Genetic Variation of the Normal Human EEG

Abstract

Since the EEG plays a key role in the diagnosis and classification of epilepsies, and since it shows an appreciable variability in the “normal” range, the question as to the causes of this “normal” variability might be of interest for our understanding of epilepsy. It is well-known that external influences (such as sensory input), changes in the chemical parameters of the blood (such as decrease of CO2 level, hypoglycemia, or altered metabolite levels due to malfunction of the liver) may lead to transitory or even long lasting modifications of the EEG (see [2]). Here, the EEG indicates an abnormal state of brain function. But even under normal conditions in individuals who are awake, relaxed, and resting with closed eyes, there is still appreciable interindividual variability, whereas the EEG of the same individual remains practically constant even over long time periods. Characteristic changes occur both during childhood and youth, and with adults of advancing age. This interindividual variability of the resting EEG in the normal range is largely genetically determined, as has been shown by studies on monozygotic (MZ) twins (for the older literature see [16]; for reviews see [19, 20]: 1. Resting EEGs of healthy MZ twins are as similar as consecutive EEG records from the same individual under constant conditions (Fig. 1). This impression is corroborated by computerized quantitative EEG analysis [15]. 2. Even MZ twins separated in infancy or childhood show very similar [7] or practically identical [9] EEG records. 3. Speed of EEG maturation during childhood and youth also shows genetic variability, as indicated by the EEG similarity in MZ twins in contrast to otherwise considerable interindividual variability. 4. Of the well-known EEG changes in advanced age, slowing of the alpha rhythm, decrease in amplitude, or increase in theta activity in the anterior leads are concordant in MZ twins. On the other hand, discordances in mild anomalies, such as temporal focal abnormalities or short bursts of bilaterally synchronous theta waves, are common [6]. 5. Discordance in neurotic symptoms of MZ twins does not lead to a discordant EEG [23]. 6. EEG changes under controlled external and experimental conditions, such as during nocturnal sleep [16, 26], EEG synchronization under hyperventilation, and alcohol ingestion [12], are under genetic control. 7. High concordance in MZ twins has also been described for visually and auditory-evoked EEG potentials [3, 1] (Fig. 2).