1. Cognitive Science 0515, University of California, San Diego, La Jolla, CA 92093-0515, USA.
2. Department of Neurophysiology, University of Oslo, 0316 Oslo, Norway.
3. Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology, Boston, MA 02115, USA.
4. Massachusetts General Hospital Nuclear Magnetic Resonance Center, Charlestown, MA 02129, USA.