The Alpha Rhythm

When electroencephalographers sit down to review a normal study, they are usually drawn first to the prominent background alpha rhythm. As with all repeating rhythms, it is most important to note the frequency and the location of the activity, its amplitude, and its reactivity. The alpha rhythm oscillation is between 8 and 13 Hz and is most prominent over the more posterior aspects of the head. Its amplitude or voltage in a bipolar (P4-O2) derivation is anywhere from 15 to 65 ^V. This is reactive or responsive to mental activity and to eye opening or closure. The rhythm is partially or completely blocked by mental activity or by eye opening (3). Likewise, it is enhanced by relaxation and by eye closure (Fig. 1).

For healthy adults, there is a bell-shaped distribution curve for the alpha frequency, centering around 10.0 Hz. The frequency is significantly affected by cerebral blood flow and may vary by up to 2 Hz in any individual, based on flow changes. It normally does not vary by more than 1 Hz during the course of the record and shows very little change during long time spans. Although it has been reported that the alpha rhythm may slow by 1 Hz every 10 yr after the age of 50 yr, in more recent studies that controlled for subtle disease states, such changes did not appear. In a few healthy subjects (<2%), no apparent alpha rhythm can be seen. In a few more subjects (<7%), a very low-voltage alpha rhythm is observed. Because recordings are bipolar, one can improve on alpha detections by increasing the inter-electrode distances. When this is done, the finding of an absent alpha rhythm is actually quite low. The frequency may also vary slightly in relationship to the menstrual cycle, where it is faster by up to 0.3 Hz during the follicular phase. This variation is probably below the level that can be visually recognized.

The alpha rhythm is most prominent over the posterior aspects of the head. In some individuals, it is overwhelmingly occipital in its distribution. In approximately one-third of healthy adults, the alpha rhythm is widely distributed and incorporates parietal and posterior temporal regions.

Fig. 1. This recording shows the presence of a normal alpha rhythm present over the posterior head regions. Note how the alpha rhythm attenuates just before a spontaneous eye opening marked by the eye movement artifact noted over the frontal electrodes, approximately midway through the recording.

There may be a side-to-side amplitude asymmetry, noted in approx 60% of people, in which the right side tends to be of slightly higher amplitude. The left side may be more predominant in left handed individuals. The difference in amplitude between the two hemispheres is not more than 50%. If it is more than that, it is almost always related to an identifiable abnormality. The skull can act as an amplitude attenuator and, therefore, as one ages and as one's skull thickens, the amplitude of the alpha rhythm tends to diminish.

The state of the eyes is important to the alpha rhythm. The alpha rhythm is the rhythm of the awake, resting, eyes-closed individual. In most people, the alpha rhythm will be blocked or significantly attenuated when the eyes are open (Fig. 1). Similarly, mental effort, such as doing simple math, likewise blocks alpha rhythm in approximately three-fourths of healthy subjects.

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