Chapter 1: Normal and Benign Variants

Normal Alpha Rhythm and Squeak Effect. An alpha rhythm appears immediately after eye closure and disappears with eye opening. Immediately after eye closure, alpha frequency may be accelerated for 0.5–1 sec. Therefore, alpha frequency assessment should not be done during this period. This is called the “squeak effect.”¹

Alpha Rhythm in Subdural EEG. Subdural recording shows the alpha rhythm in the right occipital lobe with reaction to eye opening and eye closure. Harmonic of the alpha rhythm is frequently seen in an intracranial EEG. In addition, alpha rhythms usually are sharper in morphology because the scalp and skull act as a high-frequency filter and pass lower frequencies more efficiently than higher frequencies.² All the normal EEG rhythms seen in the scalp EEG can be seen in the intracranial EEG.³

Beating (Waxing and Waning of Amplitude). The beating or waxing and waning of the alpha rhythm is the effect of two separate alpha frequencies.

Alpha and Mu Rhythm. Eye opening (open arrow) attenuates the alpha rhythm but reveals a prominent mu rhythm (C3 and C4) at the same frequency (11 Hz). Note lateral eye movement (X) after the eye opening.

Mu is an arc-like central rhythm with negative sharp component and positive slow component. The frequency is similar to alpha rhythm and it is intermixed with 20-Hz beta activity. It is located at the C3, C4, and Cz electrodes. It is not blocked by eye opening but is attenuated by movement of extremities or thinking about moving with greater effect on opposite hand. The apiculate phase may resemble spikes.

Alpha and Mu Rhythm. Eye opening attenuates the alpha rhythm (open arrow), and eye closure accentuates the alpha rhythm. Eye opening or closure does not affect the mu rhythm (C3 and C4). Mu is an arc-like central rhythm with negative sharp component and positive slow component. The frequency is similar to alpha rhythm and it is intermixed with 20-Hz beta activity. It is located at C3, C4, and Cz electrodes and is not blocked by eye opening but attenuated by movement of extremities or thinking about moving with greater effect on opposite hand. The apiculate phase may resemble spikes.