1. Research Institute of Electrical Communication, Tohoku University, 2-2-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
2. Department of Physical Medicine and Rehabilitation, Tohoku University Graduate School of Medicine, 2-1 Seiryo-machi, Sendai 980-8575, Japan
3. Graduate School of Biomedical Engineering, Tohoku University, 2-1 Seiryo-machi, Sendai 980-8575, Japan