genated hemoglobin, and subsequent conversion to its deoxygenated
form. This change in blood oxygenation can be measured by looking
at the magnetic resonance signal of hydrogen atoms in HO molecules
that are in the vicinity of hemoglobin molecules in the blood. This
change is captured in a parameter called the BOLD signal, where BOLD
stands for blood-oxygen-level dependence. The BOLD signal is essen-
tially a measure of the increased flow of blood into regions of the brain
that are more neurally active.
The spatial resolution of fMRI is pretty good; areas of increased
neural activity can, in the best of circumstances, be localized down to
within about 1 millimeter. The time resolution of fMRI at present is
several seconds. EEG has much faster time resolution (milliseconds)
but relatively poor spatial resolution (several centimeters). An innova-
tive technique records measurements from both fMRI and EEG at the
same time and makes correspondences between them. This requires
special attention to the many technical issues of how the gadgets in-
volved in fMRI and EEG impact each other’s function.
This table summarizes the relative spatial and temporal resolu-
tions of several methods of functional brain imaging.Imaging method = Spatial resolution Temporal resolution
EEG several cm milliseconds
MEG mm milliseconds
fMRI mm seconds
PET cm seconds to minutesPicture of the brain—
x-ray, CAT, and MRI.