Nat. Neurosci. 15, 1683–1690 (2012); published online 11 November 2012; corrected after print 9 January 2013

In the version of this article initially published, the computation performed to yield the values on the x axis of Figure 6c was incorrectly defined in the text and on the axis label as the absolute difference between Aθ and Aφ (mod 90°). The correct computation is 90° – ||AθAφ| – 90°|, which yields values near 0° for parallel gradients and values near 90° for perpendicular gradients. The error has been corrected in the HTML and PDF versions of the article.