Correction to: Nature Neuroscience https://doi.org/10.1038/nn956, published online 15 October 2002. Following publication of this article, the authors were alerted to an image duplication in Fig. 4, panels g and i. Due to the age of the article, the original confocal images cannot be retrieved to confirm or correct either panel. The authors acknowledge the error in the representative images and note the quantitation of multiple animals in Fig. 4k supports the key results.