1. Institute for Adaptive and Neural Computation, Division of Informatics, University of Edinburgh, 5 Forrest Hill, Edinburgh EH1 2QL, Scotland
2. Division of Biology 216-76, California Institute of Technology, Pasadena, CA 91125, USA
3. Theoretical Neurobiology, Born-Bunge Foundation, University of Antwerp, Universteitsplein 1, 2610 Wilrijk, Belgium
4. Department of Electrical and Computer Engineering 425 UCB, University of Colorado, 425 UCB, Boulder, CO 80309, USA