Slow alpha variant: a report of a 3 for 1 ratio subharmonic.

are often seen and make interpretation more difficult. Some are not understood on the basis of physiology and clinical implication. For others, no pathological substrate or epileptic condition explains these patterns. Background activity refers to the EEG rhythm corresponding to the patient’s age and state of consciousness. The alpha rhythm is in the range of 8 to 12 Hertz (Hz), has a sinusoidal waveform, blocks with eye opening and alertness and is usually pronounced in posterior regions1. In 1947, Goodwin first called attention to a slow alpha variant rhythm, which predominates in the occipital and adjacent posterior areas2. Ten percent of normal subjects’ EEG contained alpha variant rhythm. Another source reported it in 0.8% of asymptomatic people3. When it has once been observed, it may be expected to reappear more or less regularly in EEG recordings. Alpha variant can be fast, corresponding to twice the usual alpha frequency, or slow. The slow alpha variant has a frequency half of the alpha rhythm, usually in the range of 4-5 Hz. It is the definition of a harmonic pattern4. It could be, but not invariably, synchronous and symmetrical. The notched appearance waveforms can be more or less continuous while alternating with the regular alpha activity, or even paroxysmal. They often show a strong blocking to eyes opening and, as a rule to photic stimulation, being replaced by a driving effect. It is thought to represent a physiological variation of the basic cortical rhythm. According to Goodwin, in a few cases, the alpha variant rhythm ratio was 3 to 1 and on rare occasions 4 to 1 compared to the basic rhythm2. However, he did not provide any EEG sample and that has never been reported thereafter to our knowledge. We therefore present the first EEG sample of a rhythm showing the usual pattern of the slow alpha variant, but with a rhythm ratio of 1 to 3.