Which EEG rhythm is characteristically SUPPRESSED when a subject opens their eyes and engages in visual processing?

• A Theta rhythm (4–8 Hz)
• B Beta rhythm (13–30 Hz)
• C Delta rhythm (0.5–4 Hz)
• D Alpha rhythm (8–13 Hz) ✓

Explanation

The alpha rhythm (8–13 Hz) is the dominant EEG pattern in awake, relaxed adults with eyes closed and is most prominent over the occipital cortex. On eye opening, visual stimulation causes 'alpha blocking' (alpha desynchronisation), with the alpha rhythm replaced by lower-amplitude, higher-frequency beta activity. This alpha-blocking response is a normal finding and is used to verify that EEG electrodes are functioning. Delta is seen in deep sleep; theta in drowsiness and temporal lobe regions; beta predominates during active cognition.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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