1Physikalisch Technische Bundesanstalt (PTB), Berlin, Germany, 2Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 3Department of Psychiatry, Psychotherapy and Psychosomatics, Hospital of Psychiatry, University of Zurich, Zurich, Switzerland, 4Max Planck Institute for Biological Cybernetics, Tuebingen, Germany