Electrophysiological brain responses as neural markers of depression and aging

Abstract

Depression is one of the most common mental disorders. Currently, the diagnosis of depression is based on a clinical interview, and no reliable objective depression biomarkers have been recognized. Brain’s event-related potentials (ERPs) could potentially be used as neural markers for diagnosing and planning treatment options for depression. ERPs could also potentially be used to differentiate depression-related alterations from aging-related alterations. In study I, ERPs to sound intensity changes were compared between first-episode and recurrent depression groups and non-depressed controls. Larger brain responses (N1 response) to rare sounds were found in the first-episode depression group, compared to the other groups. Sound intensity processing has been suggested to reflect monoaminergic neurotransmission and therefore the enlarged responses can reflect monoaminergic deficits. In study II, ERPs to sound intensity changes were compared between younger and older depressed adults and age-matched non-depressed adults. Augmented N1 responses were found in both older adults and depressed adults, indicating similar effects for aging and depression on intensity processing. The augmentation could index an inability to suppress activity in response to irrelevant stimuli (also called sensory gating). In study III, ERPs to emotional facial expressions were compared between depressed and non-depressed groups. In addition, changes in the brain responses were investigated by conducting follow-up measurements after 2 and 39 months. It was also investigated whether the baseline brain responses were associated with later response to a brief psychological intervention. Larger P1 responses to sad faces, compared to neutral faces, were found in the depressed group, indicating a negative bias in the automatic processing of facial expressions. The bias was corrected in the follow-up measurements after symptom reduction. Compared to the non-depressed control group, a larger negative bias was found in the group that did not recover after the intervention. However, the recovered and non-recovered groups did not differ in brain responses. The results of this thesis indicate that electrical brain responses related to early information processing can be used to study depression- and aging-related alterations in brain function and the brain responses can reflect illness state. Therefore, these responses have the potential to be further developed for clinical practice as neural markers for depression.