1. Department of Computer Science, University of Toronto, Toronto, ON M5S 3G4, Canada,
2. Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, MO 63110, U.S.A.
3. Centre for Theoretical Neuroscience, Departments of Philosophy and Systems Design Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada,