1. Department Physiology of Cognitive Processes, MPI for Biological Cybernetics, 72076 Tübingen, Germany, Centre de Biophysique Moléculaire, CNRS, rue Charles Sadron, 45071 Orléans Cedex 2, France, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland, Imaging Science and Biomedical Engineering, University of Manchester, Manchester, United Kingdom, and Division of Cyclotron and Radiopharmaceutical Sciences, INMAS, Timarpur, Delhi 110054, India