This study is designed to identify changes in the brain that underlie symptoms of combat-related PTSD using brain imaging (fMRI). Then, the investigators will administer mild electrical stimulation to the side of the head (using a method called tDCS) in an attempt to reduce the symptoms of PTSD.

Being involved in combat is a horrific experience that substantially increases the risk of developing posttraumatic stress disorder (PTSD). Although several effective treatments have been identified for PTSD a substantial number of patients (up to 50%) continue to experience symptoms. The field of neuroscience has revealed that patients with PTSD demonstrate altered functioning within, and interactions between, several brain regions; findings that are consistent with animal models of chronic stress. Despite this evidence, existing treatments are generally not designed using this neuroanatomical knowledge. The central premise of the proposed study is that neuroscientifically-based information can be used to develop more precise and effective treatments. Transcranial direct current stimulation (tDCS) will be used in an attempt to "correct" the dysfunctional brain regions (and communication between these regions), with the expectation that this modulation will result in symptom improvement.

The primary goals of the study are to verify the maladaptive brain networks and then establish evidence that tDCS modulates these networks. Subsequent studies, performed during the later study years, will examine dose-response relationships and synergistic effects of tDCS and existing treatments. Outcome will be assessed using a multi-method approach that includes functional connectivity using resting-state functional magnetic resonance imaging data, neuropsychological tests, and self-report measures of emotional functioning. The combined results will provide vital methodological, mechanistic, and practical information necessary for a formal clinical trial of tDCS in PTSD.

Inclusion Criteria:

• Male
• Combat Veteran - both Veterans with and without PTSD will be included at different points in this study
• Right-handed
• Between the ages 18-88
• Stable on medications for a minimum of 2 weeks

Exclusion Criteria:

1. a history of neurological disease (e.g., dementia, epilepsy, stroke, moderate - severe traumatic brain injury)
2. "severe" Axis I diagnoses (e.g., bipolar disorder, schizophrenia)
3. presence of Axis II disorders
4. current alcohol or drug abuse/dependence (in the past 8 weeks)
5. participants will also be excluded if they are not MRI compatible (assessed using the guidelines of the American College of Radiology)
6. Imminent risk of harm to self or others
7. history of HIV or sickle cell anemia, as these can cause neuropsychological issues .