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.

This research study will examine how the brain electrical activity of elderly depressed patients is related to how quickly and how well they respond to antidepressant medication treatment. Brain electrical activity will be assessed using electrophysiological tests. Elderly patients with depression will be treated for 12 weeks with an antidepressant medication commonly used in clinical practice called escitalopram (Lexapro). Before they start treatment, patients will be asked questions about their depression to measure how severe it is. They will also be asked to have their brain electrical activity recorded while they perform tests on a computer screen. The questions about their depression and the brain electrical recordings will be repeated regularly during the 12 weeks they will be treated with antidepressant medication. The researchers are interested to see whether the brain electrical activity of elderly people with depression before they start the medication can tell us who amongst them and to what extent will finally improve with treatment with escitalopram (Lexapro). They will also determine whether patients' brain electrical activity, during the 12 weeks they will be receiving the medication treatment, will change in any way and whether this change will be linked with the change in the severity of their depression. It is hoped that information gained from this study will help the investigators to better understand the brain processes associated with depression and its successful treatment.

Inclusion Criteria:

• Diagnosis: Major depression, unipolar without psychotic features (by DSM-IV criteria) -Severity of depression: A 24-Item HDRS above 19 at screening and at baseline -
• Level of Executive Dysfunction: Two strata within each age stratum: Stroop Color-Word scores - one half of the sample < 26, one of half ≥ 26.
• Capacity to provide informed consent.

Exclusion Criteria:

• High suicide risk, i.e. intent or plan to attempt suicide in near future
• Presence of any current Axis I psychiatric disorder (other than unipolar major depression or specific phobias) including substance abuse (those with a history of substance abuse must be abstinent for at least 3 months prior to entry)
• Axis II diagnosis of antisocial personality disorder, mental retardation and pervasive developmental disorder (DSM-IV)
• History of psychiatric disorders such as psychotic depression, primary psychotic disorder, or bipolar spectrum disorder (bipolar disorder and hypomania are exclusions)
• Cognition: MMSE scores below 24 or diagnosis of dementia by DSM-IV
• Acute or severe medical illness, i.e., delirium, metastatic cancer, decompensated cardiac, liver or kidney failure, major surgery, stroke or myocardial infarction during the three months prior to entry; or drugs known to cause depression, e.g., reserpine, alpha-methyl-dopa, steroids, sympathomimetics withdrawal
• Presence of a significant neurological disease such as Parkinson's disease, primary or secondary seizure disorders, intracranial tumors, severe head trauma; neurodegenerative diseases i.e. MS
• History of failure to respond to escitalopram (Lexapro) (20mg/day for 6 weeks or longer) during the current or previous depressive episodes
• History of intolerance to escitalopram (Lexapro) or use of concomitant drugs that may provide reason to believe that escitalopram is contraindicated. Active treatment with fluoxetine at the time of screening
• Patients' unwillingness or inability to gradually withdraw all other psychotropic medications (except for the following: Low and stable doses of opiates and non-benzodiazepine hypnotics (e.g. zolpidem (5 or 10 mg), zaleplon (5 or 10 mg), or eszopiclone (1 or 2 mg).Low and stable doses of diazepam and other anxiolytics should be tapered at screening; inability to tolerate taper is not an exclusion criteria.
• Inability to perform any of the ADLs (MAI: ADL subscale) even with assistance, e.g. walking with a cane is not an exclusion criterion
• Inability to speak English
• Aphasia

14 weeks