On an EEG recorded in a relaxed but awake adult with eyes closed, the dominant rhythm is 10 Hz. When the patient opens his eyes, this rhythm is immediately suppressed. What is this EEG phenomenon called?
- A Delta suppression
- B Alpha blocking (alpha attenuation) ✓
- C Beta enhancement
- D K-complex generation
Correct answer: B. Alpha blocking (alpha attenuation)
Explanation
The dominant 8–13 Hz rhythm over posterior scalp regions (occipital) in a relaxed, eyes-closed adult is the alpha rhythm. When the patient opens their eyes — or performs mental activity — visual cortex activation causes immediate suppression of the alpha rhythm, known as alpha blocking or alpha attenuation (Berger effect). This reflects the shift from synchronised thalamocortical oscillation to desynchronised cortical activity during sensory processing.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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