Correction to: Nature Neuroscience https://doi.org/10.1038/nn.4231, published online 25 January 2016.
In the version of this article initially published, the right panel in Fig. 2b was duplicated from the corresponding panel in Fig. 2c, and some data points in Fig. 3b were duplicated from Fig. 3a. None of the conclusions in the paper are affected. The errors have been corrected in the HTML and PDF versions of the article, and source data have been posted for the revised panels.