The aim of this study is to examine the effects of estrogen and serotonin on cognition, emotional processing, and brain activation. The investigators will study the effects of acute tryptophan (TRP) depletion on cognition and mood in healthy menopausal women before and after estrogen replacement treatment (ERT). Using functional magnetic resonance imaging (fMRI), the investigators will identify differences in brain activation during memory tasks with and without TRP depletion and before and after estrogen therapy in order to determine which brain regions and cognitive functions are affected by each manipulation.