Escitalopram (Lexapro) and citalopram (Celexa) are similar selective serotonin reuptake inhibitors that alter blood flow to the amygdala and other brain structures involved in regulating mood. Escitalopram consists of S-citalopram while citalopram contains both S-citalopram and R-citalopram (racemic citalopram). There is evidence that R-citalopram may block the effects of S-citalopram. The hypothesis being tested is that because of the antagonist effect of R-citalopram, S-citalopram will have a greater effect on the mood circuit than racemic citalopram when equal doses of S-citalopram are administered. The study design consists of a two week medication period followed by blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) while viewing affective visual stimuli.