Lab. for Mathematical Neuroscience, RIKEN Brain Science Institute, Saitama 351-0198, Japan. hiro@brain.riken.go.jp|Lab. for Mathematical Neuroscience, RIKEN Brain Science Institute, Saitama 351-0198, Japan. hiro@brain.riken.go.jp|Lab. for Mathematical Neuroscience, RIKEN Brain Science Institute, Saitama 351-0198, Japan. hiro@brain.riken.go.jp|Lab. for Mathematical Neuroscience, RIKEN Brain Science Institute, Saitama 351-0198, Japan. hiro@brain.riken.go.jp|Lab. for Mathematical Neuroscience, RIKEN Brain Science Institute, Saitama 351-0198, Japan. hiro@brain.riken.go.jp