Anomalies in beta-waves have been found to be associated with mental depression. However, little is known about the causality of cortical inter- and intrahemispheric dependence of depressed patients in various cognitive states. This study investigates the possibility of beta-wave interdependence in depressed patients subjected to mental arithmetic tasks following relaxation. The EEGs of 12 depressed patients (age=37.2+/-11.8 years from 15 to 74 years, male/female=7/5) and 12 healthy volunteers (age=37.5+/-11.7 years from 22 to 58 years, male/female=6/6) were analyzed using a causality measure, partial directed coherence (PDC). The depressed patients showed lower frontal cortical interdependence in both the resting and mental arithmetic task states, confirming the histopathological evidence which shows that supra- and infragranular layers of the dorsolateral prefrontal cortex of depressed patients decrease both in density and in the size of neurons and glial cells. On the other hand, the mental arithmetic task was found to enhance inter- and intrahemispheric interactions in both groups, and such hemispheric hyperactivation is consistent with findings from functional imaging. The results presented here indicate that PDC can be a useful tool for evaluating changes in cortical interdependence in different psychotic or mental states and might well be used for diagnosis and therapeutic assessment in affective disorders.