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three-dimensional picture showing the most likely paths of nerve
fibers through each area, and then stitch together the information
from multiple points to create a wiring map.
The strength of the diffusion signal---the extent to which it
reveals a clear direction---is used to gauge how organized the
fibers of the white matter are. A stronger di usion signal may
indicate more fibers or thicker myelin; scientists don't yet know.
But the newer di usion imaging methods have revealed a strong
correlation between the strength of this signal---what researchers
refer to as the "integrity" of the white matter---and performance
on a standard IQ test. "DTI turns out to be one of the most sensi-
tive MRI measures we have for cognitive function," says Vincent
Schmithorst, a neuroscientist at Cincinnati Children's Hospital.
Thompson refers to his di usion maps as "pictures of mental
speed." Previous research has repeatedly linked IQ to processing
speed, and other studies show that processing speed in turn is
tightly linked to the quality of one's white matter. Does that mean
intelligence is determined by how fast the brain works? If so, does
finding the key to processing speed in the brain mean researchers
have finally found the secret to intelligence?
In reality, speed is probably not the only determinant of
IQ. "One of the things that is important for IQ is frontal-lobe
function, which is involved in planning, decision making,
and weighing evidence," Thompson says. "I wouldn't think
of those skills as being entirely reliant on mental speed."
Some of the newest theories of intelligence suggest
that the crucial factor may be how e ciently information
moves around the brain, rather than just how quickly. In a
recent study led by Martijn P. van den Heuvel, a neurosci-
entist at University Medical Center Utrecht, in the Neth-
erlands, researchers defined e ciency as the number of
links it takes to get from one node to another---both in spe-
cific brain areas and all over the brain. Just as a direct flight
from Paris to Chicago would be considered more e cient
than one with a layover in London, a direct link between
two parts of the brain would be more e cient than an indi-
Van den Heuvel and colleagues found that people with
above-normal IQs of 120 and up had the most efficient
brain networks. "Our hypothesis is that IQ is about how the
human brain can integrate di erent types of information,
how easily it can get information from one brain region to
another," van den Heuvel says. "These activity patterns are
highly influenced by white-matter structures in the brain, how
the brain is connected."
Richard Haier and his collaborators are now working on a new
method of measuring information flow around the brain using
magnetoencephalography, or MEG. MEG measures the mag-
netic fluctuations around neurons as they fire, allowing scientists
to track the millisecond-scale sequence of neural signaling in the
brain as people perform di erent tasks, such as pressing a button
in response to a light. Researchers hope to figure out how the flow
of these signals di ers with intelligence---whether smarter people
follow the same sequence but faster, for example, or whether their
brains skip a few steps in a circuit. "When you add the timing of the
nodes and networks," says Jung, "then we're really talking about
how the brain works in real time."
If white matter plays a key role in intelligence, is there a way to
enhance it? Does it give us ways to make ourselves smarter, or
to help people with neurological and psychiatric disorders that
a ect cognitive skills?
It's likely that the quality of white matter is at least partly geneti-
cally determined and, therefore, di cult to change. The size of
the corpus callosum, the thick tracts of white matter connect-
ing the two hemispheres of the brain, is about 95 percent genetic.
And about 85 percent of the white-matter variation in the parietal
lobes, which are involved in logic and visual-spatial skills, can be
PLOTTING DIFFUSION Water molecules in the brain diffuse along
neural wires, allowing scientists to create, essentially, circuit diagrams. First,
the most likely directions of diffusion are calculated for each two-cubic-
millimeter patch of the brain, generating 3-D shapes for each point, as
shown above. Specialized software calculates the path of neural wires on
the basis of these shapes. This method can distinguish overlapping wires,
while more conventional imaging cannot.
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