Relational color constancy in achromatic and isoluminant images

Relational color constancy, which refers to the constancy of perceived rela tions between surface colors under changes in illuminant, may be based on t he computation of spatial ratios of cone excitations. As this activity need occur only within rather than between cone pathways, relational color cons tancy might be assumed to be based on relative luminance processing. This h ypothesis was tested in a psychophysical experiment in which observers view ed simulated images of Mondrian patterns undergoing colorimetric changes th at could be attributed either to an illuminant change or to a nonilluminant change; the images were isoluminant, achromatic, or unmodified. Observers reliably discriminated the two types of changes in all three conditions, im plying that relational color constancy is not based on luminance cues alone . A computer simulation showed that in these isoluminant and achromatic ima ges spatial ratios of cone excitations and of combinations of cone excitati ons were almost invariant under illuminant changes and that discrimination performance could be predicted from deviations in these ratios. (C) 2000 Op tical Society of America. [S0740-3232(00)01502-7] OCIS codes: 330.0330, 330 .1720, 330.1690, 330.4060.