1. M. U. Gillette is in the Department of Cell and Developmental Biology and the Neuroscience Program, University of Illinois, Urbana-Champaign, IL 61801, USA.
2. T. J. Sejnowski is in the Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, CA 92037, USA and Department of Biology, University of California, San Diego, La Jolla, CA 92093, USA.