Chromatic discrimination data are presented for pulsed and steady stimuli as a function of surround chromaticity and structure. All stimuli and surrounds were at equiluminance and at a constant level of short-wavelength-sensitive cone excitation. The test stimulus was a square army of four 1 degrees squares. A 0.07 degrees crosshair of the same chromaticity as the surround separated the squares. Both the test stimuli and the surrounds varied in relative excitation of the long-wavelength and middle-wavelength sensitive cones. When stimuli briefly replaced a portion of a steadily viewed background (the Pulse Paradigm), the discriminations were optimal at the background chromaticity and degraded for chromaticities away fr om the background The discrimination steps were independent of the background size, which varied from a spatially extensive display to one matching exactly the appearance of the test array. Discrimination was determined only by the spatio-temporal chromatic contrast of the stimulus relative to the background. When the stimuli were presented continuously within a surround (the Pedestal Paradigm), discrimination was still determined by the surround chromaticity, independent of the surround size. Evert a narrow 0.07 degrees crosshair was sufficient to establish optimal discrimination at the crosshair chromaticity. With the surround and crosshair dark, spectral opponent channels maintained an intrinsic normalization near equal energy while. There tvas little indication of adaptation to the test stimuli. The data were fit by a model of spectral opponency linking detection and discrimination as a function of both retinal illuminance level and chromaticity. The model is explicitly based on observations of the behavior of retinal ganglion cells of the Macaque retina. The model incorporates well-accepted psychophysical concepts that adaptation in cone spectral-opponent channels occurs at multiple sites both before and after spectral opponency. (C) 2000 John Wiley & Sons, Inc.