[1] Boston Univ, Sch Med, Dept Neurol, Neurol Epidemiol & Genet Div, Boston, MA 02118 USA
[2] Boston Univ, Sch Publ Hlth, Dept Biostat, Boston, MA USA
[3] Boston Univ, Dept Math & Stat, Boston, MA USA
[4] Univ Calif Davis, Dept Neurol, Sacramento, CA 95817 USA
[5] Univ Calif Davis, Ctr Neurosci, Sacramento, CA 95817 USA