1. ATR Human Information Processing Research Laboratories, Kyoto 619-0288;
2. Graduate School of Science and Technology, Kobe University, Hyogo 657-0013;
3. Kawato Dynamic Brain Project, ERATO, Japan Science and Technology Corporation, Kyoto 619-0288;
4. Department of Information and Computer Sciences, Toyohashi University of Technology, Aichi 441-8540; and
5. NTT Communication Science Laboratories, Nippon Telegraph and Telephone Corporation, Kanagawa 243-0198, Japan