Institute for Adaptive and Neural Computation, University of Edinburgh, Edinburgh, United Kingdom. abarrett@inf.ed.ac.uk|Institute for Adaptive and Neural Computation, University of Edinburgh, Edinburgh, United Kingdom. abarrett@inf.ed.ac.uk|Institute for Adaptive and Neural Computation, University of Edinburgh, Edinburgh, United Kingdom. abarrett@inf.ed.ac.uk|Institute for Adaptive and Neural Computation, University of Edinburgh, Edinburgh, United Kingdom. abarrett@inf.ed.ac.uk