1. Department of Mathematics, University of British Columbia, 1984 Mathematics Rd #121, Vancouver, British Columbia V6T 1Z2, Canada
2. Department of Physiology, McGill University, 3655 Promenade Sir William Osler, Montreal, Quebec H3G 1Y6, Canada
3. Department of Physics, McGill University, 3600 rue University, Montreal, Quebec H3A 2T8, Canada