[1] Montreal Neurol Inst, Dept Neurol & Neurosurg, Montreal, PQ H3A 2B4, Canada
[2] McGill Univ, Dept Polit Sci, Ctr Study Democrat Citizenship, Montreal, PQ H3A 2T7, Canada
[3] Massachusetts Gen Hosp, Gerontol Res Unit, Charlestown, MA 02129 USA
[4] Massachusetts Gen Hosp, Dept Neurol, Charlestown, MA 02129 USA
[5] Harvard Univ, Sch Med, Charlestown, MA 02129 USA