nytimes.com
How Much Give Can the Brain
Take?
By GEORGE JOHNSON
What makes the brain unique among bodily organs
is that its cells are arranged in a very specific
manner -- in delicate constellations that somehow
reflect the knowledge gained from being alive. The
result is a world of individuals, a place where the very
idea of a brain transplant is absurd. You are what you
think, and what you think is laid out in your own neural
structures.
But just how malleable is this cellular filigree? How
easily can a person overcome the forces -- genetic and
environmental -- that shape a creature from birth? Over
the last few weeks, evidence has emerged from several
laboratories that throws these questions into a new
light. And though the experimenters don't always
welcome the moral burden, their findings are destined
to be incorporated into longstanding debates over
issues as practical as the importance of early childhood
education and as profound as the perfectibility of
mankind.
The decisions every society must make over how to
rear and educate its young or how to care for its
neurologically disabled are ultimately political.
But the politicians look to the experts and the experts
look to the data. And what they find is a confusing
tension between the brain's need to be malleable -- or
"plastic" -- enough for learning to occur and stable
enough for the learning to solidify into wisdom. The
question is where nature has placed the fulcrum. And
what is ultimately being weighed is what it means to be
human.
Earlier this month, researchers at Princeton University
issued a report that threatens (or promises) to upset
what has been one of science's seeming certainties: that
the adult brain cannot form new neurons. Common
wisdom holds that people are born with a fixed
allotment of these cells, which then die off one by one.
While the brain continues to form new connections, or
synapses, between the pre-existing components -- a
process long believed to be the basis of memory --
once a neuron is lost it is gone.
Backed by experimental evidence, the principle also
seemed to have logic on its side: If the essence of a
brain is the precision of its wiring, then generating
hordes of new neurons might gum up the works.
Earlier experiments had begun chipping away at the
certainty that the ban on adult neurogenesis, as it is
called, is absolute, at least in lower vertebrates or in
the human brain's more primitive regions. (Birds
apparently generate neurons to encode new songs.) But
the Princeton findings went further: Dr. Elizabeth
Gould and Dr. Charles G. Gross found that thousands of
new neurons a day were being formed in the brains of
monkeys, migrating to areas including the prefrontal
cortex, the seat of intelligence and decision-making.
If a steady stream of fresh brain cells is continually
arriving to be incorporated into new circuitry, then the
brain is more malleable than hardly anyone has
realized. Memories may be formed not just by forging
new synapses between old neurons but by weaving in
new ones as well.
No one really knows what these new neurons do. Stuck
at the end of the Princeton paper is an arresting
speculation: that the continuum of new neurons, arriving
in one batch after another, might be the brain's way of
storing memories chronologically, forming the pages of
the neurological book of life.
he notion of adult brains cranking out a steady
stream of new parts is especially startling since it
has become almost gospel that the most important
neurological action occurs in the first three years.
During this developmental window, the brain explodes
with a profusion of neural connections that is then
pruned back by experience into precise circuitry. Eyes
deprived of light during this crucial period will not
stimulate the formation of the neural pathways that
allow the brain to see.
Even worse, it has been feared, the damage might be far
more subtle. Without enough stimulation -- classical
music and shining objects in the crib -- children might
be permanently saddled with inferior brains.
Like most scientific folk wisdom ("You use only 10
percent of your brain power," "Right-brained people
are more creative") the Mozart effect, as it is
sometimes called, is extrapolated from research whose
meaning is open to debate. While there is indeed a
developmental window when the infant brain is
forming, it's not clear whether heroic efforts need to be
made to encourage the process.
In "The Myth of the First Three Years" (Free Press,
1999), Dr. John T. Bruer, president of the James S.
McDonnell Foundation, which supports neuroscientific
research, argues that only the most severe deprivation
causes real damage, and that even that can sometimes
be reversed.
"The brain is not 'cooked' by age 3 or age 10," he
writes. "Our brains remain remarkably plastic and we
retain the ability to learn throughout our lives."
The new Princeton results will inevitably be taken to
support this more sanguine view. But the arguments are
only beginning. Last week the Abecedarian Project, a
long-term study tracking children of African-American
families in Chapel Hill, N.C., since 1972, found strong
evidence that high-quality day care indeed enhanced not
only the ABC's and mathematical skills but one's
success in adulthood. But it's difficult to gauge how
much of the gains came directly from neural
stimulation, putting the issue more in the political than
the scientific realm.
No matter how far scientific opinion swings toward the
image of a more fluid brain, doctors will still be left
with the harsh truth that neural wounds do not heal so
readily as wounds to the skin or bones. To a small
extent, brains can recover from injuries, especially
early in life. Lost functions like language can be
transferred to other undamaged regions. But a report to
appear in next month's issue of the journal Nature
Neuroscience serves as a reminder of how intransigent
some trauma can be.
A study led by Dr. Antonio Damasio at the University
of Iowa College of Medicine found that two people
who suffered injuries in infancy, when the brain is
supposed to be at its most resilient, seemingly lost the
ability to tell right from wrong. Raised in good homes,
they were otherwise intelligent. But early damage to the
prefrontal cortex seemed to have disrupted some kind
of morality circuit. If new neurons have indeed been
migrating to the decision-making centers of their brains,
there is apparently a limit to the curative powers.
Since the study involved only two people -- childhood
injuries of this sort are thankfully rare -- the
researchers may have accidentally stumbled across
exceptions to some yet-to-be-discovered rule. But if the
trend of research keeps pointing toward more
plasticity, scientists will have to learn to harness and
enhance the process before the words "permanent brain
damage" come to have a less horrifying ring.
No matter how malleable the brain turns out to be, it is
still governed by its genetic heritage. For new
connections to form with ease, the underlying
machinery must be in order. And the design of these
biochemical cogs is encoded in the genes.
But with advances in genetic engineering, even these
constraints are not necessarily permanent. Maybe the
most striking of the recent experiments was another one
from Princeton in which a scientist, Dr. Joe Z. Tsien,
seemingly enhanced the memories of mice by tinkering
with their DNA.
His lab raised mice with extra copies of a gene that
carries the instructions for making an important piece of
a tiny structure called an NMDA receptor, which acts
like a memory switch. When two neurons connected by
the same synapse fire at the same time, the receptors
unleash chemical reactions that strengthen the
connection. This might be how two stimuli are linked
into a memory.
Mice with the genetically souped-up receptors
performed better on learning basic tasks -- like
remembering the location of a submerged platform --
inspiring discussions about the possibilities of treating
people with memory disorders or even enhancing
normal brains.
And so the brain, barely at the level of understanding
itself, may be on the verge of changing its own
architecture.
It will be up to us all to decide whether that is an
intelligent thing to do. |
