1. Institute of Medical Psychology and Behavioral Neurobiology, University of Tübingen, Germany, 2Section for Experimental MR of the CNS, Department of Neuroradiology, University of Tübingen, Germany, 3Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London, London, UK, and 4High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany