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Understanding brain development and early learning
New research better informs the ‘nature vs. nurture’ issue
By Bruce Murray
FACSNET Managing Editor
Nature vs. nurture
Advances in brain science understanding are reshaping the “nature
vs. nurture” debate closer to a “nature AND nurture” consensus.
“Human development is a blend of nature and nurture, genes and environment,”
said Dr.
Lise Eliot of the Chicago School of Medicine. “There is no cognitive,
perceptual, emotional, or motor skill that is not influenced by both of
these factors.”
Eliot spoke at the March 2 FACS conference, “Brain Science, Children
and the Future of Learning” in Cincinnati.
“We can’t do much about the genes we are born with -- or the
genes we transfer to our children, but we do influence environment. We
know the effect of environment on early experience is to actually change
the structure of individual brain cells and neurons,” she said.
“We know from 50 years of research in neuroscience that an infant’s
experience can have permanent effects on the wiring of the brain.”
[back to top]
Critical periods and plasticity
The brain’s ability to change from experience is known as plasticity.
The human brain is especially plastic early in life, which is why the
“nurture” part of the equation is so important.
Throughout life the brain continues to be plastic -- this is the mechanism
of learning – but plasticity declines in adulthood, Eliot said.
“We can continue to learn throughout life, but it is more difficult
to acquire a whole new skill. You can become better and better at what
you already do, but it is difficult to learn a new language as an adult,
or develop a new tennis stroke,” she said.
As a child’s brain develops, it goes through several “critical
periods,” a developmental phase in which the brain requires certain
environmental input or it will not develop normally.
“This is a term some people outside of biology are not comfortable
with. Some people prefer ‘sensitive periods,’ which makes the
issue more open-ended,” Eliot said. “At a circuitry level, we
know there truly are critical periods – stages of development –
in which the brain needs certain types of experience, or the circuits
don’t get put together properly.”
Eliot cited the landmark work of David
Hubel and Torsten Wiesel, who earned the 1981 Nobel Prize for Physiology
for their research on the effects of early visual experience on the brain.
Hubel and Wiesel studied the visual cortex of the brain to better understand
how the brain processes vision.
They conducted experiments on kittens, which involved suturing shut one
eye and comparing the kittens with others of the same litter left alone.
The sutured eye, when opened after two months, would remain functionally
blind for life. The blindness was not due to a defective eye, but because
the other eye had established most of the pathways in the brain to compensate.
“We know there a critical period: If child or animal, from birth,
does not have a normal visual experience with both eyes, he or she is
at risk of having permanent visual deficits,” Eliot said.
The experience of sight changes the fundamental architecture of the brain
because of the brain’s plasticity, Eliot said. The effect is similar
in all mammals.
Not only sight, but hearing, touch and the other senses and functions
go through different critical periods.
“What we are learning through neuroscience is that each different
part of the brain carries on a different cognitive function, whether it’s
perception, movement, emotion, language or memory. Each has a different
developmental time table, and along with that different critical periods,”
she said. [Back to top]
Neurons and synapses
The brain is made up of neurons,
or brain cells, which connect to one another through synapses. Synapses
are physical gaps between neurons, like the gaps between the electrodes
in spark plugs, through which nerve impulses travel.
Neurotransmitters carry signals between brain cells. They diffuse across
the synapse and trigger the electrical activity that transmits information
through the brain. There are many different types of neurotransmitters,
such as dopamine, serotonin and glutamate.
“If two neurons are synaptically coupled, and they are both electrically
active at the same time, the connection between them gets stronger. Whereas
if two neurons are synaptically coupled, but they are not electrically
active together, then those synapses are pruned or lost,” Eliot said.
This is how experience literally wires the brain.
The catch phrase in neuroscience is, “Cells that fire together, wire
together”; or otherwise stated, “Use it or lose it.”
Infants go through a critical period of “synaptical exuberance”
or a “synaptical surge” immediately after birth and during the
first year of life. After that begins the “pruning”
process that ends some time at the beginning or end of puberty. An adult
is left with fewer synapses than an infant.
“The first surge in synapses early in life has the effect of dramatically
improving vision. Within the visual cortex, the pruning process largely
complete by age 8,” Eliot said. “If a vision problem, such as
eye misalignment or congenital cataracts, is not corrected early, a child
can suffer permanent visual impairment.”
Synaptic growth corresponds with the growth of “dendrites,”
or neuron branches that receive and process signals from other brain cells.
The greatest growth of dendrites and synapses occurs during the first
five years of life. By age 5, a child’s brain weight is almost the
same as an adult, and all of the basic neurons are in place, Eliot said.
The human brain contains about 200 billion neurons. The number of neurons
doesn’t change after birth; the growth occurs in the dendrites, which
appear very much like branches on a tree. [Back
to top]
Gray matter
An important ingredient in the brain’s makeup is myelin
– a white, fatty material that is wrapped around brain axons –
parts of the neuron that transmit messages to other neurons, muscles or
glands.
Myelin acts like a plastic insulator in an electrical wiring system, Eliot
said. It prevents the leakage of the electrical current from the axon.
Myelin is crucial for the brain’s function. It prevents “cross-talk’
between adjacent axons. If there were no myelin, electric current would
leak throughout the brain, and information would become scrambled.
Myelin allows the brain processes information at the speed at which it
does. Children’s brains are very slow compared with adult brains
because of the relative state of myelination.
Myelination begins at birth -- first in the spinal cord and brain stem,
and then into the brain. Myelination of the cerebral cortex is very gradual,
and it continues in the frontal lobe into the 20s.
Myelin inhibits plasticity. Once an axon is myelinated, it has much less
ability to branch out and connect with other neurons.
“Myelination is prolonged to promote plasticity,” Eliot said.
“This is crucial to understanding what children are ready for at
different ages.” [Back to top]
Babies in motion
The vestibular system is located in the inner ear, which senses movement
of the head and allows control of balance, posture and reflexes. The bouncing,
rocking and sometimes even head-banging young children engage in is connected
to activity of the vestibular system.
The critical period for maturation of the vestibular system is from 6-12
months.
“Because the vestibular system is so overcharged at this age, children
really like to activate it with the most potent form of stimulation,”
Eliot said. “All of the motion babies experience -- whether it’s
in the womb or being carried around -- the reason it is so comforting
to children is because the vestibular system is so advanced early on,
way before hearing and vision.”
Pediatricians commonly test the integrity of the vestibular system and
brain stem by turning an infant’s head and watching to see if the
baby can keep its eyes focused forward. [Back
to top]
Touch
The sensory system matures early, and its development begins before birth.
The brain’s sensory cortex maps the entire body so it can determine
which part of the body is receiving a sensation.
Environmental stimulation of the sensory system is critical to its development.
Experiments show that if rodents’ whiskers are clipped before day
five, they never develop that part of the cortex that receives input from
the face. But if the whiskers are cut off after day 12 or beyond, they
simply grow back, and the rodent still has touch sensation from the face.
“Touch is a very fundamental form of bonding and communication –
kids need it; adults need it,” Eliot said. [Back
to top]
Motor skills
Motor development also happens early. It begins with control of the neck,
the face and smiling. In five to six months, children can control their
grasp – picking things up and holding them.
Toward the end of the first year, they gain control of their legs –
from crawling, independent standing and walking.
In the brain, motor development takes place in the posterior of the frontal
lobe.
“There is a predictable time table of motor skill maturation, which
coincides with myelination and synaptic density increases in the motor
cortex,” Eliot said.
Eliot said evidence has shown that baby walkers – devices that prop
babies up and allow them to move across a floor – interfere with
motor development by delaying the onset of independent walking and sitting.
“The hardest thing about walking is being able to balance on two
feet; so if you’re in a walker that’s holding up your body,
you don’t have to develop the muscle strength and coordination,”
she said. “Walkers also solve the main motivation for walking –
to get across the room to get that toy. If you have a walker, you don’t
have that motivation. Not only can you get across the room, but you don’t
have to worry about balance.” [Back to
top]
Emotion and memory
Emotion and memory are controlled by the limbic
system, a network of interconnected structures located deep within
the brain, just above the brain stem. Limbic literally means “border”
-- in this case it is the border between the cerebral cortex and the lower
brain.
“This is where most of what happens in terms of a child’s social
and emotional experience and development is happening,” Eliot said.
Between birth and age 3 is the critical period for development of the
limbic system.
The orphans during the Ceausescu
era in Romania inadvertently provided a good scientific case study
of the effects of neglect on young children.
In notoriously poorly managed orphanages, children were often left in
cribs with little attention, stimulation or even basic hygienic care.
After the Ceausescu regime fell, many of the orphans were adopted in the
United States. The children showed a variety of common behavioral and
developmental problems as they grew up in adopted homes.
“Development of the limbic system depends on experience. Just like
a child has to see in order to develop the proper wiring of the visual
part of the brain, so do they have to have normal social interaction to
develop a normal limbic system,” Eliot said.
“The most important form of stimulation is social. It’s not
the toys and gadgets that you buy, but the quality of interaction between
the parent and child.”
Memory is processed in a portion of the brain called the hippocampus.
Eliot said the hippocampus is the like the brain’s “tape recorder”
– the first part of the brain that processes memory before it goes
to the cortex for long-term storage.
The hippocampus is not well developed in infants, which is why most people
cannot remember early life events. “The hippocampus is just not mature
enough to store those conscious memories,” she said.
Understanding of the development of the brain’s memory system has
shed some light on old psychological debates, such as the Freudian notion
that all of a person’s early experiences are stored, but repressed.
“We now know that infantile amnesia is a true amnesia, and the memories
were never stored in the first place. It’s not that they are there
and can’t be retrieved,” Eliot said. “Babies learn implicit
memories, not explicit, conscious memories. They learn through conditioning
and association.” [Back to top]
Behavior and inhibition
The brain’s frontal lobe is important in regulating and inhibiting
movement, behaviors and thoughts.
“People who suffer strokes in the frontal lobe often have profound
changes in personality. A person who had been perfectly polite can suddenly
become rude,” Eliot said.
The frontal lobe is also key to focusing and attention.
“There is a growing consensus that ADHD (attention deficit hyperactivity
disorder) is due to lack of inhibition,” Eliot said. “Hyperactivity
comes when children can’t inhibit their movements. The lack of attention
comes because they can’t filter out all of the competing thoughts
and stimuli.”
Lack of development in the frontal lobe explains why young children get
more active when they are tired, because newly developed functions, such
as inhibition, are harder to do early in life.
“Hopefully, this understanding of inhibitory control will help our
interaction with kids,” Eliot said. [Back
to top]
Stress
How a mother’s stress can affect an unborn child is subject of debate.
Eliot said stress hormones can cross the placenta in small measure. These
hormones can cause a cleft
palate or affect the limbic system.
But usually only very severe stress can have a measurable impact.
“Generally the kind of stress that is most detrimental is not a busy
job or home life, but the chronic stress of poverty -- not knowing where
your next meal coming from,” she said. “There is no evidence
that day-to-day stress is detrimental.”
Defining stress and determining cause and effect is a problem in measuring
the impact of stress.
“We don’t know, for example, if a woman is clinically depressed
during her pregnancy, and her child later shows these tendencies, is it
because of prenatal exposure or the genes the mother passed on?”
[back to top]
Set in our ways?
Plasticity declines in adulthood, but it does continue. This is how adults
continue to learn. But learning certain basic skills, such as language
and music, becomes much more difficult with age. For example, Eliot said
developing perfect pitch is difficult if a child hasn’t begun music
training before age 9.
“This is the conflict in parenting: You don’t want to pressure-cook
your kids, but you do want to expose them to things early,” she said.
Adults attempting to learn a new skill or language may be discouraged,
but it is not impossible. “If you are highly motivated, you can learn
a second language. You may not speak it with perfect pronunciation, but
you can achieve a degree of fluency,” Eliot said.
Learning in adulthood is limited by “neural competition” among
all of the well-developed systems. Therefore, in order to effectively
learn a second language, leave the country.
“In order to re-sculpture your brain pathways to take on a new skill
in adulthood, you’ve really got to reduce the opportunity to continue
practicing the things you have been doing for the past 20 to 30 years,”
she said. [Back to top]
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Posted March 10, 2003
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