Cover legend: In each panel, the radial distance from the center represents contrast along a particular color direction. The saturation of the color at a point in the plane indicates the strength of the response of neurons tuned to that color direction. Different panels show the population response either in the control state (center panel) or after prolonged exposure to color modulation along four different directions in the isoluminant plane. Left and right panels of the center row show the effects of habituation that stimulates only L and M cones (left–right); top and bottom panels of the middle column show the effects of habituation that stimulates only S cones (top–bottom); panels at top left and bottom right, and bottom left and top right, show effects of habituation to intermediate directions that stimulate all three classes of cones. For more information, see the article by Tailby et al. in this issue (pages 1131–1139).