1. Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
2. Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Howard Hughes Medical Institute, Cambridge, MA 02139, U.S.A.
3. Max-Planck Institute for Medical Research, D-69120, Heidelberg, Germany
4. Departments of Brain and Cognitive Sciences and Physics, Massachusetts Institute of Technology, Howard Hughes Medical Institute, Cambridge, MA 02139, U.S.A.