1. Institute for Adaptive and Neural Computation, School of Informatics, University of Edinburgh, Edinburgh EH1 2QL, U.K.
2. Department of Computing Science and Mathematics, University of Stirling, Stirling FK9 4LA, U.K.
3. Centre for Integrative Physiology, School of Biomedical Sciences, University of Edinburgh, Edinburgh EH8 9XD, U.K.