This study seeks to examine the effects of treatment with a selective serotonin reuptake inhibitor (SSRI), escitalopram, a first-line treatment for depression, in combination with placebo or with extended-release memantine, on neuropsychological function, regional brain activity assessed by functional magnetic resonance imaging, and depressive symptoms, in participants with Major Depressive Disorder. Escitalopram is administered in an open-label fashion in this study; extended release memantine is administered in a double-blind, randomized, placebo-controlled manner.