Nat. Med. 17, 1121–1127 (2011); published online 28 August 2011; corrected after print 16 September 2011

In the version of this article initially published, the top electrophysiological trace of Figure 4a was inadvertently repeated as the bottom electrophysiological trace of Figure 4b. The scientific conclusions of the paper were not affected by the error. The error has been corrected in the HTML and PDF versions of the article.