The purpose of this study is to examine the relationship between brain electrical activity in elderly depressed patients and response to antidepressant medication treatment. Elderly patients with depression will be treated for 12 weeks with an antidepressant medication commonly used in clinical practice called escitalopram (Lexapro). Brain electrical activity will be assessed using electrophysiological tests. Researchers are interested in whether the brain electrical activity of elderly people with depression before they start the medication can tell us who amongst them will improve with antidepressant treatment and to what extent. They will also determine whether patients' brain electrical activity during the 12 weeks of medication treatment will change in any way and whether this change will be linked with a change in the severity of their depression. Researchers hope that information gained from this study will help to better understand the brain processes associated with depression and its successful treatment.