Vol. 209, No. 2, February 13, 2012. Pages 275–289.
The authors regret that they incorrectly described the error bars as indicating the mean in several legends in the original manuscript. The error bars indicate the standard deviation. In addition, minor errors in panel letter order (Fig. 4) and a figure legend title (Fig. S1) were noted. These errors have been corrected in the html and pdf versions of this article.