1. Department of Cell and Developmental Biology, UCL, London WC1E 6BT, UK
2. Institute of Cognitive Neuroscience, UCL, London WC1N 3AR, UK
3. Institute of Neurology, UCL, London WC1N 3BG, UK
4. Department of Psychology, University of Durham, Durham DH1 3LE, UK
5. Centre for Mathematics and Physics in the Life Sciences and Experimental Biology, UCL, London WC1E 6BT, UK