Inclusion Criteria:  Age greater than or equal to 60 years DSM5 Diagnosis of Major Depressive Disorder (MDD), Persistent Depressive Disorder, or Depression Not Otherwise Specified (NOS) Hamilton Rating Scale for Depression (HRSD) score ≥ 20 Decreased processing speed (1 SD below age-adjusted norms on the WAIS-IV Digit Symbol Coding Test) Access to a computer with daily internet access Willing to and capable of providing informed consent and complying with all study procedures. At the end of the CCT phase (week 4), depression severity will be reassessed. To be eligible for Phase 2 (SSRI trial), participants will be required to have an HRSD score ≥ 14.  Exclusion Criteria:  Diagnosis of substance abuse or dependence (excluding Tobacco Use Disorder) within the past 12 months History of psychosis, psychotic disorder, mania, or bipolar disorder Primary neurological disorder, including dementia, stroke, Parkinson's disease, epilepsy, etc. Mini Mental Status Examination (MMSE) score less than 24 HRSD suicide item greater than 2 or Clinical Global Impressions (CGI)-Severity score of 7 at baseline History of allergic or adverse reaction to escitalopram, or non-response to adequate trial of escitalopram (at least 4 weeks at dose of 20 mg) during the current episode Current or recent (within the past 2 weeks) treatment with psychotherapy, antidepressants, antipsychotics, mood stabilizers Contraindication to MRI scanning (such as metal in body) or inability to tolerate the scanning procedures History of significant radioactivity exposure (nuclear medicine studies or occupational exposure).