1. From the Oxford Centre for Functional MRI of the Brain (FMRIB), John Radcliffe Hospital (R.P., S.T.P., S.S., D.F., P.M.M.), and the MRC Biochemical and Clinical Magnetic Resonance Unit, Department of Biochemistry, and the John Radcliffe Hospital (S.T.P., A.M.B., P.S.), Oxford, UK. Drs Pineiro and Pendlebury contributed equally to the article.