiar ways. "They put their faces very close to
the child," she reports. "They use shorter utterances, and
they speak in an unusually melodious fashion." The heart
rate of infants increases while listening to Parentese,
even Parentese delivered in a foreign language. Moreover,
Fernald says, Parentese appears to hasten the process of
connecting words to the objects they denote. Twelve-month-
olds, directed to "look at the ball" in Parentese, direct
their eyes to the correct picture more frequently than when
the instruction is delivered in normal English.
In some ways the exaggerated, vowel-rich sounds of
Parentese appear to resemble the choice morsels fed to
hatchlings by adult birds. The University of Washington's
Patricia Kuhl and her colleagues have conditioned dozens of
newborns to turn their heads when they detect the ee sound
emitted by American parents, vs. the eu favored by doting
Swedes. Very young babies, says Kuhl, invariably perceive
slight variations in pronunciation as totally different
sounds. But by the age of six months, American babies no
longer react when they hear variants of ee, and Swedish
babies have become impervious to differences in eu. "It's
as though their brains have formed little magnets," says
Kuhl, "and all the sounds in the vicinity are swept in."
TUNED TO DANGER
Even more fundamental, says Dr. Bruce Perry of Baylor
College of Medicine in Houston, is the role parents play in
setting up the neural circuitry that helps children
regulate their responses to stress. Children who are
physically abused early in life, he observes, develop
brains that are exquisitely tuned to danger. At the
slightest threat, their hearts race, their stress hormones
surge and their brains anxiously track the nonverbal cues
that might signal the next attack. Because the brain
develops in sequence, with more primitive structures
stabilizing their connections first, early abuse is
particularly damaging. Says Perry: "Experience is the chief
architect of the brain." And because these early
experiences of stress form a kind of template around which
later brain development is organized, the changes they
create are all the more pervasive.
Emotional deprivation early in life has a similar effect.
For six years University of Washington psychologist
Geraldine Dawson and her colleagues have monitored the
brain-wave patterns of children born to mothers who were
diagnosed as suffering from depression. As infants, these
children showed markedly reduced activity in the left
frontal lobe, an area of the brain that serves as a center
for joy and other lighthearted emotions. Even more telling,
the patterns of brain activity displayed by these children
closely tracked the ups and downs of their mother's
depression. At the age of three, children whose mothers
were more severely depressed or whose depression lasted
longer continued to show abnormally low readings.
Strikingly, not all the children born to depressed mothers
develop these aberrant brain-wave patterns, Dawson has
found. What accounts for the difference appears to be the
emotional tone of the exchanges between mother and child.
By scrutinizing hours of videotape that show depressed
mothers interacting with their babies, Dawson has attempted
to identify the links between maternal behavior and
children's brains. She found that mothers who were
disengaged, irritable or impatient had babies with sad
brains. But depressed mothers who managed to rise above
their melancholy, lavishing their babies with attention and
indulging in playful games, had children with brain
activity of a considerably more cheerful cast.
When is it too late to repair the damage wrought by
physical and emotional abuse or neglect? For a time, at
least, a child's brain is extremely forgiving. If a mother
snaps out of her depression before her child is a year old,
Dawson has found, brain activity in the left frontal lobe
quickly picks up. However, the ability to rebound declines
markedly as a child grows older. Many scientists believe
that in the first few years of childhood there are a number
of critical or sensitive periods, or "windows," when the
brain demands certain types of input in order to create or
stabilize certain long-lasting structures.
For example, children who are born with a cataract will
become permanently blind in that eye if the clouded lens is
not promptly removed. Why? The brain's visual centers
require sensory stimulus--in this case the stimulus
provided by light hitting the retina of the eye--to
maintain their still tentative connections. More
controversially, many linguists believe that language
skills unfold according to a strict, biologically defined
timetable. Children, in their view, resemble certain
species of birds that cannot master their song unless they
hear it sung at an early age. In zebra finches the window
for acquiring the appropriate song opens 25 to 30 days
after hatching and shuts some 50 days later.
WINDOWS OF OPPORTUNITY
With a few exceptions, the windows of opportunity in the
human brain do not close quite so abruptly. There appears
to be a series of windows for developing language. The
window for acquiring syntax may close as early as five or
six years of age, while the window for adding new words may
never close. The ability to learn a second language is
highest between birth and the age of six, then undergoes a
steady and inexorable decline. Many adults still manage to
learn new languages, but usually only after great struggle.
The brain's greatest growth spurt, neuroscientists have now
confirmed, draws to a close around the age of 10, when the
balance between synapse creation and atrophy abruptly
shifts. Over the next several years, the brain will
ruthlessly destroy its weakest synapses, preserving only
those that have been magically transformed by experience.
This magic, once again, seems to be encoded in the genes.
The ephemeral bursts of electricity that travel through the
brain, creating everything from visual images and
pleasurable sensations to dark dreams and wild thoughts,
ensure the survival of synapses by stimulating genes that
promote the release of powerful growth factors and
suppressing genes that encode for synapse-destroying
enzymes.
By the end of adolescence, around the age of 18, the brain
has declined in plasticity but increased in power. Talents
and latent tendencies that have been nurtured are ready to
blossom. The experiences that drive neural activity, says
Yale's Rakic, are like a sculptor's chisel or a dressmaker'
s shears, conjuring up form from a lump of stone or a
length of cloth. The presence of extra material expands the
range of possibilities, but cutting away the extraneous is
what makes art. "It is the overproduction of synaptic
connections followed by their loss that leads to patterns
in the brain," says neuroscientist William Greenough of the
University of Illinois at Urbana-Champaign. Potential for
greatness may be encoded in the genes, but whether that
potential is realized as a gift for mathematics, say, or a
brilliant criminal mind depends on patterns etched by
experience in those critical early years.
Psychiatrists and educators have long recognized the value
of early experience. But their observations have until now
been largely anecdotal. What's so exciting, says Matthew
Melmed, executive director of Zero to Three, a nonprofit
organization devoted to highlighting the importance of the
first three years of life, is that modern neuroscience is
providing the hard, quantifiable evidence that was missing
earlier. "Because you can see the results under a
microscope or in a PET scan," he observes, "it's become
that much more convincing."
What lessons can be drawn from the new findings? Among
other things, it is clear that foreign languages should be
taught in elementary school, if not before. That remedial
education may be more effective at the age of three or four
than at nine or 10. That good, affordable day care is not a
luxury or a fringe benefit for welfare mothers and working
parents but essential brain food for the next generation.
For while new synapses continue to form throughout life,
and even adults continually refurbish their minds through
reading and learning, never again will the brain be able to
master new skills so readily or rebound from setbacks so
easily.
Rat-a-tat-tat. Rat-a-tat-tat. Rat-a-tat-tat. Just last week,
in the U.S. alone, some 77,000 newborns began the
miraculous process of wiring their brains for a lifetime of
learning. If parents and policymakers don't pay attention
to the conditions under which this delicate process takes
place, we will all suffer the consequences--starting around
the year 2010.
-----8<-----------------------------------------------------
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