Nat. Methods 12, 140–146 (2015); published online 22 December 2014; corrected after print 6 February 2015

In the version of this article initially published, the size of the scale bar reported in the legend of Figure 3a was incorrect. The correct size is 100 μm, not 50 μm. In addition, the volume of injected virus in the Online Methods section “Titration of calcium indicator expression” had the incorrect unit. The correct volume is 100 nl, not 100 μl. The errors have been corrected in the HTML and PDF versions of the article.