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Unlike EEG and MEG, fMRI is a secondary measure of brain activation.
Functional MRI works by imaging the level of blood oxygenation, a technique
called BOLD (Blood Oxygen Level Dependence) contrast. The
properties of hemoglobin (molecules of red blood cells that carry oxygen) change
depending on whether it is carrying oxygen or whether it has given up oxygen.
Oxygenated hemoglobin is diamagnetic while deoxygenated hemoglobin is paramagnetic.
When neurons fire, they consume oxygen causing an initial reduction in local
oxygenation levels. This decrease is then followed by an increase above the
resting level as nearby capillaries dilate to allow more oxygenated blood into
the active area, overcompensating for the initial decrease. As a result, activation
of neurons in a particular area of the brain results in an increase of the recorded
signal levels. Areas of the brain associated with particular tasks can then
be determined by comparing the MR signal intensities measured during task conditions
compared to a resting control.
The
animation shows areas of the brain that were activated during the performance
of a finger sequencing task. The activations revealed with fMRI (colored regions)
are shown on a functional scan of the subjects head.
The subject's task was to continually touch the thumb to the fingers of the
right hand (e.g. 5-3-4-2 with 5 as the pinky) in a predefined sequence for thirty
second periods. These periods were then compared to a thirty second period of
rest. This task results in the activation of bilateral primary sensorimotor
areas which is much stronger on the contralateral side. Also activated were
several areas involved in motor planning such as the SMA and bilateral premotor
regions. Finally we also see strong activation in the ipsilateral cerebellum
a region involved in motor preparation and timing.
Functional
Magnetic Resonance Imaging (fMRI) is a recent noninvasive imaging technology
that can be used to identify regions in the brain that are activated during
the performance of particular tasks. In contrast to conventional MRI scans showing
brain structure, fMRI provides information about brain function.
