The goal of this research is to elucidate the neurophysiological abnormalities associated with processing emotionally valenced stimuli in MDD. Neuroimaging technology has led to the identification of specific abnormalities in the amygdala, ventral striatum and medial prefrontal cortex in response to affective pictures. Much of the knowledge involving these structures in emotion processing has been elucidated through animal models including classical fear conditioning and reversal-learning paradigms. Functional magnetic resonance imaging (fMRI) studies have applied similar techniques in humans to study emotional processing. The amygdala has been shown to play a role in the learning of fear (aversive) conditioning to emotional stimuli, the ventral striatum has been shown to play a role in appetitive conditioning, and areas of the prefrontal cortex are recruited for extinction of conditioned responses. These areas have also been implicated as key structures in the abnormal expression of emotion in MDD. Dysregulation of connections between the amygdala and prefrontal cortex has been hypothesized to influence depressed subjects' tendency to ruminate on emotional events and memories, which may be associated with an inability to properly regulate emotional processing through habituation and extinction mechanisms. Research suggests that depressed individuals have an impaired ability to disengage from or habituate to emotional stimuli perceived as sad.