Biomedical Engineering Reference
In-Depth Information
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Figure 1.7
Alpha rhythm (potential) waveforms recorded along the midline with data transformed
to the average reference using (1.7). Nine out of a total of 111 channels (electrodes) are shown. The
two dashed lines on each waveform indicate fixed time slices and show phase differences recorded
from different locations. The sources appear to originate from very large dipole layers, perhaps an
anterior-posterior standing cortical wave. (
From:
[2]. © 2006 Oxford University Press. Reprinted with
permission.)
channels of averaged reference potentials recorded in the eyes-closed resting state.
Two dashed vertical lines are drawn on each waveform to indicate common time
slices. At these instances electrode sites over occipital cortex (electrodes 1, 2, and 3)
yield a peak positive potential of the alpha rhythm, while electrode sites near the
frontal pole (electrodes 8 and 9) show a peak negative potential at the same time
slice. Such 180
phase shift between anterior and posterior regions is often observed
in alpha rhythm dynamics. The back-to-front spatial distribution of the alpha
rhythm suggests very low spatial frequency source activity (associated with large
dipole layers), with minimum potential magnitude at electrodes near the vertex. A
number of other theoretical and experimental studies suggest that these layers con-
sist of standing waves of cortical source activity along the anterior-posterior direc-
tion, perhaps corresponding to a global cortical\white matter fundamental mode of
oscillation [2, 4, 16-18].
Figure 1.8 shows the spline Laplacian for identical data at these same 9 midline
electrodes (estimated using all 111 electrode sites). The Laplacian oscillations are
largest at electrode sites 5 and 6 near the vertex. The waveforms differ at these elec-
trodes, with a peak occurring at the first time slice in electrode 6 with no corre-
sponding peak at electrode 5. The electrode sites over occipital and frontal areas
show the strongest alpha signal in the potential plots, but much smaller amplitude
Laplacian signals.
Similar differences between potential and Laplacian waveforms were observed
in other scalp locations (not shown), again indicating the simultaneous presence of
both small and large dipole layers. Occipital cortex, for example, showed several
regions (off midline) with large Laplacians and other regions with small Laplacians.
Other studies of alpha spectra and coherence measures indicate that the low spatial
frequency (large dipole layer) alpha activity typically tends to oscillate near the low
end of the alpha band (near 8 Hz). By contrast, the more local alpha activity often
°
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