Background: Cognitive theories of depression predict mood-congruent negative biases already at automatic stages of processing, although several behavioral studies seem to contradict this notion. That is, depression should potentiate emotional reactivity to negative emotional cues, whereas it should reduce reactivity in response to positive emotional stimuli. Assessing neurobiological substrates of automatic emotion processing might be a more sensitive challenge for automatic negative bias in depression than behavioral measures.
Methods: In 30 acutely depressed inpatients and 26 healthy control subjects, automatic amygdala responses to happy and sad facial expressions were assessed by means of functional magnetic resonance imaging (fMRI) at 3 Tesla. To examine automatic responses, a presentation paradigm using subliminal, backward-masked stimuli was employed. A detection task was administered to assess participants' awareness of the masked emotional faces presented in the fMRI experiment.
Results: Detection performance was at chance level for both patients and healthy control subjects, suggesting that the neurobiological reactions took place in absence of conscious awareness of the emotional stimuli. A robust emotion by group interaction was observed in the right amygdala. Whereas healthy control subjects demonstrated stronger responses to happy faces, depressed patients showed the opposite. Furthermore, amygdala responsiveness to happy facial expression was negatively correlated with current depression severity.
Conclusions: Depressed patients exhibit potentiated amygdala reactivity to masked negative stimuli along with a reduced responsiveness to masked positive stimuli compared with healthy individuals. Thus, depression is characterized by mood-congruent processing of emotional stimuli in the amygdala already at an automatic level of processing.