The purpose of this study is to find out if atomoxetine causes a change in the biologic markers (substances that may indicate the presence of a disease) in the cerebrospinal fluid (CSF) of participants diagnosed with Mild Cognitive Impairment (MCI). In this research study, the spinal fluid of subjects with MCI who take atomoxetine will be compared to spinal fluid of those who take capsules containing inactive material, also known as placebo. At the six-month timepoint, subjects who were taking placebo during the first six months will be placed on active study medication, and those who received active study medication will be reassigned to placebo. After completion of the study, subjects will be able to receive open-label atomoxetine.