1. Physiological Laboratory, Cambridge CB2 3EG, UK
2. Department of Physiology, Anatomy and Genetics, University of Oxford, Sherrington Building, Oxford OX1 3PT, UK
3. Max Planck Institute for Brain Research, Deutschordenstrasse 46, Frankfurt am Main, D 60528 Frankfurt am Main, Germany
4. Centre de Recherche Cerveau et Cognition, Toulouse, France