Nat. Neurosci. 17, 506–512 (2014); published online 9 March 2014; corrected after print 30 July 2014

In the version of this article initially published, the wrong image was included as Figure 5g, top left panel. The image that appeared was the bottom left image from Supplementary Figure 9a; the correct image is the top right one from Supplementary Figure 9a. The error has been corrected in the HTML and PDF versions of the article.