Department of Mathematics and Statistics, Boston University, Boston, Massachusetts, United States of America.|Department of Neurology, Massachusetts General Hospital, Boston, Massachusetts, United States of America, Harvard Medical School, Boston, Massachusetts, United States of America.|Department of Neurology, Massachusetts General Hospital, Boston, Massachusetts, United States of America, Harvard Medical School, Boston, Massachusetts, United States of America.|Department of Mathematics and Statistics, Boston University, Boston, Massachusetts, United States of America.|Department of Mathematics and Statistics, Boston University, Boston, Massachusetts, United States of America.