I would like to thank Sandro Mussa-Ivaldi, Ted Milner, Emilio Bizzi, Richard Lippmann, Marc Raibert, and the reviewers for their comments and criticism. This report describes research done within the laboratory of Dr. Emilio Bizzi in the department of Brain and Cognitive Sciences at MIT. The author was supported during this work by the division of Health Sciences and Technology, and by NIH grants 5R37AR26710 and 5R01NS09343 to Dr. Bizzi.