Asymmetry of occipital alpha-rhythm under resting conditions: a neurophysiological measure of "hemisphericity".

Using the resting EEGs of 20 healthy subjects in semirecumbent position with eyes closed and left undisturbed, we studied the question whether and, if so, to what extent, lateralization of occipital alpha-rhythm depends on level of vigilance. As a simple indicator of vigilance, the relative alpha-power for each 2-sec. epoch of 02-CZ (occipital right) was calculated. Lateralization was defined as the ratio of absolute alpha-powers in 01-CZ and 02-CZ (occipital left and right). This was also calculated epoch by epoch. The results of individual analysis of data from each subject allowed categorization into 3 types. This typological differentiation obtained quantitatively could be supported by visual pattern analysis. The findings are discussed in relation to the state- vs trait-problem which constantly confronts us when we interpret experimentally obtained functional asymmetries of hemispheres.