Nat. Neurosci. 6, 1253–1254 (2003)

In the version of this brief communication initially published online, the Supplementary Methods file included some incorrect figure callouts. Callouts to Supplementary Fig. 1a–c have now been corrected to read Supplementary Fig. 2a–c, and callouts to Supplementary Fig. 2 have now been corrected to read Supplementary Fig. 1.