Image Processing Reference
In-Depth Information
In addition, SAM filtering results in sharper peaks of the distribution of neuronal
activity index over the volume [43].
Having computed and using SAM or LCMV for the two experimental
conditions, passive ( ) and active ( ), it is possible to compute a pseudo-
t
value
for each location across the two conditions
ν
q
ν
ε
p
a
t
νν
νν
ε
()
a
−
+
( )
p
ˆ
q
q
t
=
(8.14)
()
a
( )
p
ε
Such an approach provides the possibility of considering experimental design
in the analysis of E/MEG localization.
Unlike ECD, beamforming does not require prior knowledge of the number
of sources, nor does it search for a solution in an underdetermined linear system
as does DECD. For these reasons, beamforming remains the favorite method of
many researchers in EMSI and has been suggested for use in the integrative
analysis of E/MEG and fMRI, which we cover in Section 8.5.
8.3
MULTIMODAL EXPERIMENTS
Obtaining noncorrupted simultaneous recordings of EEG and fMRI is a difficult
task due to interference between the strong MR field and the EEG acquisition
system. Because of this limitation, a concurrent EEG/fMRI experiment requires
specialized design and preprocessing techniques to prepare the data for the analysis.
The instrumental approaches described in this section are specific to collecting
concurrent EEG and fMRI data. For obvious reasons MEG and fMRI data must
be acquired separately in two sessions. However, even when MR and MEG are
used sequentially, there is the possibility of contamination from the magnetization
of a subject's metallic implants, which can potentially disturb MEG acquisition
if it is performed shortly after the MR experiment.
8.3.1
M
EASURING
EEG
DURING
MRI: C
HALLENGES
AND
A
PPROACHES
Developing methods for the integrative analysis of EEG and fMRI data is difficult
for several reasons, not the least of which is that the concurrent acquisition of
EEG and fMRI itself has proved challenging. The nature of the problem is
expressed by Faraday's law of induction: a time-varying magnetic field in a wire
loop induces an electromotive force (EMF) proportional in strength to the area of
the wire loop and to the rate of change of the magnetic field component orthogonal
to the area. When EEG electrodes are placed in a strong ambient magnetic field
resulting in the EMF effect, several undesirable complications arise:
•
Rapidly changing MR gradient fields and RF pulses may induce
voltages in the EEG leads placed inside the MR scanner. Introduced
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