[1] Univ Strathclyde, Dept Phys, Glasgow G4 0NG, Lanark, Scotland
[2] UCL, MSSL, Dorking RH5 6NT, Surrey, England
[3] Sheffield Hallam Univ, Sch Sci & Math, Sheffield S1 1WB, S Yorkshire, England
[4] UCL, Gatsby Computat Neurosci Unit, London WC1N 3AR, England