This study aims to improve understanding of how people with low mood and negative feelings (known as dysphoric) may be different from people with normal mood and feelings (nondysphoric) when responding to a variety of social and emotional information. The study will look at the patterns of activity in peoples' brains in situations (presented as a battery of tests) after treatment with a medicine (escitalopram) or a placebo. The results from this study will help to gather information about the effectiveness of the various tests being used in this study in detecting any changes due to treatment with an antidepressant. Half the volunteers taking part in this study will be dysphoric (mildly depressed) whilst the other half of volunteers will be healthy volunteers. It is hoped that the results of this study will provide guidance for assessing effectiveness of new medicines and potentially help with the treatment of depression.