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those diseases, however, Thompson and his collaborators tested
cognitive function in their subjects. When they began to look
more closely for variables that correlated with brain structure,
they found that intelligence seemed to be among the most signifi-
cant. "IQ came in as a key factor that determines how the brain
looks," Thompson says.
Scientists who study intelligence typically define it in com-
parative terms, as a general cognitive ability measured against a
mean. A quantifiable "general intelligence factor," known as g, can
be statistically extracted from scores on a battery of intelligence
tests. While some people clearly have particular areas of talent,
those who score well on one test are likely to score well on others
as well, reflecting a higher g.
Researchers have yet to find a simple neural explanation for g.
In 2001, Thompson showed that it is correlated with volume in the
frontal cortex, a result consistent with a number of studies that
have linked intelligence to overall brain size. But size is a crude
measure: while larger brains may be smarter on average, it's not
clear if that's because they have more nerve cells, more connections
between cells, or more of the fibers that carry neural signals. Any
of these factors can result in a larger brain or thicker cortex, but
neither of these things is necessary for great intelligence. Stud-
ies of Albert Einstein's brain, for example, have found that it was
typical in size, or even a bit on the small side. (It was missing a
wrinkle in the inferior parietal lobe, which is behind the frontal
cortex; some have speculated that this quirk allowed the neurons
in that region to communicate more e ectively.)
As structural brain imaging has become more sophisticated,
scientists have focused on sections of the brain involved in spe-
cific tasks, including sensory processing, memory, attention, and
decision making. Di erent studies have connected di erent areas
with intelligence, however, making it di cult to come to an over-
arching conclusion about its anatomical basis.
But what if the key to intelligence is neither an individual area
of the brain nor its total volume but the network over which infor-
mation is transmitted and integrated? In 2007, Jung and Richard
Haier, now professor emeritus of psychology at the University of
California, Irvine, developed the first comprehensive theory drawn
from neuroimaging of how the brain gives rise to intelligence. Gath-
ering information from 37 published papers that had used imaging
to study intelligence, they mapped out the brain areas that had been
pinpointed in at least a third of the studies to sketch a network of
regions spanning the frontal and parietal lobes.
The network consists of about 10 nodes, or clusters of cells, that
had been linked to attention, working memory, and facial recogni-
tion, among other cognitive functions. Applying existing theories
of how information flows in the brain, Jung and Haier hypoth-
esized that neural signals travel from nodes near the back of the
brain, where sensory data is collected and synthesized, to those in
the frontal lobes, which are responsible for decision making and
planning. The connections between these nodes, they argued, are
just as critical as the nodes themselves. "If the nodes of a network
aren't communicating e ectively and e ciently, then the network
won't function e ciently," says Jung.
The theory was provocative, but the data used to develop it had
a major limitation: the published studies had focused primarily on
gray matter. As for the connecting white matter, Jung and Haier
inferred its paths from the locations of the key nodes and existing
maps of neural anatomy. They didn't look directly at the white mat-
ter itself, largely because they lacked the technology to do so.
By volume, gray matter makes up roughly half the human brain.
The other half is white matter, consisting of filament-like neural
projections wrapped in a fatty material called myelin; such a high
proportion of white matter appears to be unique to humans. As
we "evolved from worms to humans," says George Bartzokis, a
professor of psychiatry at UCLA, the number of non-neural cells
in the brain increased 50 times more than the number of neurons.
He adds, "My hypothesis has always been that what gives us our
cognitive capacity is not actually the number of neurons, which
can vary tremendously between human individuals, but rather
the quality of our connections."
Thanks to their layer of insulation, which prevents leakage of
electrical impulses, myelinated nerve fibers can send signals about
100 times as fast as unmyelinated ones. The myelin also allows
more information to be sent per second by reducing the waiting
time between signals. The result is that neurons can process 3,000
times as much information as would otherwise be possible. That
capacity, Bartzokis believes, is crucial for speaking and process-
The type of MRI typically used for medical scans does not show
the finer details of the brain's white matter. But with a technique
called di usion tensor imaging (DTI), which uses the scanner's
magnet to track the movement of water molecules in the brain,
scientists have developed ways to map out neural wiring in detail.
While water moves randomly within most brain tissue, it flows
along the insulated neural fibers like current through a wire.
Most DTI scans break the MRI image into tiny areas and mea-
sure the di usion of water molecules through each one in six to
12 directions, which is su cient for detecting thick bundles of
neural fibers. But places where wiring overlaps appear as a blur.
Newer variations of di usion imaging measure di usion in 50 to
500 directions. Computer algorithms synthesize this data into a
To see 3-D video of Emily Singer s brain scan and an
interview with neuroscientist Richard Haier, visit
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