[1] Univ Toronto, Dept Comp Sci, Toronto, ON M5S 3G4, Canada
[2] Gatsby Computat Neurosci Unit, London WC1N 3AR, England
[3] Univ Toronto, Dept Mol & Med Genet, Toronto, ON M5S 1A8, Canada
[4] Univ Toronto, Banting & Best Dept Med Res, Toronto, ON M5G 1L6, Canada