1 Department of Anesthesiology, Children's Hospital and Medical Center, University of Washington, School of Medicine, Seattle, WA 98195 U.S.A. Department of Anatomy and Developmental Biology, University College London, London WC1E 6BT U.K. Department of Psychology, Dalhousie University, Halifax, NS, B3H 4JI Canada Children's Hospital of Western Ontario, Child Health Research Institute, University of Western Ontario, London, Ont., N6C 2V5 Canada.