1. Dynamic Brain Imaging Laboratory, Departments of Neurology and Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461;
2. Nuclear Magnetic Resonance Center, Massachusetts General Hospital, Charlestown, Massachusetts 02129;
3. Department of Radiology, Helsinki University Central Hospital;
4. BioMag Laboratory, Medical Engineering Centre, Helsinki University Central Hospital; and
5. Cognitive Brain Research Unit, University of Helsinki, Helsinki, Finland