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Functional magnetic resonance imaging, or fMRI, is a type of neuroimaging used to obtain
images of brain activity.
fMRI involves exposing the brain to multiple magnetic fields, and relies
on the observation that protons in the nuclei of hydrogen atoms respond to this procedure
by emitting an electromagnetic signal that can be detected by the fMRI scanner.
The fMRI scanner is capable of determining some of the properties of the tissue the signal
came from, and can use this information to reconstruct a high-resolution image of the
brain.
Additionally, the fMRI scanner can detect differences in the magnetic properties of
oxygenated vs. deoxygenated blood, and thus can identify changes in levels of oxygenated
blood in different regions of the brain using a method called blood-oxygen-level-dependent,
or BOLD, contrast.
BOLD is typically what enables us to identify which brain areas are most active in fMRI.
Areas of the brain that are more active tend to receive higher levels of oxygenated blood.
Thus, higher levels of oxygenated blood in a particular brain region are believed to
correspond to higher neural activity in that region.
On a typical fMRI image, color-coding is used to represent differences in the level of oxygenated
blood and thus differences in activity.
Activity in those areas can then be associated with whatever task was performed at the time
of the scan.
fMRI was developed in the early 1990s and since has become a very popular neuroimaging
method.
Nevertheless, in its short history fMRI research has been plagued by issues like small sample
sizes, the use of methods that lead to a high number of false positives, and a small proportion
of study results that have been independently reproduced.
As the field has become more aware of these problems, many researchers have begun to adjust
their approach to address them.