92 CHAPTER 3
Functional magnetic resonance imaging (fMRI) is currently the most widely
used method for measuring human brain function. Neuroimaging with fMRI typi-
cally relies on three facts: (1) Iron affects the way that the hydrogen atoms in
water molecules respond to the magnetic fi elds used in fMRI. (2) The presence of
oxygen diminishes this effect of iron. (3) Hemoglobin, the crucial component of red
blood cells that allows them to carry oxygen throughout the body and brain, has
iron atoms in its structure. The key here is that the effects of the iron are different
when the hemoglobin carries oxygen than when it has been stripped of its oxygen
(because the oxygen was used for cell metabolism). When a region of the brain
is activated, it draws blood more quickly than the oxygen carried by the hemo-
globin in the blood can be used. This means that red blood cells with oxygenated
hemoglobin accumulate in the activated region—and this increase is what is mea-
sured in an fMRI scan. Brain regions that are not activated (or are activated less
strongly) when a person is performing a particular task (such as speaking in pub-
lic or looking at pictures) draw less blood, and the oxygen carried by the blood
gets used up. The difference in oxygen levels due to brain activity is refl ected in
the fMRI images.
The advantages of fMRI over PET include the absence of radiation and
the ability to construct brain images showing activation that occurs in just a
few seconds. Disadvantages include the requirement that a participant must lie
very still in the narrow tube of a noisy machine (which some people find un-
comfortable); in addition, any metal object containing iron cannot be brought
into the machine.
Neither PET nor fMRI simply detects which parts of the brain are “on” or
“off” at a particular time. Even at rest, the brain is never completely “off,” so
activity in the brain during a particular task is always measured relativeto activ-
ity during another condition. In many neuroimaging studies, researchers compare
activity when the participants are performing a given task with activity when they
are “resting.” However, researchers cannot know what is happening in the brain
while a particular participant is at rest. For instance, even if two participants are
both daydreaming while supposedly at rest, they may be thinking about differ-
ent things, which would produce different resting states in their brains. A person
with depression might be thinking about how hopeless everything seems, whereas
someone with schizophrenia may be thinking about a particular delusion. The
same stimulus in the task itself might affect the two participants’ brains differ-
ently, but the two resting states are already different. When the brain activity dur-
ing performance of the task is compared to that in the resting state, how much
of the observed difference is due to the stimulus in the task and how much is due
to differences in the resting state? Designing proper comparison tasks is a major
challenge in functional neuroimaging studies.Neurotransmitter and Hormone Levels
Many researchers study how biochemical imbalances contribute to mental illness.
However, researchers are just beginning to develop reliable ways to assess neu-
rotransmitter levels in functioning human brains (Gujar et al., 2005). Magnetic
resonance spectroscopy (MRS) relies on magnetic resonance to assess levels of
neurotransmitter substances in the brain. For example, Smith and her colleagues
(2001) used MRS to scan the brains of children whose mothers used cocaine
while they were pregnant, thereby exposing their unborn infants to this drug.
Even though the children were, on average, 8 years old at the time of scanning,
they had abnormally high amounts of creatine (a chemical involved in supply-
ing energy to neurons and muscles) in their frontal lobes. High levels of this
substance indicate increased numbers of glial cells, which may be attempting
to repair damaged tissue. If, in fact, the children’s frontal lobes were damaged,
the damage could contribute to a range of psychological disorders—including
problems inhibiting aggression.Functional magnetic resonance imaging (fMRI)
A neuroimaging technique that uses MRI to
obtain images of brain functioning, which
reveal the extent to which different brain
areas are activated during particular tasks.
Magnetic resonance spectroscopy (MRS)
A neuroimaging technique that uses magnetic
resonance to assess levels of neurotransmitter
substances in the brain.