1. From the Cardiovascular Research Laboratory (J.W.H., C.O.T., J.A.T.), Spaulding Rehabilitation Hospital, Boston, Mass; Physical Medicine and Rehabilitation (C.O.T., J.A.T.), Harvard Medical School, Boston, Mass; Harvard-MIT Division of Health Sciences & Technology (K.L.), Massachusetts Institute of Technology, Cambridge, Mass; and the Department of Cognitive and Neural Systems (M.A.C.), Boston University, Boston, Mass.