Alpha activity in major depressive disorder: hemispheric laterality and beyond

Numerous EEG investigations into major depressive disorder (MDD) have documented a lateralised imbalance in resting-state alpha activity over prefrontal regions. This pattern has been has been interpreted in the context of theories of hemispheric laterality and emotional processing and is thought to reflect an aberrant affective processing style. Anterior alpha asymmetry is discussed extensively in the biological psychiatry literature and has had a practical influence in the way that MDD is treated with brain stimulation techniques. However, a number of studies have failed to detect lateralised differences in alpha activity in MDD and the prevalence and significance of alpha asymmetry in MDD remain unclear. Relatively recent methodological and theoretical advances may shed light on the associations between aberrant alpha activity and MDD. Methods have been developed that enable delineation of personalised alpha bands based upon individual electrophysiological characteristics. In addition, the traditional conceptualisation of alpha being inversely related to cortical activity has been revised, and there is evidence to suggest that there may be multiple functionally distinct subbands within the broader alpha range which are uniquely sensitive to different aspects of cognition. The current thesis aimed to revisit and extend upon previous findings of abnormal alpha activity in MDD. A series of studies were conducted to determine whether a) the application of individualised alpha bands or narrow alpha subbands provide a more sensitive measure of resting-state anterior alpha asymmetry in MDD, and b) whether, when considered in the context of revised theoretical perspectives, investigation of activity in narrow alpha subbands during active cognitive processing could more directly establish relationships between electrophysiological abnormalities and specific aspects of depressive psychopathology. The aims of this thesis were achieved. It was shown that resting anterior alpha asymmetry does not reliably differentiate between healthy and depressed individuals, irrespective of whether fixed, individualised, broad or narrow alpha bands are applied. The results further indicated that during active cognitive processing, activity within specific alpha subbands is sensitive to altered working memory systems and affective information processing biases in MDD. The findings have uncovered new associations between neural oscillations and cognitive and emotive symptoms in MDD, and have implications for ongoing EEG research into this devastating mental illness.