The amplitude may vary over a wide range, from a few microvolts to several hundred microvolts. Generally, alpha rhythm (8 to 13 Hz posterior dominant rhythm) in adults is usually less than 100 µV. Most of the alpha amplitude in adults falls within the 40 to 100 µV range and is higher (100 to 200 µV) in children. Abnormal activity could reach as high as 1,000 µV. An example of this is hypsarrhythmia, which is characterized by extremely high-amplitude slow waves, seen in children with infantile spasms or West syndrome (see “Infantile Spasms, Salaam Spasms, West Syndrome,” Chapter 10; see also Figs. 10-25A and B, 10-26, and 10-27A and B). Generally speaking, faster frequency activity has lower amplitude, and slower frequency activity has higher amplitude.

It should be noted that the amplitude measured in a bipolar recording does not reflect the true (absolute) amplitude of activity from either electrode; one should always keep in mind that the EEG records the amplitude difference between the two electrodes. A more accurate amplitude measurement may be made by using a referential derivation, assuming that the reference electrode is relatively “inactive.” The amplitude of the alpha rhythm from the occipital electrode is thus measured more accurately, for example, from O1 to A1 than from O1 to T5 or O1 to P3 (see Fig. 7-1A and B). With an ipsilateral ear reference recording, however, T3 and T4 amplitudes are erroneously low because of a cancelation effect due to the short interelectrode distances between T3 and T4 and their respective ipsilateral ear electrodes (see Fig. 7-1A). Conversely, frontocentral alpha activity will show lower amplitude on a bipolar than a referential recording (see Fig. 7-1A and B). This occurs because of the cancelation effect from electrodes having similar activities, such as between Fp1 and F3.

An amplitude asymmetry between homologous electrodes in a bipolar montage may be due to technical problems rather than a true amplitude asymmetry. These technical problems may include an erroneously short interelectrode distance (see Fig. 15-48A and B), a salt bridge (shorting of two electrodes by excessive electrode paste), or mistakenly recorded parameters, such as sensitivity or filter settings (see Figs. 15-32A and B and 15-33A and B). Depression of amplitude noted in a bipolar derivation, especially if only in one channel, must be confirmed by a referential recording (see “Technical Pitfalls and Errors,” Chapter 15; see also Fig. 15-47A and B).

The frequency of EEG activity is conventionally classified as follows:

delta waves < 4 Hz, theta waves 4 to 7.5 Hz, alpha waves 8 to 13 Hz, beta waves 14 to 30 Hz, and gamma waves 30 to 80 Hz.

The alpha rhythm is specifically designated as the posterior background rhythm and is attenuated by eye opening (see Fig. 7-1A and B). Most normal adults have an alpha rhythm faster than 9 Hz during the awake state. An alpha rhythm consistently at or less than 8 Hz during the awake state is abnormally slow at any age except in children less than 3 or 4 years old. The frequency may be measured or estimated by counting the number of waves occurring within 1 second, as long as waves with the same frequency appear repeatedly. If the appearance of waves is sporadic or a series of waves is irregular, the frequency must be determined by measuring the duration, which can be converted to the frequency (see Figs. 2-6 and 2-7).

Many EEG patterns have characteristic waveforms. Alpha rhythm usually appears as a repetition of similar waveforms or rhythmically recurring waves, with waxing and waning amplitude changes, but not exactly sinusoidal. Alpha activity in the central electrodes usually has a different waveform than the posterior alpha rhythm. This has a pointed negative peak followed by a rounded positive phase forming a wicket-shaped
appearance often referred to as “mu rhythm” (synonyms: wicket rhythm, comb rhythm, and rythme en arceau) (Fig. 6-1; see also Figs. 7-18 and 7-19A and B). Sometimes, posterior alpha rhythm may also appear as mu-shaped form when associated with beta activity (Fig. 6-2). This occurs because mu waveform is a mixture of alpha and beta activities.

Slow waves, usually consisting of delta activity, are categorized into two forms. One is serial rhythmic (or monomorphic) or intermittent rhythmic delta activity (IRDA), which consists of a series of slow waves having similar wave form, amplitude, and frequency. An example of this is FIRDA (frontal intermittent rhythmic delta activity) or frontally predominant RDA^* often seen in patients with diffuse encephalopathy (Fig. 6-3; see also Fig. 8-14). Another example of IRDA is OIRDA (occipital intermittent rhythmic delta activity) or occipitally predominant RDA^*, which is commonly seen in children, especially in patients with absence seizure (see “Intermittent rhythmic delta activity,” Chapter 8; see also Fig. 8-15; see also Video 10-6).

The other form of slow waves is serial irregular (or polymorphic), which consists of a series of slow waves having different amplitude, frequency, or waveform (Fig. 6-4). This pattern is often seen as a localized (focal) slow wave in patients with a focal brain lesion or pathology (see “Focal Delta Activity,” Chapter 12).

Transients and bursts are paroxysmal activities that appear and disappear suddenly and are clearly distinguishable from the ongoing and sustained EEG activities. A transient is a single wave consisting of a brief mono-, bi-, or triphasic waveform. A burst is a group of mixed waves that could be either a stereotyped sequence of waves (monomorphic, monorhythmic, or serial rhythmic) or a mixture of various waveforms (polymorphic or serial irregular). Examples of transients are spike or sharp discharges, which are defined as sharply contoured waves (distinguishable from the background activity) with waveform duration of less than 70 ms for spike (Fig. 6-5A) and 70 to 200 ms for sharp waves (Fig. 6-5B). The spike discharge has higher correlation with seizure diagnosis than the sharp discharge. Some sharp discharges show a waveform having a steep declining phase, which are considered to be “spike-equivalent” potentials, and have equal clinical significance as spike discharge (Fig. 6-5C).