1. Center for Information and Neural Networks, Advanced ICT Research Institute, National Institute of Information and Communications Technology and Osaka University, 588-2, Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, Hyogo 651-2492, Japan
2. Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan
3. Research Fellow for Young Scientists of the Japan Society for the Promotion of Science, 5-3-1, Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan