Cover legend: Graphical representation of eight colors used by Brouwer and Heeger. Colors are of equal luminance and saturation, and vary only in hue. As depicted in the image, these colors form a circular color space, matching the perception of similarity between these colors of human observers: the organization of the colors form a logical progression through color space, from red, to orange, yellow, green, blue, purple and back to red. Responses to these colors were measured using functional magnetic resonance imaging (fMRI). In visual area V4, these responses reveals a progression through this perceptual color space, with perceptually similar colors evoking the most similar pattern of responses. The image was rendered with Radiance (Larson and Shakespeare, 1997. Rendering with Radiance. San Francisco: Morgan Kaufmann. Oxford UP). For more information, see the article by Brouwer and Heeger in this issue (pages 13992–14003).