Nat. Neurosci. 15, 1683–1690 (2012); published online 11 November 2012; corrected after print 3 December 2012

In the version of this article initially published, the scale bar length for Figure 1e was misstated as 500 μm. The correct length is 50 μm. The error has been corrected in the HTML and PDF versions of the article.