Visualization in Connectomics - Scientific Visualization: Uncertainty, Multifield, Biomedical, and Scalable Visualization - page 211

tissues (e.g., brain, skull, skin) to a recording electrode on the scalp. The measured
EEG is mainly generated by neuronal (inhibitory and excitatory) postsynaptic poten-
tials and burst firing in the cerebral cortex. Measured potentials depend on the source
intensity, its distance from the electrodes, and on the conductive properties of the
tissues between the source and the recording electrode.
Several visualization methods are applied to assist in the interpretation of the
EEG [ 93 ]. In a conventional EEG visualization, the time-varying EEG data are rep-
resented by one time series per electrode, displaying the measured potential as a
function of time. Synchronous activity between brain regions is associated with a
functional relationship between those regions. EEG coherence, calculated between
pairs of electrode signals as a function of frequency, is a measure for this synchrony.
A common visualization of EEG coherence is a graph layout. In the case of EEG,
graph vertices (drawn as dots) represent electrodes and graph edges (drawn as lines
between dots) represent similarities between pairs of electrode signals. Traditional
visual representations are, however, not tailored for multichannel EEG, leading to
cluttered representations. Solutions to this problem are discussed in Sect. 21.8 .
21.4.2 MRI
Inmagnetic resonance imaging, orMRI, unpaired protons, mostly in hydrogen atoms,
precess at a frequency related to the strength of the magnetic field applied by the
scanner. When a radio-frequency pulse with that specific frequency is applied, the
protons resonate, temporarily changing their precession angle. They eventually regain
their default precession angle, an occurrence that is measured by the scanner as
an electromagnetic signal. By applying magnetic field gradients throughout three-
dimensional space, protons at different positions will precess and hence resonate at
different frequencies, enabling MRI to generate volume data describing the subject
being scanned.
21.4.2.1 Diffusion-Weighted Imaging
Water molecules at any temperature above absolute zero undergo Brownian motion
or molecular diffusion [ 23 ]. In free water, this motion is completely random, and
water molecules move with equal probability in all directions. In the presence of
constraining structures such as the axons connecting neurons together, water mole-
cules move more often in the same direction than they do across these structures.
When such a molecule moves, the two precessing protons its hydrogen nucleus con-
tains move as well. When this motion occurs in the same direction as the diffusion
gradient q (an extra magnetic field gradient that is applied during scanning) of a
diffusion-weighted MRI scan, the detected signal from that position is weakened.
By applying diffusion gradients in a number of different directions, a dataset can be