1. Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba-ken 277-8561,
Japan
2. Center for Advanced Intelligence Project,
RIKEN, 1-4-1 Nihonbashi, Chuo-ku, Tokyo 103-0027, Japan
3. Department of Mechatronics, Nanzan University,
18 Yamazato-cho, Showa-ku, Nagoya 466-8673, Japan
4. Graduate School of Information Science and Technology, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan