Abstract
PURPOSE: Chromatic assimilation usually is studied by measuring color-appearance shifts caused by introducing context. In this study, however, assimilation is assessed by temporal nulling of the perceived assimilation from a slowly varying inducing field. This technique, which measures chromatic assimilation without requiring a color judgment, is used to test whether spread light can account for chromatic assimilation. METHODS: Using a calibrated CRT, a 3 deg, 4 cd/m2 circular field was presented with thin inserted 4-min wide concentric inducing rings (8-min separation between rings). These rings in steady viewing cause chromatic assimilation. In experiments here, the chromaticity of the rings was slowly modulated (1.2 Hz) in the S-cone direction, which caused the circular field to vary in color appearance. The task was to make the circular field appear steady (that is, null the temporal variation in assimilation) by adjusting the modulation amplitude of light added to the circular field. PROCEDURES AND RESULTS: The first experiment verified that modulation of the inducing rings indeed modulates assimilation. For assimilation, the shift in appearance of the circular field is in phase with the modulation of the rings. The nulling stimulation, therefore, should be 180 deg out of phase. Observers made pairwise comparisons between nulling modulation at 180 deg (assimilation) versus 0, 22.5, 45, 67.5, 90, 112.5, 135, 157.5 or 180 deg. A further condition compared 90 deg to the other phase differences. Modulation at 180 deg gave the best null. In the second experiment, nulling modulation at 180 deg was adjusted in amplitude to cancel the modulation induced by the rings, which were at either 2.67 or 6.0 cd/m2. Measurements show a far smaller change in modulation amplitude with luminance than predicted by spread light. CONCLUSION: Temporal nulling of assimilation shows that spread light cannot fully explain chromatic assimilation.