HOW THE BRAIN MAKES POTENTIAL 191de cade or so, the fi eld has been boosted by the use of MRI scans in what
is called “brain scanning” or “brain mapping.”
Th ese scans are basically of two types. In structural MRI, the scan ma-
chine creates a varying magnetic fi eld in the target tissue, thus agitating
protons in the atoms within it. When the magnetic fi eld is turned off , the
protons gradually return to normal, giving off an electrical signal that
can be mea sured. Th is happens at diff er ent rates, depending on the density
of the tissues. So the scan can distinguish, for example, between brain and
bone (skull); or between gray matter (such as densely packed neurons)
and white matter (more dispersed axons sheathed in fatty glial tissue).
Th is method has been mainly used for mea sur ing brain size. A num-
ber of studies using it have suggested correlations of 0.2–0.4 between IQ
and brain size. Th ese values have been widely accepted.
Other research has used “functional” (fMRI). It has focused on more
specifi c aspects of brain, like ce re bral connectivity, cortical surface area
or cortical thickness in attempts to relate them to cognitive per for mance
(e.g., taking an IQ test). In a review in 2012, Ian Deary claimed that such
studies are “exemplifying key empirical advances in the association be-
tween brain structure and functions.”^25
Most reports are, like this one, quite bullish, adopting the promissory
posture of the “genes for IQ” researchers. Richard Haier, for example,
hosts an expansive website making confi dent claims. One of these is that
the density of gray and white matter in regions of the brain is related to
diff erences in how people score on IQ tests. In a glowing note, not un-
typical of the fi eld, he says that “researchers are now on their way to a de-
tailed scientifi c explanation of what defi nes intelligence, where it comes
from, and how it operates in the brain.”^26
EMPIRICAL PROB LEMSTh ere are many prob lems with such claims, and some make easy targets.
One of these is the use of IQ- type tests as mea sures of brain functions. It
is a major error to relate per for mances on such tests as if they directly
refl ect a ladder of brain function. As described in chapter 3, they test for
the learned factual knowledge and cognitive habits more prominent in
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