1. Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London WC1N 3BG, U.K.
2. Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London WC1N 3BG, and Institute of Philosophy, School of Advanced Studies, University of London EC1E 7HU, U.K.
3. Institute of Cognitive Sciences and Technologies, National Research Council, 7-00185 Rome, Italy
4. Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London WC1N 3BG, U.K., and Division of Sleep Medicine, Harvard Medical School, Boston, MA 02215, U.S.A.