1. Mayo Clinic Graduate School of Medicine, Rochester, MN 55905, U.S.A.
2. Department of Neurology, Mayo Clinic, Rochester, MN 55905, U.S.A.
3. Division of Cardiovascular Diseases, Mayo Clinic, Rochester, MN 55905, U.S.A.
4. Division of Preventive, Occupational and Aerospace Medicine, Mayo Clinic, Rochester, MN 55905, U.S.A.
5. Department of Surgery, Mayo Clinic, Rochester, MN, U.S.A.
6. Department of Physiology and Biomedical Engineering, Mayo Clinic, Rochester, MN 55905, U.S.A.