1. Institute of Neuroinformatics, University and Eidgenössische Technische Hochschule, Zürich 8006, Switzerland
2. Computation and Neural Systems Program, 139-74, California Institute of Technology, Pasadena, CA 91125, USA.
3. Center for Biological and Medical Systems, Imperial College, London SW7 ZBT, UK.
4. Medical Research Council Anatomical Neuropharmacology Unit, Oxford OX1 3TH, UK.