Image Processing Reference
In-Depth Information
Fig. 21.1
Different brain imaging modalities and their spatial and temporal resolutions. For con-
nectomics, light-(
LM
) and electron microscopy (
EM
) are mostly performed in vitro. The color
indicates functional versus structural information in the acquired data
connectivity, and finally fMRI for functional connectivity. Besides the visualization
approaches discussed here, the reader is also referred to Sect.
21.8
for more detail
on network analysis and comparative visualization techniques.
21.4.1 EEG and MEG
Developed in the 1920s, electroencephalography (EEG) is the oldest noninvasive
functional neuroimaging technique, which records electrical brain activity from elec-
trodes on the scalp. Nowadays, the number of electrodes can be as large as 128 or
even 512; in that case one speaks of
multichannel
or
high-density
EEG [
81
,
93
].
By contrast, magnetoencephalography (MEG) measures magnetic fields outside the
head induced by electrical brain activity [
35
]. The temporal frequency of these sig-
nals ranges from less than 1 Hz to over 100 Hz. The spatial resolution is lower than
that of fMRI. Sometimes, MEG is preferred over EEG because the electrical signals
measured by EEG depend on the conduction through different tissues (e.g., skull and
skin). However, EEG has much lower costs and higher equipment transportability
than MEG (and fMRI). Moreover, EEG allows participants more freedom to move
than MEG and fMRI. In Sect.
21.8
we will discuss the use of EEG to discover func-
tional brain networks. Therefore, we will focus on EEG for the remainder of this
section.
Electrical potentials generated within the brain can be measured with electrodes
at the scalp during an EEG recording. The measured EEG signals reflect rhythmical
activity varying with brain state. Specific brain responses can be elicited by the pre-
sentation of external stimuli. For EEG analysis, one often studies activity in various
frequency bands, such as alpha, beta, theta or delta bands. As a result of
volume con-
duction
, an electrical current flows from the generator in the brain through different
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