1. Computational Neurobiology Laboratory, Salk Institute, La Jolla, CA 92037, U.S.A.,
2. Institute for Neural Computation, University of California at San Diego, La Jolla, CA 92093, U.S.A.,
3. Computational Neurobiology Laboratory, Salk Institute, La Jolla, CA 92037, U.S.A., and Department of Biology, University of California at San Diego, La Jolla, CA 92093, U.S.A.,