1. D. Contreras, A. Destexhe, M. Steriade, Laboratoire de Neurophysiologie, Faculté de Médecine, Université Laval, Quebec G1K 7P4, Canada.
2. T. J. Sejnowski, Howard Hughes Medical Institute, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037, and Department of Biology, University of California San Diego, La Jolla, CA 92093, USA.