1. N. C. Spitzer is in the Department of Biology and Center for Molecular Genetics, University of California at San Diego, La Jolla, CA 92093, USA.
2. T. J. Sejnowski is at the Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, CA 92037 and Department of Biology, University of California at San Diego, La Jolla, CA 92093, USA.