Selective Serotonin Reuptake Inhibitors (SSRIs) are used to treat depression. It is thought that it takes several weeks for them to take effect but some studies have suggested that they work much quicker than that. Both depression and anxiety have been associated with over-activity in a part of the brain called the amygdala and one way it is thought that SSRIs may be effective is in damping down this over-activity. One of the ways in which the amygdala becomes over-active is when responding to threatening faces or situations. Researchers from Warneford Hospital in Oxford and Oxford University used brain scans to study the effect of the SSRI antidepressant citalopram on 26 healthy volunteers. They gave half of them citalopram and half a placebo then, after three hours, they showed them a series of facial expressions while giving them an MRI scan. They found that the participants who took citalopram showed much lower amygdala activity in response to the fearful faces than those who took the placebo.
Murphy, Susannah E. ... [et al] - Effect of a single dose of citalopram on amygdala response to emotional faces British Journal of Psychiatry June 2009, 194(6), 535-540