1. Ricoh Institute of Information and Communication Technology, Research and Development Division, Ricoh Company, 2-7-1 Izumi, Ebina 243-0460, Japan
2. Graduate School of Engineering, Tohoku University, 6-6-05 Aoba, Sendai 980-8579, Japan
3. Graduate School of Biomedical Engineering, Tohoku University, 6-6-05 Aoba, Sendai 980-8579, Japan