Nat. Neurosci. 16, 1392–1400 (2013); published online 25 August 2013; corrected after print 1 June 2015

In the version of this article initially published, the image in Figure 2c described as being from 20-month-old transgenic mice was actually from 2-month-old control mice. New representative images showing a wider field of view have been provided for all of Figure 2c, and Figure 2d has been replaced by a new quantification performed independently of the original one. This quantification (n = 3) yielded a P value for the comparison between the 2-month-old control and transgenic of <0.001, as compared to <0.01 in the original. The errors have been corrected in the HTML and PDF versions of the article.