Neurobiology. In the articles “An empirical basis for Mach bands” by R. Beau Lotto, S. Mark Williams, and Dale Purves, which appeared in number 9, April 27, 1999, of Proc. Natl. Acad. Sci. USA (96, 5239–5244), and “Mach bands as empirically derived associations” by R. Beau Lotto, S. Mark Williams, and Dale Purves, which appeared in number 9, April 27, 1999, of Proc. Natl. Acad. Sci. USA (96, 5245–5250), the following correction should be noted. The reproduction of some of the figures in these papers was unsatisfactory due to the presence of moiré patterns and other deficiencies in the published versions. Given the difficulty in faithfully reproducing gradients in print, readers may wish to view the electronic versions of the figures at purveslab.neuro.duke.edu.