Image Processing Reference
In-Depth Information
protocol does have drawbacks. Specifically, it imposes a limitation on
the amount of subsequent EEG activity that can be monitored if the EEG
high-pass filters do not settle down soon after the MR sequence is
terminated [70]. In this case, EEG hardware that does not have a long
relaxation period must be used. Another drawback with this approach is
that it requires online EEG signal monitoring to trigger the fMRI acqui-
sition in case of spontaneous activity. Often, experiments of this kind
are called EEG-correlated fMRI due to the fact that offline fMRI data
time analysis implicitly uses EEG triggers as the event onsets [46].
•
Interleaved EEG/f MRI:
The experiment protocol consists of time
blocks and only a single modality is acquired during each time block
[61-71]. This means that every stimulus has to be presented at least
once per modality. To analyze ERP and fMRI activations, the triggered
fMRI protocol can be used with every stimulus presentation so that
EEG and MR are sequentially acquired in order to capture a clean
E/MEG signal followed by the delayed HR [72].
•
Simultaneous f MRI/EEG:
Preprocessing of the EEG signal mentioned
in Section 8.1 is used to remove the MR-caused artifacts and to obtain
an estimate of the true EEG signal. However, neither of the existing
artifact-removing methods has been proved to be general enough to
work for every type of EEG experiment and analysis. It is especially
difficult to use such an acquisition scheme for cognitive experiments
in which the EEG-evoked responses of interest can be of small ampli-
tude and be completely overwhelmed by the MR noise [73].
8.4
MULTIMODAL ANALYSIS
There is an increasing number of reported E/MEG/fMRI conjoint studies that
attempt to gain the advantages of a multimodal analysis for experiments involving
perceptual and cognitive processes: visual perception [59,72,74,75] and motor
activation [59], somatosensory mapping [65,76], fMRI correlates of EEG rhythms
[41,71,77-79), auditory oddball tasks [63], passive frequency oddball [80], illu-
sory figures in visual oddball tasks [81], target detection [62,82], face perception
[64], sleep [70], language tasks [74,83], and epilepsy [66,67,69,84-87].
This section begins with an explanation of the role of anatomical MRI in
multimodal experiments followed by a description of multimodal analysis meth-
ods used in the above-mentioned studies or test-driven on the simulated data.
8.4.1
U
SING
A
NATOMICAL
MRI
The difference in captured MRI contrasts (proton densities [PD] or T1, T2 relax-
ation times) for different types of organic tissue makes possible the noninvasive
collection of information about the structural organization of the brain. In addi-
tion, a regular gradient or spin-echo EPI sequence is capable of detecting transient
or subtle changes of the magnetic field in cortical tissue caused by neuronal
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