Kelly Patricia Cosgrove, Ph.D., Yale University, will examine differences in brain chemistry between individuals with and without PTSD using a combination of brain imaging techniques: positron emission tomography (PET), which measures molecules of interest in the living brain; and functional magnetic resonance imaging (fMRI) which measures brain activation in response to a task. They will focus on levels of an enzyme called 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1), an enzyme in the stress pathway that modulates the amount of stress hormones present in the brain, as well as activation of the amygdala relative to subjects’ recollection of a traumatic event in their life.