Answer
Normal beta activity has the following characteristics:
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Frequency (by definition) greater than 13 Hz - Common 18-25 Hz, less common 14-16 Hz, and rare 35-40 Hz [2]
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Location - Mostly frontocentral but somewhat variable; some describe various types according to location and reactivity: generalized, precentral, and posterior
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Morphology - Usually rhythmic, waxing and waning, and symmetric
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Amplitude - Usually range of 5-20 mV
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Reactivity - Most common 18- to 25-Hz beta activity enhanced during stages I and II sleep and tends to decrease during deeper sleep stages; central beta activity may be reactive (attenuates) to voluntary movements and proprioceptive stimuli; in infants older than 6 months, onset of sleep marked by increased beta activity in central and postcentral regions [3]
A sample of awake EEG showing the normal or usual amount of beta activity. As shown here, beta activity is often easier to identify during relaxed wakefulness or early drowsiness.
This is the normal amount of beta activity, frontally predominant, with waxing and waning amplitude.
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A 10-second segment showing a well-formed and well-regulated alpha rhythm at 9 Hz. Note that it is very regular, rhythmic, waxing and waning, and posterior dominant. The contrast between the first and second halves of the page illustrates the reactivity of a normal alpha rhythm, with attenuation upon eye opening.
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Fleeting alpha. At times, as shown here, the alpha rhythm can be identified only in very brief bursts and often immediately after eye closure. If normal in frequency, this is normal.
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This is an example of an alpha rhythm with a wider distribution than is typical. If frequency and reactivity are normal, this is another variation of normal. A similar EEG pattern can be seen in patients in a coma (ie, alpha coma), but in these situations it is usually unreactive.
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This is an example of "slow alpha variant." The patient's alpha rhythm at 12 Hz is seen in the second half of the sample. The first half shows a subharmonic at half that frequency, and this is the "slow alpha variant."
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A sample of awake EEG showing the normal or usual amount of beta activity. As shown here, beta activity is often easier to identify during relaxed wakefulness or early drowsiness.
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Mu rhythm over the left (greater than right) central region. To be absolutely certain that this is a mu rhythm, reactivity should be tested. However, morphology (not absolutely typical but fairly so), frequency, and distribution strongly suggest that this is a mu rhythm.
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An example of a typical normal alpha rhythm, showing clear attenuation upon eye opening (second half of page).
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This is the normal amount of beta activity, frontally predominant, with waxing and waning amplitude.
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Alpha rhythm with somewhat "spiky" or sharply contoured morphology. When fragmented (eg, in drowsiness), this can be misinterpreted as sharp waves.
