1. Department of Electrical Engineering, University of Toronto, Toronto, Ontario, Canada, M5S 1A4 and Department of Electrical Engineering/Dynamic Graphics Project, Universitty of Toronto
2. Apple Computer Inc., 20525 Mariani Ave MS 27A0, Cupertino, CA and Human Interface Group, Apple® Computer Inc.
3. Institute for Cognitive Science, C-015, University of California, San Diego, La Jolla, CA and Institute for Cognitive Science, University of California, San Diego