Nat. Neurosci. 12, 1062–1068 (2009); published online 20 July 2009; corrected after print 9 September 2009

In the version of this article initially published, the last sentence of the second paragraph in the right column on page 1065 read “that is, the following term was added to the RT prediction: ρ[σslow (s,t) – σfast (s,t)], where ρ is a free parameter.” A variable in the equation contained in this sentence was incorrect. The sentence should read “that is, the following term was added to the RT prediction: ρ[μslow (s,t) – μfast (s,t)], where ρ is a free parameter.” The error has been corrected in the HTML and PDF versions of the article.