Nat. Methods 5, 881–886 (2008); published online 21 September 2008; corrected after print 29 September 2008.

In the version of this article initially published, the time scale reported in the Figure 2d legend is incorrect. The correct time scale should be 3 μs. Additionally, the time delay of 320 ms reported in Figure 5b is incorrect. The correct time delay is 200 ms. These errors have been corrected in the HTML and PDF versions of the article.