1. Department of Statistical Modeling, Institute of Statistical Mathematics, Tokyo 190-8562, Japan
2. FIRST, Aihara Innovative Mathematical Modelling Project, Japan Science and Technology Agency, Tokyo 153-8505, Japan, and Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan
3. Department of Statistics and Center for the Neural Basis of Cognition, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A.
4. Department of Physics, Kyoto University, Kyoto 606-8502, Japan