Nat. Neurosci. 17, 367–376 (2014); published online 26 January 2014; corrected after print 27 February 2014; corrected after print 18 June 2014

In the version of this article initially published, the line graphs presented in Figure 1g,h were switched with ones from a different experiment.

The error has been corrected in the HTML and PDF versions of the article.