The brain of a deaf person watching videos of people
demonstrating sign language also reorganizes. In what Japanese researchers
this year called "striking evidence of neural plasticity," brain
scans in such a man showed that a part of the brain normally used for
hearing and understanding spoken language was recruited to help him decipher
All this evidence "has led us to believe that how you use your brain
physically helps determine how the brain is organized," said Guy
McKhann, director of the Mind/Brain Institute at Johns Hopkins University.
"The brain is much more plastic than we thought."
The new findings are tantalizing. If the brain can figure out ways of
reorganizing itself to recover lost or damaged functions, this offers
tremendous hope for better treatments of stroke, epilepsy, Alzheimer's
disease, Parkinson's disease and spinal cord injury. Yet medicine's
knowledge of how to spur this kind of brain reorganization is still
Why one stroke patient, for example, regains use of the left hand, while
another doesn't, remains largely a mystery, McKhann said. If scientists knew
the answer, they'd be closer to solving the next big challenge: how to
promote such recovery in stroke patients and others.
Once scientists understand how the brain accomplishes its plasticity,
they may be able to develop more effective therapies – such as drugs or
physical exercises that mimic or enhance the natural process.
Plasticity's implications touch "virtually every major category of
human illness, from mental retardation at one end of the age spectrum to
senility and Alzheimer's at the other," Black said.
Recovery of function – how the brain repairs or limits the damage from
a major illness or injury – has become the overriding challenge for
neuroscience, said Edward Taub, a behavioral brain scientist at the
University of Alabama at Birmingham. "It's almost like the Holy
The drastic surgery that cured Maranda Francisco's epilepsy has been
performed on about 80 children at Johns Hopkins over the past 15 years.
In carefully selected young patients, neurologist Freeman said, the
hemispherectomy can improve lives by ridding them of disabling seizures
without causing unacceptable brain damage. "It's better to have no
hemisphere than to have a dysfunctional hemisphere," he said.
"When you take out the bad one, the good one finds new pathways."
The remarkable plasticity that allows the operation to work has
"vast implications" for eventual treatment of other brain
illnesses, Freeman said.
"All of these things are sort of works in progress."
'A Real Trickster'
When a leg is amputated or disabled by a stroke, suddenly the brain no
longer can receive signals from it. That part of the cortex goes silent,
"What does the brain do when part of itself is silent?" said
Lucien M. Levy, a neuroradiologist and chief of the spectroscopy unit at
NINDS. "It basically reorganizes – just like any big organization.
Like the NIH!"
In the reorganization, the silent parts are recruited for another
purpose. Doctors used to think of the nervous system as fixed, so that
"if you lose brain cells, that's it," Levy said. "But the
more we look at it, the more we learn that the brain can adapt and
That's where the analogy between the brain and the computer breaks down.
A hard-wired computer adapts only if you change its software. "But in
the human brain, the hard wiring itself changes," Levy said.
The brain, in other words, is a computer that can learn.
A computer can be programmed to play chess by recognizing the patterns of
play. But its expertise is limited to bits of data governed by the rules of
Humans, because they have brains instead of a microchip, look at a
chessboard differently, McKhann said. They see a king, for example, and
think of a castle in Scotland – which in turn reminds them of a music
festival in Edinburgh.
"It's that kind of activity that our brains do all the time,"
McKhann said, "and that a computer can't do." Plus, each brain is
unique. "You might think of the Edinburgh music festival, and I might
think of a local pub."
The brain's built-in plasticity helps it respond quickly to new demands
– from memorizing a phone number to practicing the violin – or setbacks
like the loss of a limb.
"You cannot have a brain without [plasticity]," said Alvaro
Pascual-Leone, a neuroscientist at Harvard Medical School and director of
behavioral neurology at Beth Israel Deaconess Medical Center in Boston.
"That's why the analogy with the computer is not a good one."
The brain's ability to adapt has limits, of course. Normal plasticity
cannot fully compensate for a severe "insult" to the brain, such
as a damaging head injury, tumor or stroke.
Plasticity in the brain can also go awry, overreacting to a stimulus or
reacting in a chaotic way. It leads amputees to feel "phantom
pain" in a missing leg, as impulses from nerves in the remaining stump
are misinterpreted as coming from the amputated limb. Writer's cramp and
musician's cramp are forms of a repetitive stress injury that results from
brain plasticity – when an often-repeated motion is learned so well that
the fingers cannot unlearn it to perform other routine tasks.
And no one would want the brain to become totally plastic. Then, instead
of relying of memory and habit, it would be constantly reorganizing even in
response to the most trivial experiences – with chaotic results.
When the brain tries to make up for an injury by shifting the operations
of the damaged area to an unaffected area, the compensation sometimes comes
with a cost – a kind of neurological traffic jam.
NINDS's Grafman recalls an adolescent boy who had suffered a severe brain
injury when he was very young. The injury destroyed much of the boy's right
parietal lobe, including the part of the brain involved in spatial
processing, which helps people locate themselves in the world and keep a
sense of direction. By the time Grafman saw him, the boy had made
substantial progress in navigating his way through daily life, as the left
side of his brain apparently had taken over part of the right's
But the recovery had a price. The boy now had serious difficulty using
numbers or making simple calculations. Apparently the spatial location
function had crowded out his mathematical ability from the left side of the
brain. There wasn't room for both.
Another form of brain plasticity involves what Grafman calls a
"compensatory masquerade." For example, people use different
strategies, without even being aware of it, when driving from home to work.
One person may rely mainly on street signs, following their explicit verbal
cues. Another may rely more on a general sense of direction and spatial
location, without reading specific signs. If a brain injury harms one
ability – the sign-reading capacity or the more general sense of spatial
location – the brain may shift to the other approach, enabling the person
to navigate the route. But the brain's ability to shift strategies may
simply mask the damage, misleading a patient or family to underestimate the
"The brain is a real trickster," Grafman said.
One of Pascual-Leone's patients was a man who loved to paint pictures.
The man suffered a stroke in his fifties and suddenly was unable to speak or
move his right hand. His speech returned by the time he left the hospital,
but it was only with great difficulty and long rehabilitation that he
gradually recovered the ability to manipulate a paintbrush with his right
That progress, however, left a bizarre result: The man could no longer
speak and paint at the same time.
A PET scan showed that both speech and hand movement had relocated in his
brain – and now occupied the same exact site on the cortical map.
"The brain was struggling to find other ways to get his hand to move
and his voice to work properly," Pascual-Leone said.
Plasticity also plays a role after more mundane injuries. Within minutes
after a broken leg is set and immobilized in a plaster cast, the part of the
brain representing the leg starts to reorganize. It's as if the brain said:
"I don't need the brain cells for that leg anymore, so I'll use them
for something else."
What intrigues neuroscientists is that immobilizing the leg appears to
affect the brain even before the muscles start to wither from lack of use.
Finding a way to prolong activity in that part of the brain – the part
controlling the leg – might help "shorten the rehabilitation down the
road," said Pascual-Leone.
'A House With a Gazillion Doors'
Plasticity gives new meaning to the maxim, "Use it or lose it."
Within limits, the human brain has the capacity to remodel itself at any
age, said Michael M. Merzenich, a brain scientist at the University of
California at San Francisco. Merzenich's tests on monkeys in the 1980s
provided some of the most compelling early evidence of brain plasticity.
All experience, good and bad, drives the brain's continual remodeling,
Merzenich said. Even illness or injury can be seen as a kind of
"learning" for the brain. The ultimate therapy for a degenerative
disease like Parkinson's, which embeds itself harmfully in the brain,
Merzenich suggested, may come not from a pill but from an intense adaptive
"learning" process that somehow helps the patient
"unlearn" the illness.
Human learning, Merzenich said, is "lifelong and continuous."
Yet as the Baby Boom generation is discovering, remodeling is more
difficult in an adult brain than in a child's. Some kinds of learning do get
harder, and memory does deteriorate with aging. Even the old saw, "You
never forget how to ride a bicycle," is not perfectly true. (You may
not fall off, Merzenich said, but you probably won't ride as well as you did
as a child unless you keep practicing.)
The study of children learning to play the violin showed that the changes
in their brains were greatest in those under age 13. And the hemispherectomy
surgery that can cure severe epilepsy in some children would be unethical in
an adult because of the severe and irreparable brain damage it would wreak.
"The younger you are, the more plasticity can work," Grafman
said. "The hemisphere is less 'committed.' There's less knowledge in
there. There's more room for these new connections to emerge."
While there is some physical deterioration and loss of memory in the
adult brain over time, a bigger impediment to continued learning is the
patterning that already occupies the brain.
Consider a person learning a second language. Every language has its own
distinctive sound patterns, and in trying to learn new words, the brain goes
through trial-and-error guesses based on knowledge of the first language.
The older the student, the more "embedded" the first language is,
and the harder it is to override expectations about what a new language will
"It's not that the brain is full," Merzenich said. "It's
that the brain operations around the first language are so powerfully
embedded that it impedes your ability to learn the second."
"Imagine," said Pascual-Leone, "a house with a gazillion
doors." He was trying to describe the adult brain.
The often-used doors in this mansion are kept unlocked or even open, with
easy access and a free pathway. But with the passage of time, unused doors
stay closed or blocked and eventually get stuck.
Is it possible to reopen a door that has been stuck closed for years?
"Sure," he said, "but you may have to call a
The loss of brain cells, contrary to popular belief, is not the hallmark
of aging – or the reason doors get stuck. For better and for worse, it's
more complicated. Aging involves constant gain and loss, the brain's
shifting synaptic patterns that reminded Sherrington of "an enchanted
loom" and Pascual-Leone of the doorways in an infinite house.
As a mentally active person gets older, Pascual-Leone said, the
connections between nerve cells become more numerous and far-flung.
"They have more breadth and sophistication."
Perhaps this is what enabled artists like Monet, Picasso and Georgia
O'Keeffe to keep reinventing themselves throughout long productive lives.
Neurons have branches, like trees. Each time a nerve cell branches out,
it allows for more subtle connections with other faraway neurons, not just
the nearby ones that do similar things. That gives the mind of an adult more
reach and richness, a larger view of the world.
"So even though you may lose some of the details [as you age],"
Pascual-Leone said, "you get the big picture.
"Maybe that is wisdom."
© 1999 The Washington Post Company
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