Technical Tips: Identification of Focal Slowing in EEG


Author: A. Todd Ham, BS, R. EEG T., CLTM

In anticipation of the new ABRET EEG Neuroanalyst credential being formally introduced in the near future (which I am proud to have had the honor of helping to construct), I am going to discuss an analytical EEG skill that many say is one of the most difficult to acquire.  Focal slowing can be more challenging to identify for reasons including overlying faster frequencies which obscure it and the default timebase not being ideal for such a slow frequency.  But essentially, it is due to the inherent subtlety of the associated EEG slow waveforms.  Paroxysms such as spikes, spike-and-waves, and lateralized periodic discharges (LPDs) more obviously contrast against the adjacent background, including within the very channel (electrode pair) that we see them.  With focal slowing, we are looking at how the particular region contrasts against the other regions of the brain.

Figure 1

Historically, I’ve referenced The Matrix movies to analogize the skill required for properly identifying focal slowing.  It was explained to Neo, the protagonist, that what he perceived as chaotic green ‘1’s and ‘0’s flickering down and across the monitor were to the more acclimated character the equivalent of a video feed of “real” life, as it existed in the Matrix realm.  So, the more EEGs that one reviews, the more developed their analytical prowess will be. 

In addition to manipulating the timebase, there are other actions that can be performed to accentuate slowing, including lowering the low frequency (high pass) filter to accentuate slower activity.  Figure 1 (top) provides an example of focal [left anterior temporal] intermittent slowing.  Visualize a car which is about 30 millimeters (mm) in length (it just fits between two successive vertical green lines) that is traveling from left to right on our EEG road (EEG channel).  Try picturing a car which is even smaller (Figure 1 bottom) – perhaps 500 milliseconds (ms) in duration.  Note that the magnitude of path disruption increases relative to a smaller car.  Which channels would be likely to cause the car to bottom out?  Which channels would only be felt by the driver as a bumpy, but nonetheless drivable, road? 

The grey boulders are strategically placed over some of the delta waves to highlight the activity.  Smaller “pebbles” over the midline chain, and even over the T5 channel, are not representative of slowing but rather normal alpha and theta-range activity (perhaps midline theta rhythm along with posterior dominant rhythm). Note that A1 is contaminated; this is why we see similar activity, though of opposite phase, in the other channels.  Imagining some analogy other than a car may serve you better.  Flowing water which follows the contours?  A runner who will trip over the big “humps”? 

Though slow rolling eye movements of drowsiness are also better appreciated with a compressed timebase (and decreasing the low frequency filter), cortical slowing tends to have a field; that is, the slowing should cogently extend across multiple adjacent electrodes in a sensical distribution, just like spikes and sharp waves.   The bifrontal “slowing” artifact associated with drowsiness has an artifactual field that is confined to F7 and F8; the polarities are also opposite between F7 and F8 as the positively charged cornea slowly moves in the horizontal plane back and forth. 

In our first example, we see that the top 3 channels all captured the same general signature of the slowing, though perhaps more prominent in the F7-T3 and T3-T5 channels.  For an additional example, take a look at Figure 2 A. and compare the left temporal chain to the right temporal chain; note that the montage channels are arranged so that the left temporal chain is adjacent to the right temporal chain (as are the parasagittal chains).  Such an arrangement can help articulate focal slowing as it provides a method to contrast two homologous regions side-by-side.  As is often true with so many things, we can assign properties for something based on how it relates to something else.  Where would the red lined-tracings best fit over?  The left temporal chain, right temporal chain, left parasagittal chain, right parasagittal chain, or midline chain?  Figure 2 B. shows that the best location to superimpose the red-lined slowing is over the right temporal chain.  When the timebase is changed to 15 mm/sec., we can better appreciate the right temporal slowing (Figure 2 C.).

Figure 2 A.
Figure 2 B.
Figure 2 C.

Such compression of the viewable epoch is analogous to a roller coaster (Figure 3 A. and B.).  Imagine you are standing very close to a large roller coaster; you look up and while you can appreciate some curvature, the overall pattern of the roller coaster track is difficult to see because you are too close.  Now imagine backing up, backing up, backing up more and more until you start to see enough of the roller coaster’s contours filling your field of vision so that the overall pattern is appreciable.

Figure 3 B.

Figure 3 A. and B.  Note that “zooming out”, or lowering the timebase, has no effect whatsoever on the amplitude, or height of the roller-coaster (or brain waves).  The exact same concept applies to EEG timebases in relation to waveform recognition.  If slow waves are the object of inquiry, then the timebase should be lowered (compressed).  Lowering a timebase (compressing it) equates to lowering the numerical distance represented by 1 second and “zooming out” – in the roller coaster example, the horizontal dimension is simply the scale (i.e., 30 feet/inch).  Imagine that the roller-coaster track represents some delta waves of an EEG.

For emphasis, if we trim the high frequency filter to 5 Hz, thus significantly attenuating all activity 5 Hz and above; and decrease the low frequency filter to 0.5 Hz, thus accentuating all slow activity 0.5 Hz and above, there is a striking difference when comparing that slow right temporal chain to the left temporal chain, especially in the compressed timebase setting (Figure 4).

Figure 4. Low frequency filter = 0.5 Hz; high frequency filter = 5 Hz.

In conclusion, the most important thing that can be done to improve the ability to recognize focal slowing is to review several EEGs; the more EEGs reviewed, the easier it will be to see the slowing.  Sitting alongside an attending epileptologist during EEG review is also very beneficial.

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