Current Practice of Clinical Electroencephalography by Ebersole John S. & Husain Aatif M. & Nordli Douglas R

Author:Ebersole, John S. & Husain, Aatif M. & Nordli, Douglas R.
Language: eng
Format: epub
Publisher: LWW
Published: 2014-04-10T04:00:00+00:00

Figure 12.3: Left: Scalp EEG of a right temporal spike with a horizontal, radial field orientation. Cursor denotes 0 millisecond latency. Top right: Sequential voltage topography of the spike. Note stable shape of the fields over 40 milliseconds. Bottom right: Single and moving dipole models of the spike. Note that voltage field stability and tight cluster of the moving dipole model suggest a simple source for which a single-dipole model is appropriate.

Figure 12.4: Left: Scalp EEG of a left temporal spike with an elevated, combined radial/tangential field orientation. Cursor denotes 0 millisecond latency. Top right: Progressive change in the sequential voltage topography of the spike is evident. Bottom right: Single and moving dipole models of the spike. Single dipole at spike peak is oblique in orientation, while moving dipole model shows a progression in dipoles from vertical tangential to horizontal radial, suggesting propagation from base to lateral temporal cortex.

The direction of dipole movement can convey useful information about the likely path of spike propagation, as long as it is simple and unidirectional. Moving dipoles that spiral in a loop or make sudden turns in direction or position are usually the result of fields created by superposition of early source repolarization and later source depolarization or several different asynchronous sources. Therefore, one-dipole solutions can be misleading, particularly at later latencies when propagation is likely to have occurred, resulting in increased source complexity. Several investigators have concluded that modeling the rising phase of the spike is more likely to represent the initial spike source (2,3,19,20). Obviously, there is a trade-off because the earlier time points typically have lesser signal-to-noise (S/N), which may make modeling less accurate. Averaging closely similar spikes or sequential ictal waveforms can improve the S/N, which will provide a more confident solution (21).

Spatiotemporal Multiple-Dipole Model

Although the moving single-dipole model will characterize simple propagation, it is overly simplistic to think that small brain regions are activated briefly in sequence. It is more reasonable that adjacent cortical areas are activated for a longer period, but not synchronously. Source modeling with single instantaneous dipoles cannot take into consideration such an overlap of activity of multiple generators, nor can it decompose the voltage fields produced by this superposition. Spatiotemporal source modeling is, however, an approach based on this rationale (22–24). Dipoles are fixed in location and orientation but can vary over time in strength and polarity to explain the temporal evolution of a voltage field. Sufficient degrees of freedom are obtained to calculate multiple dipoles from this positional constraint and from modeling over several time points. The solution for a given data set reveals not only a best-fit location for the model, but also the putative activity of each dipole over time in the form of a source potential (Fig. 12.5).

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