Brain/drug effects; Chemical Warfare Agents/toxicity; Cognition/drug effects; Gulf War; Magnetic Resonance Imaging; Military Personnel; Organophosphorus Compounds/toxicity; Persian Gulf Syndrome/chemically induced; Sarin/toxicity; Veterans; Adult; Analysis of Variance; Attention/drug effects; Brain/pathology; Brain/physiopathology; Case-Control Studies; Chi-Square Distribution; Cross-Sectional Studies; Dose-Response Relationship, Drug; Female; Humans; Image Interpretation, Computer-Assisted; Inhalation Exposure; Linear Models; Male; Memory/drug effects; Middle Aged; Neuropsychological Tests; Occupational Exposure; Persian Gulf Syndrome/pathology; Persian Gulf Syndrome/physiopathology; Persian Gulf Syndrome/psychology; Predictive Value of Tests; Risk Assessment; Risk Factors; Time Factors; United States