1. Cincinnati Veterans Affairs Medical Center and Departments of Neurology and Physiology/Biophysics, University of Cincinnati College of Medicine, Cincinnati, OH 45220 (U.S.A.)
2. Departments of Neurological Surgery and Neuroscience, University of Florida, College of Medicine, Gainsville, FL 32610 (U.S.A.)
3. Department of Radiology, University of Florida College of Medicine, Gainsville, FL 32610 (U.S.A.)