Monitoring slow motions: Action disturbances in major depressive disorder

Major depressive disorder (MDD) is one of the most widespread psychiatric ilinesses. Besides depressive cognitions, psychomotor retardation is one of the core clinical features of a depressive episode. This retardation is the result of a slowing in the performance of an action due to disturbances in the processes responsible for its planning and execution. However, besides an appropriate planning and execution, an optimal action outcome also requires a good action monitoring (AM), i.e. the online evaluation and correction of the ongoing and executed behaviour. Accordingly, optimal planning and execution (psychomotor processes) as well as an adequate AM are prerequisites for an adequate performance. The current thesis focussed on the integrity and significance of both the psychomotor and AM processes in MDD.Part I was dedicated to studies examining psychomotor features in varied samples of MDD patients. The therapeutic, diagnostic and pathophysiological value of psychomotor symptoms in MDD was demonstrated. Furthermore, beneficial psychomotor effects of the antidepressant sertraline were demonstrated, the pattern of retardation was compared between patients with MDD and with chronic fatigue syndrome, and the amount of stereotyped responding in MDD was documented.In Part II, studies were presented that charted deviations in AM in severely depressed patients. In the presented studies, the electrophysiological registration of the error-related negativity (ERN) and error positivity (Pe) was used to investigate the integrity of the AM process. The ERN and Pe were recorded in a large group of severely depressed patients, showing clear alterations relative to the healthy controls. Furthermore, the impact of depressive symptom reduction and perfectionism on AM disturbances were also examined.This thesis was concluded by a discussion section in which a better delineation of the construct 'psychomotor' was proposed. Moreover, the importance of psychomotor symptoms in the choice of the most efficient antidepressant treatment was discussed. The ERN and Pe findings in our severely depressed patients were compared with the divergent findings of other research groups in moderately depressed patients. Possible explanations for these divergences were suggested. The discussion was ended with the proposition of an explanatory, theoretical model in which most of the existing knowledge on the ERN in MDD was integrated. Finally, future research directions were proposed.