1. Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, 1432 Aas, Norway, and Theoretical Neuroscience Group, RIKEN Brain Science Institute, Wako-City, Saitama 351-0198, Japan
2. Theoretical Neuroscience Group, RIKEN Brain Science Institute, Wako City, Saitama 351-0198, Japan, and Bernstein Center for Computational Neuroscience, Albert-Ludwigs University, 79104 Freiburg, Germany