1 Wolfson Centre for Prevention of Stroke and Dementia, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9DU, UK.
2 Wolfson Centre for Prevention of Stroke and Dementia, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9DU, UK. Electronic address: peter.rothwell@ndcn.ox.ac.uk.