By determining the locations of boundaries between colour categories, we measured changes in the colour appearance of test-reflectances as a function of the simulated illumination. Test-reflectances were displayed against a variegated background of reflectance samples. Under prolonged adaptation to each illuminant, observers demonstrated a high degree of appearance-based colour constancy. By using backgrounds that consisted of chromatically biased sets of reflectances, we tested whether this stability depends on estimates of the illuminant’s cone-coordinates based on simple scene statistics. The chromatic bias of the background had only a small effect on the classification of test materials. To compare the roles of spatially local and spatially extended estimation processes, we then (unknown to the observer) simulated different illuminants on the test and on the background. Observers continued to demonstrate reasonable colour constancy. To examine the relative roles of automatic adaptation and perceptual strategies, we reduced the duration of exposure to the test compared to exposure to the background (under the conflicting illuminant). The results suggest that mechanisms that preserve information across successive test-presentations (e.g. spatially local adaptation with a time course of a few seconds, and perceptual adjustments to levels of reference) are key determinants of the stability of colour appearance.

*illuminant*’s cone-coordinates to those of an equal energy illuminant, also transform the cone-coordinates of

*surfaces*to approximately their cone-coordinates under the equal-energy illuminant. The left-hand panels of Figure 2 help to illustrate why this simple transform will work. The illuminant (indicated by a black cross within a red circle) plots at the extreme end of the line of reflectances. Multiplying each cone-coordinate by the ratio of the illuminant cone-coordinates will transform most cone-coordinates to the unit diagonal, thus equating neural signals under the two illuminants. Mathematically, the Ives transform consists of multiplying all cone-coordinates by the same diagonal matrix and has been widely analyzed in the computer vision literature where it is misnamed the Von Kries transform. Von Kries’ original transform multiplies each local cone-coordinate by a scalar depending only on its

*local*magnitude, and thus shifts all colours towards a neutral colour (Vimal, Pokorny, & Smith, 1987; Webster, 1996) rather than achieving the required transformation to an equal energy illuminant.

*chromaticity*that elicited the percept of neither red nor green (or neither yellow nor blue) was substantially different for the two illuminant-conditions, while the classification of

*materials*was largely unaffected.

^{2}) equiluminant plane of the MacLeod-Boynton chromaticity diagram. Traces of classification boundaries are plotted for three observers. Red lines represent boundaries obtained under sunlight illumination; blue lines represent boundaries obtained under skylight illumination. Clearly, the illuminant has a large effect on the location of red/green and yellow/blue boundaries in chromaticity space.

*a*

_{1}/

*a*

_{2}), which is derived from the achromatic settings, reveals the scaling factor used by the putative multiplicative neural transformation;

*b*

_{1}/

*b*

_{2}provides a summary of the colour conversion imposed by the illuminant change. For perfect constancy (

*a*

_{1}/

*a*

_{2}) = (

*b*

_{1}/

*b*

_{2}) and the index is equal to one. If there is no neural transformation due to the illuminant, the achromatic setting is determined by the cone-coordinates, therefore

*a*

_{1}=

*a*

_{2}, and the index is zero.

*C*= 0 indicates no constancy, and

*C*= 1 indicates perfect constancy. However, since the mapping between chromaticity space and so-called uniform colour space is not yet known, and is likely to be nonlinear and depend on adaptation state, no constancy index can provide an absolute measure of how steady a material will appear under an illuminant change.

^{2}, and expressed as percentages, for the 3 observers, for the L-, M- and S-cone signals and for the L/(L+M) and S/(L+M) opponent signals. Indices for achromatic points evaluated at luminances between 10 and 20 cd/m

^{2}vary by less than 6%. The data presented here show high levels of constancy, with indices ranging from 58% to 94%. For the three observers, HES, HS and JEM, mean constancy indices were 87%, 72% and 94% calculated from cone signals, and 87%, 68% and 93% calculated from opponent signals.

*are*available from the statistical properties of the sample of chromaticities presented to the observer. The mean chromaticity of a scene has been suggested as a cue to the colour of the illuminant because, for a given scene, this statistic varies systematically with changes in illumination. In Experiment 1, the mean chromaticity of the scene provides a reliable estimate of the cone-coordinates of the illuminant, so good colour constancy would be predicted by spatially extended adaptation or alternatively by a high-level mechanism that used the mean to derive an illuminant estimate.

^{2}along the L/M-opponent and S-opponent axes of MacLeod-Boynton space. Constancy indices obtained with balanced backgrounds are re-plotted for comparison (black bars). Indices obtained with red-blue and green-yellow biased backgrounds are plotted as light and dark grey bars respectively. Observers demonstrate high levels of constancy in all conditions.

*b*

_{1}and

*b*

_{2}represent the coordinates of the mean chromaticities of the backgrounds, and

*a*

_{1}and

*a*

_{2}represent the corresponding achromatic settings. Attributing the change in mean chromaticity of the background to a change in illuminant is the wrong assumption, so here the constancy index is zero for perfect constancy, and 1.0 for no constancy. For observers HES and HS, constancy indices evaluated along both axes of MacLeod-Boynton colour space are all less than 0.2 indicating good constancy. Indices for observer JEM are less than 0.2 for the S/(L+M) axis but around 0.4 for the L/(L+M) axis. Under the conditions of our experiment, colour appearance is relatively little affected by a change in the mean chromaticity of the background, and a bias in the set of reflectances available is largely not misattributed to a bias in the spectrum of the illumination.

*global*mechanism would estimate the wrong illuminant for the test, and constancy would be low. In a single trial, the observer has no information about the test-illuminant, since it falls only on a single material and there are no statistical cues to disentangle the material reflectance and the illuminant spectrum. Under this manipulation, information about the test illuminant is available only by collating information over successive trials. We ask whether the classification of test-materials in the inconsistent illuminant conditions will follow that predicted by the background illuminant, or that predicted by the test illuminant.

*b*

_{1}to

*b*

_{2}in Equations 1 and 2. In this analysis, a change in the illuminant on the test is not accompanied by a corresponding change in the illuminant on the background, so the spatial context provides no cues to the illuminant change, signalling instead either steady sunlight (light bars) or skylight (dark bars). So, if performance were determined by the spatial context, the coordinates of the achromatic point (

*a*

_{1}and

*a*

_{2}in Equations 1 and 2) should be identical, and we should measure constancy indices equal to zero. If however performance were determined by the test illuminant, constancy indices should approach one (or rather the value obtained in Experiment 1 for a global illuminant change). Since we used the same set of test-materials for all conditions, the achromatic point predicted by the background illuminant was generally outside the range of boundaries we could measure. Correspondingly, the short red lines in Figure 12 indicate the lower limit of constancy indices that we could measure. For the three conditions where constancy indices are not plotted, we can say only that they fall somewhere below the red lines. Constancy indices obtained in Experiment 3 are in all cases slightly lower than those obtained in Experiment 1, but constancy is far from abolished.