Nature Neuroscience 7, 41–47 (2004); published online 23 November 2003; corrected after print 20 June

In the version of this article initially published, the dose of urethane anesthesia in the Methods section was incorrect. The correct dose should be 1.25 g per kg of body weight. The error has been corrected in the PDF version of the article. The authors regret the error.