Title: ERG as a potential biomarker of SSRI-responsive PTSD: A pilot study Posttraumatic stress disorder (PTSD) affects many individuals who experience a traumatic event. Previous studies suggest that there is a connection between the brain and the retina, and that the electroretinogram (ERG) may be a valuable biomarker to tell us more about signals in the brain that are related to mental health disorders like PTSD. The goal of this observational study is to examine the ERG waveform in veterans with PTSD before and after a single dose of the selective serotonin reuptake inhibitor (SSRI), sertraline. Sertraline is one of two FDA-approved medications for the treatment of PTSD. The main questions this pilot study aims to answer are: 1. How does sertraline, an SSRI, influence the ERG waveform in veterans with PTSD? 2. Is there a significant correlation between baseline ERG signals and the change in ERG following SSRI treatment? Participants will be asked to: * Undergo ERG recordings before and after a single dose of sertraline. * Provide relevant clinical information related to PTSD symptoms and treatment history. Following the initial study visits, participants will enter an optional open label phase of the study in which qualifying participants who initiate antidepressant treatment through routine clinical care will be invited back for a follow-up ERG recording.