1. Department of Neurology, Division of Behavioral Neurology and Cognitive Neuroscience, University of Iowa College of Medicine, Iowa City, IA 52242, USA.
2. Department of Neurology, University of Iowa College of Medicine, and Salk Institute for Biological Studies, La Jolla, CA 92186, USA.
3. Laboratory for Information and Decision Systems, MIT, Cambridge, MA 02139, USA.