Mechanisms of change: effects of repetitive exposure to feared stimuli on the brain's fear network.

Repetitive exposure to feared stimuli is considered as the essential element in therapy with phobic patients. However, the mechanisms mediating symptom reduction and their underlying neurobiological processes are poorly understood. Therefore, we presented the same fear-relevant and neutral stimuli repeatedly to individuals with high and low fear of animals during fMRI scanning. High-, but not low-fearful individuals showed an initial fear-stimulus-related potentiation of amygdala and insula activity. Potentiation of the amygdala in the high-fearful group habituated quickly, but insula activity was still potentiated during later repetition trials. Both groups showed an initial potentiation of the dorsomedial prefrontal cortex (dmPFC) that continuously decreased in low-, but not in high-fearful participants. Thus, within-session habituation may occur on an automatic processing level (amygdala), but does not cause lasting neural changes on a higher order cortical level (dmPFC).