1 Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114; enb@neurostat.mit.edu.
2 Department of Brain and Cognitive Sciences, Institute for Medical Engineering and Science and Picower Institute for Learning and Memory, Massachusetts Institute of Technology, Cambridge, MA 02139.
3 Department of Psychology and Center for the Neural Basis of Cognition, Carnegie Mellon University, Pittsburgh, PA 15213.