Depression and functional EEG asymmetry

A new quantified EEG index (Alpha Burst Occurrence Variability Index: alpha BVI) has been devised in order to characterize sequential variations of alpha bursts in EEG. This index makes it possible to quantify slope variations of a cumulative graph (periodogram) where the rank of alpha bursts is traced as a function of their instant of arrival. Such quantitative analysis was applied to spontaneous EEG segments which were recorded in the parieto-occipital regions (P3-Fz and P4-Fz) during a rest period, in three groups of depressed patients, one being characterized by psychomotor retardation (PRM group), another one by an emotional blunting (EB group) and a group of impulsive depressed patients (I group). The patients were compared to a group of healthy control subjects. In the three patient groups, before treatment, it was observed that the right and left alpha BVI were significantly lower than those of the control subjects. The right hemisphere was involved in all cases, but the left hemisphere was involved only in severe cases. In the EB and I groups, two strong "electro-clinical" correlations were observed, each one being specific for each hemisphere, one relating the reduction of the right alpha BVI to the depressive mood, the other relating the reduction of the left alpha BVI to the degree of speech ("ideoverbal") retardation. These results emphasize the role of the right hemisphere in the probable pathogenesis of depression.