Additional Study Information: The purpose of this research study is to better understand the brain's dopamine system among older adults with depression. The dopamine system plays important roles in thinking, movement, motivation, and pleasurable or rewarding activities. In addition to performing brain scans and several different tests of your thinking and movement, we use a medication called carbidopa/levodopa (Sinemet) to investigate how the brain's dopamine levels influence behavior. We are using carbidopa/levodopa "off-label" in this study, which means that it is not approved by the Food and Drug Administration for the treatment of depression.