1. NEC Research Institute, Princeton, NJ 08540, U.S.A.
2. NEC Research Institute, Princeton, New Jersey 08540, U.S.A., and Department of Physics, Princeton University, Princeton, NJ 08544, U.S.A.
3. NEC Research Institute, Princeton, NJ 08540, U.S.A., and School of Computer Science and Engineering and Center for Neural Computation, Hebrew University, Jerusalem 91904, Israel