Nature 450, 299–303 (2007)

In this Letter, some axis labels in Figs 3 and 4 were inadvertently mislabelled. In Fig 3d, the labels on the x axes of the three graphs should read ‘Hi Low Nil’ instead of ‘Hi Low Ni’. The x axes of the bar graphs in Fig. 4a (right and left panels) and Fig. 4b (right panel) should read ‘Hi Low Nil’ instead of ‘Hi Low Hi’.