Nat. Neurosci. 14, 1202–1208 (2011); published online 14 August 2011; corrected after print 12 April 2012

In the version of this article initially published, the percentages of neurons encoding action value Qx(t) shown in Figure 7b were incorrect owing to a mistake in the computer code used to analyze the data:

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