 Carrying out this research on healthy volunteers will enable us to understand if modulating serotonin function by antidepressant administration has an effect not only on mood symptoms - as is evident in depressed patients - but also on the predisposing psychological and cognitive processes that sustain the depressed mood, such as the response to emotional stimuli. We will also be able to verify if this effect is shown early treatment and prior to any subjective changes in mood. This will be done by administering seven days of either the antidepressant citalopram or placebo to subjects with high neuroticism scores and then comparing them on a series of computer based psychological tests measuring various aspects of how emotionally salient stimuli are processed. 