thinking about anything in particular, not doing any task, your brain
is very active and using glucose at a robust rate. If you then turn
on the lights, listen to music, move your arms, read out loud, and
so forth, energy consumption in the brain increases by only a tiny
amount (a few percent, at most) in particular regions. The upshot: the
brain is working really hard all the time. What exactly all this activity
is for is currently largely unknown, and the term “dark energy,” bor-
rowed from cosmology, has been used to highlight this mystery.
The last of the dynamic brain imaging methods to be discussed here is
functional magnetic resonance imaging, or fMRI. This technique uses
the same MRI technology discussed above in the context of structural
brain imaging, but {MRI employs a new twist—collecting a series
of magnetic resonance images over time and looking at something
that changes as neural activity changes in the brain. What is that
something?
Cells derive most of their energy from the breakdown of glucose
using oxygen—similar to the process of obtaining energy by burning
carbon-containing molecules with oxygen in combustion engines,
furnaces, stoves, and campfires. As discussed in the context of PET,
the flow of blood within the brain is regulated such that flow increases
in regions of greater neural activity, delivering more glucose and oxy-
gen to active cells.
Hemoglobin is the oxygen-carrying protein in red blood cells.
Oxygen binds to the hemoglobin molecule in the lungs and comes off
the hemoglobin at locations all over the body, where oxygen is needed
to power the activity of cells. And hemoglobin produces a different
magnetic perturbation effect on its local environment depending on
whether or not oxygen is attached to the hemoglobin molecule. Re-
gions exhibiting increased neural activity have a greater influx of oxy-