Correction: In the article, “Anterior Forebrain Neurons Develop Selectively by an Intermediate Stage of Song Learning in Zebra Finches,” by M. M. Solis and Allison J. Doupe, which appeared on pages 6447–6462 of the August 15, 1997 issue, the formula for d′ on page 6449 of Materials and Methods was written incorrectly. The correct formula (and the one with which the data were calculated) should read: