Antidepressants have been shown to reduce responses of the brains limbic regions. They are known to swiftly control emotional processing, like increasing recognition of positive relative to negative emotional facial expressions.

In a double-blind study 29 patients with major depressive disorder were randomized to receive either escitalopram or placebo for one week, after which functional magnetic resonance imaging (fMRI) was performed. 

After a 1 week treatment with escitalopram, it was found that escitalopram reduces amygdala responses to fearful facial expressions in depressed patients. 

This finding recommends that even before mood improvement occurs, escitalopram modulates the way that patients neurally process complex emotional "real-to-life" stimuli.

Significantly, the difference between the escitalopram and placebo groups was seen beforehand the antidepressant had any effect on depressive symptoms or subjective affective state, suggesting a direct effect of escitalopram instead of a secondary effect arising from improved mood.