Fig. 2: Experiment 1-2.

The first row (a–d) illustrates stimulus displays and task (a), and results (c, d) for Experiment 1. Observers adapt to the inducer display for 30 s (yellow in a). Then the inducer display is replaced by a circle of background grey together with nine comparison colours (purple in a) in a circular arrangement around the grey circle. The grey circle in the centre is perceived as the induced afterimage, and observers select the comparison colour that best matches the colour they see in the centre. b gives examples of afterimage matches averaged across N = 31 participants in Experiment 1a with maximally saturated colours. Discs correspond to inducers, stars to average matches of perceived afterimages. The thin line in the background indicates the cone-opponent direction, the dashed line the prediction of the cone-adaptation model (see Figs. S2a and S3 for results with the other four colours). c, d illustrate results from N = 52 participants in Experiment 1b. The second row (e–h) corresponds to Experiment 2a with the chaser-like paradigm. In (e), the chromatic ring is the inducer. Participants fixate the centre, and the moving grey circle on the ring reveals the afterimage. Observers adjusted hue and chroma of the centre circle to match the moving one. Average afterimage matches from N = 10 participants and overall N = 45 measurements per colour (black curve in f) closely correspond with predictions by cone adaptation (red outline), yielding a correlation (bottom left) between measured and simulated afterimage intensity (i.e., chroma) across the N = 72 inducer colours. The curves in the third column (c, g) show the deviation of the cone-adaptation model (red curve) and of the measured afterimages (black curve) from the colours opponent to the inducers. Correlations between simulated and measured deviations from opponency are shown at the bottom of the diagrams. The correlation reflects the high similarity in profile of the two curves. The hue histograms in the last column (d, h) counts hue responses (azimuth in f) and displays the resulting frequencies as a function of azimuth in a polar plot. The histogram of the measurements is shown by the grey area in the background, and the histogram of the simulated afterimages by the red outline. Panel d provides the hue histogram for the N = 52 measurements for 24 colours in Experiment 1b and (h) those for the N = 45 measurements for 72 inducer colours in Experiment 2a. In (h), a smoothed version of the hue histogram is shown by the black line. Hue histograms feature three clusters that closely correspond with model predictions (red line), hence confirming the results from (c, f, g). Additional results from Experiment 2b with stimuli in DKL space are provided by Fig. S12 of Supplementary Material. The last row illustrates average deviations of different model predictions from measured afterimage hue (i) and chroma (j). The top and bottom edges of each box are the upper and lower quartiles, the line inside each box is the median; whiskers are the minimum and maximum values that are not outliers, and dots outside the whiskers are outliers identified as values outside the interquartile range (box) by 1.5 times the size of the interquartile range. The grey symbols (Noise) provide estimates of noise calculated as the average difference of each individual's measurement from the group mean (interindividual variation) in Experiment 1 and of each single measurement from the mean of each participant (intraindividual variation) in Experiment 2. Chroma has been z-scored to focus on relative differences across hues. Supplementary Movies 1–4 visualise the differences between opponent and afterimage hues (see also section A and Fig. S1 of Supplementary Material).