1. Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9DU, UK
2. Department of Anesthesia and Pain Medicine, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA
3. Department of Neuroscience and Center for Advanced Pain Studies, University of Texas at Dallas, Richardson, TX 75080, USA
4. Brain Function Research Group, School of Physiology, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg 2000, South Africa