[ExI] Fwd: Intelligence Explained
hkeithhenson at gmail.com
Mon Oct 26 18:07:34 UTC 2009
Most of you have probably seen this, but better an extra copy than
none at all. Keith
Tracking and understanding the complex connections within the brain
may finally reveal the neural secret of cognitive ability.
By Emily Singer
A series of black-and-white snapshots is splayed across the screen,
each capturing a thin slice of my brain. The gray-scale pictures would
look familiar to anyone who has seen a brain scan, but these images
are different. Andrew Frew, a neuroscientist at the University of
California, Los Angeles, uses a cursor to select a small square. Thin
strands like spaghetti appear, representing the thousands of neural
fibers passing through it. A few clicks of the cursor and Frew refines
the tract of fibers pictured on the screen, highlighting first my
optic nerve, then the fibers passing through a part of the brain
that's crucial for language, then the bundles of motor and sensory
nerves that head down to the brain stem.
Frew is giving me a tour of my white matter--the tissue connecting the
neurons, or nerve cells, that make up gray matter. Something about the
twisting, turning neural wires that ferry information between the
neurons--their individual thickness, perhaps, or their abundance, or
the specific paths they take from one part of the brain to
another--may explain, at least in part, the variations in human
Scientists have been searching more than two centuries for the source
of intelligence--the general cognitive ability often quantified in the
form of IQ. With the advent of technologies such as magnetic resonance
imaging (MRI), researchers concentrating mainly on gray matter have
been able to map the parts of the brain that appear to play a role.
But this has taken them only so far, and the focus on gray matter has
not told the whole story. Not until the last few years, as new
variations of MRI home in on the brain's white matter, has a deeper
understanding begun to emerge. "Scientists are now able to switch the
focus from particular regions of the brain to the connections between
those regions," says Sherif Karama, a psychiatrist and a
neuroscientist at McGill University's Montreal Neurological Institute.
Their initial findings have led Karama and others to believe that
neural wiring and the way it carries information around the brain may
be crucially important to IQ.
Until fairly recently, only a few scientists were studying how brain
structure might be related to IQ, in part because the idea of a
biological and genetic basis for intelligence has long been
controversial. Since people from different ethnic groups often score
differently on intelligence tests, such studies may raise contentions
of racism, and critics fear potential abuses such as discrimination in
education or employment. Nonetheless, new imaging techniques have
allowed types of studies never before possible, and the number of
research groups focusing on this question is growing quickly. Many of
these groups are setting their sights on white matter.
The hope is that finding the brain areas and circuits involved in
intelligence will provide new insight into neurological and
psychiatric diseases that impair cognition, such as Alzheimer's and
schizophrenia. "If you want to understand cognitive decline, you need
to understand how cognition is manifested and put together in the
brain," says Rex Jung, a neuroscientist at the Mind Research Network
in Albuquerque, NM. The research may also improve understanding of
learning disabilities such as dyslexia and ADHD, perhaps leading to
better treatments. But other potential applications could be more
controversial. Some scientists envision a day when brain scans are
used to estimate IQ. Sandra F. Witelson, a neuroscientist at the
Michael G. DeGroote School of Medicine at McMaster University in
Ontario, says, "It's not a wild guess to say that sometime in the
future, brain scans will be part of a group of tools that try to
indicate what level someone's ability is going to be."
Neuroscientist Paul Thompson is one of those researchers studying
brain structure and IQ, but that wasn't what he planned on when he
started his lab at UCLA: he focused on the wave of changes in the
brain that characterize Alzheimer's and schizophrenia. Because serious
cognitive deficits accompany both of 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 significant. "IQ came in as a key factor that determines how
the brain looks," Thompson says.
Scientists who study intelligence typically define it in comparative
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. Studies 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 effectively.)
As structural brain imaging has become more sophisticated, scientists
have focused on sections of the brain involved in specific tasks,
including sensory processing, memory, attention, and decision making.
Different studies have connected different areas with intelligence,
however, making it difficult to come to an overarching 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 information
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. Gathering
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 recognition,
among other cognitive functions. Applying existing theories of how
information flows in the brain, Jung and Haier hypothesized 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
effectively and efficiently, then the network won't function
efficiently," 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 matter 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 processing language.
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
diffusion 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 measure the
diffusion of water molecules through each one in six to 12 directions,
which is sufficient for detecting thick bundles of neural fibers. But
places where wiring overlaps appear as a blur. Newer variations of
diffusion imaging measure diffusion in 50 to 500 directions. Computer
algorithms synthesize this data into a 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
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 diffusion signal may indicate more fibers
or thicker myelin; scientists don't yet know. But the newer diffusion
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 sensitive MRI measures we have for cognitive
function," says Vincent Schmithorst, a neuroscientist at Cincinnati
Thompson refers to his diffusion 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 efficiently information moves around the brain,
rather than just how quickly. In a recent study led by Martijn P. van
den Heuvel, a neuroscientist at University Medical Center Utrecht, in
the Netherlands, researchers defined efficiency as the number of links
it takes to get from one node to another--both in specific brain areas
and all over the brain. Just as a direct flight from Paris to Chicago
would be considered more efficient than one with a layover in London,
a direct link between two parts of the brain would be more efficient
than an indirect route.
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 different 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 magnetic fluctuations
around neurons as they fire, allowing scientists to track the
millisecond-scale sequence of neural signaling in the brain as people
perform different tasks, such as pressing a button in response to a
light. Researchers hope to figure out how the flow of these signals
differs 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 affect
It's likely that the quality of white matter is at least partly
genetically determined and, therefore, difficult to change. The size
of the corpus callosum, the thick tracts of white matter connecting
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
attributed to genetics, according to Thompson. But only about 45
percent of the variation in the temporal lobes, which play a central
role in learning and memory, appears to be inherited.
Thompson is now trying to identify specific genes that are linked to
the quality of white matter. The top candidate so far is a gene for a
protein called BDNF, which promotes cell growth. People with one
variation have better-organized fibers, he says.
But environmental factors also play a role. Rodents raised in a
stimulating environment have more white matter. And research suggests
that the apparent IQ difference between people who were breast-fed and
bottle-fed as babies may arise because breast milk contains omega-3s,
fatty acids involved in the production of myelin; as a result, some
baby formula now includes these compounds.
Hope remains for those who passed the baby-formula stage long ago.
Although the adult brain isn't as malleable as a young brain, and is
therefore less easily influenced by environmental factors, evidence is
growing that the adult brain is still remarkably plastic. Scientists
haven't yet studied white matter enough to know how to improve it
directly, especially in healthy people. But exercise, diet, and mental
activity have all been shown to boost brain health and decrease the
risk of dementia, a disorder that has been linked to white-matter
damage. And other studies have shown that just a few months of
practicing a new skill can enlarge certain parts of the brain,
including parts of the frontal cortex involved in motor planning and
parts of the temporal lobes that integrate visual, auditory, tactile,
and internal physiological information. Similar studies on ways to
improve the quality of white matter are under way.
Although looking at images of my own white matter was fascinating, it
was not deeply illuminating. The scan gave me no indication of how
efficient or flexible my mental processes are. And, the researchers
told me, not even the most astute neuroanatomist would be able to
glean a general sense of my cognitive capabilities from my brain scan.
Learning more about the role of white matter in intelligence will give
scientists a fuller picture of how brain anatomy influences cognition.
It could help explain how differently structured brains might produce
the same IQ, or whether particular patterns--thick white matter here,
a large chunk of gray matter there--are linked to particular cognitive
strengths and weaknesses. "One of the key findings that has come out
of the last decade of studies of intelligence is the fact that the
brain can generate the same IQ score a number of ways," says Haier.
Intelligence is characterized by "individual differences in learning,
memory, and attention and how they are integrated in any one
individual." Haier envisions a day when brain scans could alert
teachers to the cognitive strengths and weaknesses of each student, so
that lessons could be individually tailored. It might be possible to
derive much the same information from extensive cognitive testing, but
such testing is rare because it's expensive and time consuming. A
15-minute brain scan, on the other hand, might be applied much more
Although it's not yet possible to estimate someone's IQ from a brain
scan, some scientists say that day may not be far off. "For a very
simple example," says Haier, "suppose the total amount of gray matter
in several areas is a good correlate of IQ, and this correlation gets
better if we add additional scan information--perhaps the amount of
white matter in other areas or the amount of activation in certain
areas while a problem is solved. We don't yet know which combination
of brain parameters will be most predictive of psychometric IQ or
other intelligence factors or mental abilities, but we know how to
find out. Once funding is available to scan very large samples with
multiple techniques and test everyone with a battery of psychometric
measures, it's just a matter of time."
That could be a boon for physicians working with Alzheimer's patients
or others suffering from diseases that cause cognitive damage. Some
experts, however, fear it will create the sense that people's
abilities are completely predetermined. Scientists working in the
field argue that using a brain scan to quantify intelligence is really
no different from using a standardized test like the SAT. But because
a brain scan measures a physical property, it's likely to arouse even
more concern than today's testing methods. "If you can estimate
someone's IQ from a brain scan, even if it isn't any more predictive
than an SAT [score], it gives the illusion that his or her future is
fixed," says Karama.
In truth, it's not yet clear that brain scans would be any better than
SAT scores at predicting an individual's cognitive function--or
success in school, career, or life. Their value will depend on what we
do with them. Perhaps, as with the SAT, training courses will be
developed to help people improve their scores--to make better use of
the network of connections in their brains. Says UCLA's Frew, "It's
not just the tool. It's how well we are using it."
Emily Singer is Technology Review's senior editor for biomedicine.
Copyright Technology Review 2009.
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