1. a Department of Mechanical Engineering, Waseda University, 3-4-1 Okubo Shinjuku-ku, Tokyo 169-8555, Japan
2. b Graduate School of Informatics, Kyoto University, Yoshida-honmachi Sakyo-ku, Kyoto 606-8501, Japan
3. c Brain Science Institute, RIKEN, 2-1 Hirosawa Wako-shi, Saitama 351-0198, Japan
4. d Department of Mechanical Engineering, Waseda University, 3-4-1 Okubo Shinjuku-ku, Tokyo 169-8555, Japan