Electroencephalographic Localization

Focal Events

A focal EEG event is one that occurs on a limited part of the brain surface rather than over the whole brain surface. Occasionally, an event may be so focal that it occurs in only one electrode. The large majority of discharges will also be detectable, though perhaps more weakly, in adjacent electrodes. The electrode position that picks up the highest voltage, be it positive or negative, is referred to as the discharge’s maximum. Although the discharge may be best seen at the maximum, adjacent electrodes often pick up varying amounts of the discharge. The hypothetical discharge shown in Figure 4-1 shows a discharge maximum in the right parietal electrode (P4) with a field that includes the right posterior temporal electrode (T6) and, to a lesser extent, the right occipital electrode (O2). A focal discharge having a broader field, such as the one shown in Figure 4-2 , can have a maximum at one electrode (C4 in this example) but involve a substantial part of one hemisphere.

Rarely, an EEG event may occur so focally that its activity is only recorded in a single electrode, such as the example of the highly focal right parietal discharge shown in Figure 4-3 . According to the schematic, the discharge would only be detected by the single electrode (P4), and adjacent electrodes would be electrically quiet. In practice, such highly focal discharges are uncommon and represent the exception rather than the rule (although this phenomenon of highly focal discharges is more commonly seen in newborns). The first two examples given above in which a discharge is detected by a group of electrodes is the more commonly encountered situation. The pattern of how strongly the discharge is picked up in various electrodes helps define the shape of the discharge’s electric field as discussed next.

Electric Fields

Several analogies have been used to describe the shape of the electric field of a typical discharge on the scalp surface and how its intensity drops off with distance from the maximum. The analogy of a pebble dropped into a quiet pool of water can be used to describe the way a simple field’s strength dissipates as it becomes more distant from the point of highest intensity (where the pebble hit the water). The wave that is formed is strongest at the point of impact but diminishes with increasing distance from the central maximum. This example is useful because many electric fields measured on the scalp do show this radially symmetric shape but, in practice, many electric fields dissipate with varying shapes.

Rather than showing a smooth and steady decrease in voltage in every direction from the central maximum point, it is possible for fields to dissipate gently in one direction and abruptly in another. A better analogy for the shape of electric fields is the visualization of mountain peaks. Mountain peaks give a more realistic picture of EEG fields because they need not be so perfectly symmetrical in shape as the circular waves caused by a pebble hitting water. Imagining the terrain around a 5,000-foot mountain peak, we might expect that the height of land surrounding the peak will fall off with varying steepness in each direction. Likewise, electric fields may manifest a steeper slope of voltage decrease in one direction and a more gentle slope in another direction. An abrupt and immediate falloff in voltage from a central point in all directions as shown in Figure 4-4 would correspond to a thin needle of land 5,000 feet high with nothing surrounding it, an uncommon finding both in geography and in electroencephalography but akin to the discharge in Figure 4-3 .

The mountain peak analogy reminds us that an electric field can be visualized as a surface in three dimensions. Just as slope refers to the steepness of a curve imagined in two dimensions at a given point, the steepness of a surface imagined in three dimensions at a given point is referred to as the gradient at that point. (The slope of a curve at a given point is the slope of a line tangent to that curve at a given point. Similarly, the gradient of a surface at a given point is defined by the slope of an imaginary plane tangent to the surface at that given point.) The rate at which the terrain that surrounds the summit of a mountain falls off is the steepness of the terrain. The rate at which an electric field changes intensity at a particular point on a surface is called the electrical gradient; the two properties are analogous. The exercise of visualizing the shapes of electric fields is similar to visualizing the contours and steepness of a region of mountain terrain. Just as we would never confuse the area of maximum altitude of a mountain with the point of maximum steepness of a mountain (which may or may not be at the same point), so we will take care not to confuse the point of maximum voltage of an EEG event with the point of the maximum gradient of the field surrounding the maximum.

The illustration in Figure 4-4 depicts a highly focal discharge, similar to that shown in Figure 4-3 . The plane of the rectangular grid is a representation of the voltages measured on the scalp surface. In this figure, areas close to the discharge’s peak are not involved, and relatively nearby electrodes would not “perceive” any change in voltage. Such highly restricted or “punched-out” discharges are relatively uncommon (just as 5,000-foot mountains in the shape of a needle are uncommon). The field shown in Figure 4-5 is somewhat more realistic, with a peak or maximum in the same position, but a more gradual falloff in voltage as distance increases from the maximum point. In this example, adjacent electrodes would pick up increasingly weaker voltages with increasing distance from the point of maximum. Figure 4-6 shows a discharge with a broad field, and, although the middle electrode would pick up the highest voltage, the adjacent electrodes would detect a voltage intensity of more than half that detected by the middle electrode. Figure 4-7 reminds us that, quite often, the electric field can slope off asymmetrically from its peak.

Generalized Events

In contrast to the focal events described earlier, some events occur in all brain areas at once and are said to occur in a generalized distribution. Figure 4-8 depicts a discharge with a perfectly even electric field. With cerebral discharges, even in the case of generalized discharges, there is almost always some unevenness to the field, and an area of maximum intensity can still be identified. In Figure 4-9 , the discharge affects all brain areas and is, therefore, generalized, but the intensity is highest in the F3, Fz, and F4 electrodes.

The purpose of this chapter is to help the reader become adept at translating the patterns of pen deflections recorded on the EEG page into the particular localizations, polarities, and the shapes of the gradients that those patterns imply. In short, we examine the patterns that EEG pens will draw when they encounter electrical fields of various shapes, including the types depicted in the figures discussed earlier. Ideally, after analyzing an EEG discharge on an EEG page, the reader will be able to imagine a “mountain range” configuration that accurately depicts the shape and the gradients of the discharge’s electric field.