1. NTT Microsystem Integration Laboratories, NTT Corporation, 3-1 Morinosato-wakamiya, Atsugi, Kanagawa 243-0198 Japan, NTT Advanced Technology, 3-35-1 Shimorenjyaku, Mitaka, Tokyo 181-0013 Japan, and School of Bionics, Tokyo University of Technology, 1404-1 Katakura, Hachioji, Tokyo 192-0982 Japan