Nat. Neurosci. 13, 1292–1298 (2010); published online 12 September 2010; corrected after print 10 February 2011

In the version of this article initially published, there were symbols dropped from the equations in the second paragraph of the results section. The term θright should have been θrright in the first equation and the term θleft should have been θrleft in the second equation. The error has been corrected in the HTML and PDF versions of the article.