Inclusion Criteria (All Participants):  Ages 50-80 years Eligible for Washington, DC Veterans Affairs Medical Center (VAMC) research participation Capacity to provide consent to participate in research (assessment made by study neurologist and PI) Ability to read and write English  Inclusion Criteria Case Group:  • History of traumatic brain injury of sufficient severity to have resulted in medical attention ascertained via the Ohio State University TBI Identification Questionnaire (OSU TBI-ID). TBI defined by Departments of Defense/Veterans Affairs (DoD/VA) criteria.  Inclusion Criteria Control Group:  • No history of traumatic brain injury of sufficient severity to have resulted in medical attention ascertained via the OSU TBI-ID, and no TBI based upon DOD/VA criteria.  Exclusion Criteria (All Participants):  History of penetrating brain injury History or evidence of disabling neurological or psychiatric condition such as epilepsy (besides posttraumatic epilepsy), multiple sclerosis, hypoxic-ischemic encephalopathy, encephalitis, or schizophrenia History or evidence of cortical or subcortical stroke History or evidence of diabetes mellitus requiring therapy (Hemoglobin A1c > 9.0% for purposes of this study) History or evidence of uncontrolled hyperlipidemia. For the purposes of this study, "hyperlipidemia" will be defined as total cholesterol of 230 in the presence of either or both diabetes and hypertension and 300 in the absence of both of these conditions.  Statin therapy with normal cholesterol levels is allowed.  History or evidence of uncontrolled hypertension (defined as systolic pressure > 160 and/or diastolic pressure > 110 mmHg), or hypotension (systolic pressure < 110 and/or diastolic pressure < 65 mmHg). Hypertension controlled with a single anti- hypertensive medication is allowed. Untreated atrial fibrillation Active tobacco use MRI incompatibility If a participant is currently or has previously taken a phosphodiesterase inhibitor (PDESi), then a two week washout period is required immediately prior to the evaluation visit. Use of nitrates