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The use of filters in recording and displaying EEG data is an indispensable tool in producing interpretable EEG tracings. Without filters, many segments of EEG would be essentially unreadable. As we shall see in this chapter, the use of filters can affect the EEG signal in ways that range from the subtle to the dramatic. The main benefit of filters is that they can appear to “clean up” the EEG tracing, making it easier to interpret and generally more pleasing to the eye. Certain filter settings can also be used to accentuate particular types of EEG activity. Filters can, however, be used improperly, and at times their use can lead to unintended consequences.
Some consider the study of how filters work an inherently dry topic. The purpose of this chapter is to provide a simple overview of how EEG filters work so that they can be used appropriately by the EEG technologist and reader. There also follows a brief discussion of simple circuit design for analog EEG filters, a topic that has traditionally been a part of electroencephalography training. The basis of some of the techniques used to filter digital EEG signals is also introduced. Although detailed knowledge of filter design is not necessary to interpret EEGs, understanding the circuitry or algorithms used to build these filters can enhance understanding of filter behavior and increase the level of sophistication of EEG reading.
Figures 7-1 and 7-2 illustrate the impact of filters on an EEG page. Figure 7-1 shows an EEG recorded during a moderate amount of patient movement, a “raw” EEG trace displayed without the explicit use of filters. Figure 7-2 shows the same page displayed with typical filter settings. Note that, despite the fact that muscle artifact still obliterates portions of the top four and bottom four lines of the EEG (the temporal areas), in the filtered example, the amplitude of that muscle artifact is reduced, making it easier to see adjacent channels. Indeed, in the filtered example, the presence of certain waveforms can be intermittently recognized within the areas of muscle artifact (this artifact is generated by contraction of the temporalis muscles) that otherwise would not have been detectable. Also note that the baseline of each channel is flatter, allowing for easier interpretation—each channel is more likely to stay within its own horizontal area after the filters are used.
There are also potential pitfalls in choosing filter settings. When using filters on a page of EEG that has a cluttered appearance, one might think that if a given filter setting works moderately well, then even more aggressive settings might work even better. With filters, however, the strategy of “more is better” often does not hold true because implicit to the act of filtering the EEG signal is the potential loss of information. Overzealous filter settings can overly “clean” the EEG, resulting in the filtering out and disappearance of waveforms that may be of interest to the reader. As we will see, some filter settings can change the shape of brain waves in a way that might suggest the presence of waveforms that are not really there.
The most ideal filter design would be one that removes all of the electrical noise or artifact from the EEG and only allows true cerebral activity to pass through. Unfortunately, no such “smart” filters exists; filters can only remove waves according to rigid mathematical rules. Luckily, there are good rationales for filtering out certain components of EEG signals using fairly simple mathematical assumptions. These assumptions are based on the idea that the brain only generates EEG waves within a certain range of frequencies and that any activity outside that range (unusually slow activity and unusually fast activity) is not likely to be of cerebral origin. Indeed, one of the general assumptions of EEG filter design is that activities well below 1 Hz and well above 35 Hz do not arise from the brain and likely represent electrical noise or artifact. Like many assumptions, this claim is mostly true but not completely true, as we shall see. On the basis of the concept that the frequency of almost all brain electrical activity of interest lies within a particular bandwidth, EEG filters are typically set up so that one filter rejects the majority of very high-frequency activity and another filter rejects the majority of very low-frequency activity. The range of frequencies between these unwanted high and low frequencies that is allowed to pass through the filter setup is referred to as the bandpass . The way that different filter setups are associated with different bandpasses is illustrated here.
Rather than being used to reject spurious activity, occasionally filtering techniques can be used to bring out certain EEG activity that might otherwise have been hidden in other, higher voltage activity. In this application, the electroencephalographer may purposely attenuate the slow activity in a record (even though it represents true cerebral activity) to accentuate or “bring out” fast activity that would otherwise be lost in high-voltage slow waves. Examples of such special filtering techniques (which are not necessarily used during every EEG reading session) are illustrated in Figures 7-3 and 7-4 . These figures show how aggressive use of the low-frequency filter can be used to bring out the presence of spike-wave discharges.
The three commonly used filter types in clinical EEG are low-frequency filters, high-frequency filters, and notch filters. The purpose of a low-frequency filter is to filter out low-frequency activity and to leave higher frequencies as they are. Because low-frequency filters attenuate low frequencies and allow high frequencies to “pass through,” engineers often refer to low-frequency filters as high-pass filters . Likewise, high-frequency filters are designed to filter out high-frequency activity and allow low-frequency activity to pass through and are sometimes referred to by engineers as low-pass filters. Although the use of the terms high pass and low pass to name filters is more common in the world of electrical engineering, these are not the preferred terms in clinical electroencephalography. In the world of clinical EEG, the alternate terms high-frequency filter (HFF—filters out the high frequencies) and low-frequency filter (LFF—filters out the low frequencies) are used, with the terms high filter (HF) and low filter (LF) sometimes used as shorthand abbreviations. Thus, HF is synonymous with HFF and LF is synonymous with LFF.
The notch filter is the third type of filter. Its purpose is to filter out activity at a specific frequency (rather than a frequency range). Because the alternating current in standard electric outlets in North America oscillates at 60 Hz, electric fields produced by the 60-Hz activity in the environment that surrounds us in our indoor environments frequently contaminates the EEG. Sixty-hertz notch filters (filters designed specifically to filter out 60-Hz activity) are used to attenuate or eliminate this unwanted signal. In countries where line frequencies are 50 Hz, 50-Hz notch filters are used for the same purpose.
There are two different naming schemes for high- and low-frequency filters. A filter can be named after a frequency (e.g., a “5-Hz low filter”) or after its time constant (e.g., a “low-filter with time constant of 0.1 seconds”). When a filter is named after a particular frequency, this is referred to as the nominal frequency or the cutoff frequency of the filter. Whether the filters on a particular EEG machine are named according to a cutoff frequency or a time constant is the decision of the manufacturer. Because referring to filters by their cutoff frequencies is becoming more common, and also because cutoff frequencies are easier to understand, we discuss the relationship between a filter’s electrical characteristics and its cutoff frequency first.
The term cutoff frequency conjures up the image of an all-or-nothing effect at the frequency named. For example, a class may have a particular cutoff grade for passing or failing—one point below the cutoff grade, and the student does not go on. An amusement park may have a particular height cutoff to go on certain rides—all individuals below that height are excluded from the ride. The behavior of low-frequency filters in terms of their cutoff frequencies is not at all so absolute as the behavior of classroom teachers or amusement park officials. In fact, it may be surprisins to learn how little a filter affects activity at its cutoff frequency. When a low-frequency filter encounters a sine wave that happens to be exactly at its cutoff frequency, it cuts down the amplitude of that wave by approximately 30%. Sine waves at frequencies somewhat below that frequency are reduced by somewhat more than 30%—the farther the wave’s frequency is below the filter’s nominal frequency, the more it is attenuated. Perhaps more surprising, sine waves at frequencies somewhat above the cutoff frequency are also reduced in size by the filter, although by somewhat less than 30%. Again, the more the sine wave’s frequency exceeds the low-frequency filter’s nominal frequency, the less it is affected by the filter.
When standard low frequency filter settings such as a cutoff frequency of 1 Hz or below are used for the low-frequency filter, the main effect is to help keep each EEG channel within its own horizontal area, eliminating large drifts upward or downward into the space of other channels. This is because this baseline drifting actually represents a very low frequency wave. More aggressive use of the low-frequency filter (higher cutoff frequencies such as 3 Hz or 5 Hz) initially begins to attenuate delta frequencies and, when even higher cutoff frequencies are used, may almost completely eliminate some slow activity, sometimes with the advantage of bringing out other features in the EEG (see Figures 7-5, 7-6, 7-7, 7-8, and 7-9) . Examples of how different low-frequency filters might affect a simple trace of the posterior rhythm are shown in Figure 7-10 . Note that when successively more restrictive settings are used for the low filter, the baselines of each channel become straighter, but faster activity is relatively preserved.
The graph in Figure 7-11 illustrates how a 5-Hz LF would handle sine waves of varying frequencies. The curve describes what portion of a pure sine wave (y axis) at a given frequency (x axis) would be allowed to pass through the filter. Considering the example of this 5-Hz low-frequency filter in more detail, the curve shows that a 5-Hz sine wave presented to this filter will lose 30% of its amplitude after passing through the filter. (Why the amount of reduction at the cutoff frequency is specifically 30% is explained later.) If the original 5-Hz wave presented to the filter has an amplitude of 100 μV (the input wave), then the filter’s output wave would only have an amplitude of 70 μV. What does the 5-Hz LF do with waves just above and just below 5 Hz? The roll-off curve for this filter shown in this figure indicates that a 4-Hz curve would be attenuated by 33%, but a 6-Hz sine wave would only be attenuated by 26%. The type of curve shown in Figure 7-11 that shows how a given filter processes pure sine waves of different frequencies is called the roll-off characteristic of the filter. Figure 7-12 shows how a 5-Hz LFF handles input waves of 10 Hz, 5 Hz, 2 Hz, and 0.5 Hz of the same amplitude, attenuating the lowest frequency waves dramatically but only causing a mild reduction in the amplitude of the 10-Hz wave. The exact amount of reduction at each frequency is given by the roll-off characteristic shown in the previous figure. Figure 7-13 shows the roll-off characteristics of 0.1-, 1-, 5-, and 10-Hz low filters.
Of course, when filters are applied to real EEG signals the waves presented to the filter consist of mixed frequencies. The filter attenuates each frequency component of the waves according to the rule of the roll-off characteristic even in wave mixtures. Low-frequency filters are especially useful for filtering out certain artifacts caused by patient motion that might shift a channel’s baseline or other sources of low-frequency noise.
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