Biomedical Engineering Reference
In-Depth Information
As beautifully described by the late Francisco Varela [6], synchronization in the
brain can occur at different scales. For example, the coordinated firing of a large
population of neurons can elicit spike discharges like the ones seen in Figure 4.1(b,
c). The sole presence of spikes in each of these signals—or oscillatory activity as in
the case of the signal shown in Figure 4.1(a)—is evidence for correlated activity at a
smaller scale: the synchronous firing of single neurons.
The recordings in Figure 4.1 are from two intracranial electrodes in the right
and left frontal lobes of male adult WAG/Rij rats, a genetic model for human
absence epilepsy [7]. Signals were referenced to an electrode placed at the cerebel-
lum, they were then bandpass filtered between 1 and 100 Hz and digitized at 200
Hz. The length of each dataset is 5 seconds long, which corresponds to 1,000 data
points. This was the largest length in which the signals containing spikes could be
visually judged as stationary.
As we mentioned, spikes are a landmark of correlated activity and the question
arises of whether these spikes are also correlated across both hemispheres. The first
guess is to assume that bilateral spikes may be a sign of generalized synchronization.
It was actually this observation done by a colleague that triggered a series of papers
by the author of this chapter showing how misleading it could be to establish syn-
chronization patterns without proper quantitative measures [8]. For example, if we
are asked to rank the synchronization level of the three signals of Figure 4.1, it seems
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Figure 4.1
Three exemplary datasets of left and right cortical intracranial recordings in rats. (a) Nor-
mal looking EEG activity and (b, c) signals with bilateral spikes, a landmark of epileptic activity. Can
you tell by visual inspection which of the examples has the largest and which one has the lowest syn-
chronization across the left and right channels?
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