1. R. Shadmehr, Department of Biomedical Engineering, School of Medicine, Johns Hopkins University, 720 Rutland Avenue, 419 Traylor, Baltimore, MD 21205–2195, USA.
2. H. H. Holcomb, Maryland Psychiatric Research Center, University of Maryland, Post Office Box 21247, Baltimore, MD 21228, USA, and Department of Radiology, Johns Hopkins University, Baltimore, MD, USA.