Figure 1 displays the PD with a commonly used 8 channel montage (Redding and Knecht, 1984). based on 5 electrodes. Although the PD is conspicuous, the EEG does not clearly indicate whether it is focal, widespread or even possibly generalized.

This EEG was recorded from a 4 week old foal. In the center of the figure, an approximately 3 sec-duration paroxysmal discharge (PD) consisting of spike-and-wave complexes, is visible in all derivations; in some derivations the complexes are "downpointing".

The EEG is displayed in a widely-used montage in which 5 electrodes are connected to the amplifiers to form 8 derivations (Redding and Knecht, 1984) . The first 4 derivations are "common reference derivations" (i.e., all using the same reference, in this case, the vertex electrode) the last 4 are "bipolar derivations" (See Electroencephalography Electrode Map section below). (For discussion of montages and derivations, see Holliday and Williams, VNN, 1999). In this montage it appears the PD probably occurred on the right side, rostral to the right occipital electrode and caudal to the right frontal electrode. The absence of electrodes between the frontal and occipital electrodes and the absence of electrodes in more lateral sites makes it impossible to localize more precisely. (for information on "locating" spikes and waves see Holliday and Williams, 1999) Eye movements accompanied this PD, but did not regularly occur with other PD in the record. Here only the right lateral canthus (OD) electrode was functional, the left one having come loose during recording, causing excessive artifact)

The foal's behavior during this period, and the EEG background rhythms (BGR) at the time, were those of REM sleep. The BGR show the high amplitudes expected in young mammals (note display amplification, shown in calibration bar, is less than the "customary" 50 microvolts).

In Figure 2, eight electrodes were used in 3 chains extending rostrocaudally, once again displayed in 8 channels. This montage makes it possible to better localize events by forming the electrode derivations into "chains" which take advantage of "instrumental phase reversal" to help in localizing focal events (Holliday and Williams, VNN, 1999). In this montage, it is apparent the event arose focally in the right hemisphere, with highest amplitude at or near the C4 electrode. A transverse chain of electrodes might allow even more precise localization

The same epoch shown in Figure 1 is shown here after remontaging. Here the PD appears to consist of a ~3 sec burst of 6/sec spike-and-wave complexes that is most clearly defined in the C4-P4 derivation, but is obvious in others (some are downpointing).

Here the EEG is displayed in an 11-electrode montage, still using only 8 channels. The electrodes are arranged in 3 rostrocaudal chains: one 4-electrode chain on the left side, a 3 electrode chain on the midline, and a 4-electrode chain on the right side. (See Electroencephalography Electrode Map section below). The amplitudes suggest the PD occurred on the right side. A phase reversal at C4 suggests the focus is near C4 (compare channels 6 and 7). The use of additional electrodes has improved localization somewhat. However, derivations forming transverse chains across the hemispheres could be helpful in clarifying the laterality of the event. (See Figure 3)

In Figure 3, the montage used in Figure 2 has been modified by adding a chain of 3 electrodes that extends from the left side, across the midline and onto the right side, at right angles to the midline and passing through the vertex (Cz). The occipital electrodes have been omitted so that an 8 channel display is still possible. The extent and focal nature of the event are more strongly emphasized; however, it could have occurred some unknown distance lateral to the C4 electrode.

This figure shows the same epoch as that in figures 1 and 2. Remontaging has omitted the occipital electrodes and added a short transverse chain (C3-Cz; Cz-C4, channels 7 and 8)). There is phase reversal of the spike-and-wave complexes at the C4 electrode in the rostrocaudal chain (compare F4-C4 and C4-P4, channels 5 and 6). In the transverse chain, the spikes and waves are downpointing at C3-Cz (channel 7) but at Cz-C4, (channel 8) directionality is equivocal, with phase reversal of spikes but uncertain direction of the waves. An electrode lateral to C4 possibly could be helpful in interpretation. (See Electroencephalography Electrode Map section below).

In Figure 4, a more complete array of electrodes is displayed. This montage uses all the electrodes used in the previous montages plus electrodes in the temporal regions (A1, A2). It comprises 3 rostrocaudal longitudinal chains and 4 left-to-right transverse chains, the latter in rostral, central, parietal and occipital locations. Even more precise localization is possible (Figure 4).

The same epoch is shown here in a montage containing 17 EEG channels formed by 13 electrodes connected in three rostrocaudal chains: left (channels 1-3);, midline (channels 4,5) and right (channels 6-8) and 4 transverse chains: frontal (channels 9, 10); central, extending from the left temporal region to the right temporal region (channels 11-14) ; parietal (channels 14, 15); occipital (only 2 electrodes, channel 17). (See Electroencephalography Electrode Map section below).

Amplitude differences and phase reversals at the C4 electrode in the rostrocaudal chain on the right side indicate the PD is affecting the C4 electrode more than others in that chain. In the transverse central chain the downpointing spikes and waves at C3-Cz indicate Cz is affected more than C3. The "up-pointing" complexes in the C4-A2 derivation (channel 14) indicate the event affected the C4 electrode more than A2. At Cz-C4, as in Fig 3, the PD is still of equivocal direction. The event probably occurred where it affected both electrodes about equally. Importantly, though it is visible in all channels, the PD does not appear to be generalized.

Figure 5A shows an epoch of EEG displayed with a high frequency limit of 70 Hz. The recording is heavily laden with muscle artifacts, consisting of some low amplitude, steadily ongoing muscle potentials plus bursts of high amplitude, high frequency muscle potentials that occurred when the horse moved its mandibles and/or its ears.(Figure 5A)

This EEG epoch from a horse is heavily laden with ongoing high-frequency activity consisting of muscle potentials, e.g., channel 4. Superimposed on this in most channels are bursts of much higher amplitude high-frequency muscle potentials which occurred in association with ear movements and/or movements of the mandibles. Some low amplitude delta activity at about 1/sec is evident but it seems possible other slow activity is obscured by the artifacts.

The identical epoch is shown in Figure 5B except the high frequency limit has been reduced to 15 Hz. The low amplitude muscle potentials have been removed or greatly attenuated and even the high amplitude bursts are greatly reduced; this allows one to better appreciate the ongoing delta activity in the background rhythms (Figure 5B) (Readers please note: we do not advocate using such poor data in diagnosis, with or without lowering the high frequency sensitivity; we merely selected this as an extreme example.)

Figure 5B. The same epoch shown in Figure 5A is shown here after changing the high-frequency limit to 15 Hz. The artifacts have been nearly eliminated, allowing better visualization of the low frequency content of the recording. Note the delta and theta activity that was obscured or invisible before changing the frequency sensitivity. (See Electroencephalography Electrode Map section above).

"Filtering out the muscle" may appear to be "the best of all worlds", but to avoid errors of interpretation it is essential to recognize the changes filtering can induce: the "improvements" made by changing the high frequency limit are not without cost. In Figure 6, the upper trace (A) is displayed with the high frequency limit set at 70 Hz. In the lower trace (B) the high frequency limit is set at 15 Hz. Note that the spikes of the spike-and-wave complexes are obliterated or greatly attenuated and "blunted" by the filtration. This could result in failure to recognize spikes and misidentification of spike-and-wave complexes, especially if they occurred as isolated events rather than in conspicuous trains such as those in the figure (Figure 6). Take care in using filters!

A spike-and-wave complex displayed at 70 Hz high-frequency limit in the upper trace and at 15 Hz in the lower trace. Note that small spikes, such as those at A in upper trace, are eliminated or greatly attenuated at 15 Hz in lower trace (B). Larger spikes in the last 2 sec of the event are still present at 15 Hz but are smaller and not as sharp as when displayed at 70 Hz. This could lead to errors of interpretation if the frequency limits are not considered.