Nat. Neurosci. 11, 450–456 (2008); published online 2 March 2008; corrected after print 21 May 2008

In the version of this article initially published, the units for the values reported for resistance in Figure 2i,j were incorrect. The correct unit should be gigaohms. In addition, the authors did not state that the cell death experiments in Figure 6e,f were carried out using a physiological extracellular magnesium concentration of 1 mM. This error has been corrected in the HTML and PDF versions of the article.