1. Laboratory for Neuroengineering, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA
2. Dept. of Mathematics, University of Pittsburgh, Pittsburgh, PA 15260, USA
3. Dept. of Mathematics, The Ohio State University, Columbus, OH 43210-1174, USA
4. Cellular and Systems Neurobiology Section, Laboratory of Neural Control, NINDS, National Institutes of Health, Bethesda, MD 20892-4455, USA