Department of Neurophysics, Institute for Theoretical Physics, University of Bremen, Bremen, Germany. udo@neuro.uni-bremen.de|Department of Neurophysics, Institute for Theoretical Physics, University of Bremen, Bremen, Germany. udo@neuro.uni-bremen.de|Department of Neurophysics, Institute for Theoretical Physics, University of Bremen, Bremen, Germany. udo@neuro.uni-bremen.de|Department of Neurophysics, Institute for Theoretical Physics, University of Bremen, Bremen, Germany. udo@neuro.uni-bremen.de|Department of Neurophysics, Institute for Theoretical Physics, University of Bremen, Bremen, Germany. udo@neuro.uni-bremen.de|Department of Neurophysics, Institute for Theoretical Physics, University of Bremen, Bremen, Germany. udo@neuro.uni-bremen.de