1. ATR Human Information Processing Research Laboratories, 2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-02, Japan
2. The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
3. Primate Research Institute, Kyoto University, Kanrin, Inuyama-City, Aichi 484, Japan