The perceived color of a stimulus depends not only on its spectral properties, but also on those of its surround. For instance, a patch that looks gray on an achromatic surround appears reddish when surrounded by green, and greenish when surrounded by red. Previous studies showed that the effect of the surround is repulsive: It enhances the perceptual difference between stimulus and surround. Here, we performed psychophysical experiments to quantify the repulsion. To report the results, a notion of distance in color space was required. We therefore proposed an individually tailored metric in color space that captured the perceptual abilities of each observer. To define the metric, we determined the minimal chromatic difference between a stimulus and its surroundrequired by each subject to detect the stimulus. Next, observers performed discrimination experiments between two spatially localized stimuli presented on a surround of a different chromaticity . The surround color affected the discrimination thresholds. Quite remarkably, when these thresholds were expressed in the color coordinatesdefined before , the change in thresholds followed a simple law that only depended on the  distance between the surround and  the two compared stimuli. Perceptual coordinates, hence, reveal the symmetry of the repulsion effect. This finding was confirmed and modeled with a third experiment, in which subjects were asked to match the color of two stimuli surrounded by two different chromaticities  .
Our sensory systems transform external signals into neural activity, thereby producing percepts. We are endowed with an intuitive notion of similarity between percepts, that need not reflect the proximity of the physical properties of the corresponding external stimuli. The quantitative characterization of the geometry of percepts is therefore an endeavour that must be accomplished behaviorally. Here we characterized the geometry of color space using discrimination and matching experiments. We proposed an individually tailored metric defined in terms of the minimal chromatic difference required for each observer to differentiate a stimulus from its surround. Next, we showed that this perceptual metric was particularly adequate to describe two additional experiments, since it revealed the natural symmetry of perceptual computations. In one of the experiments, observers were required to discriminate two stimuli surrounded by a chromaticity that differed from that of the tested stimuli. In the perceptual coordinates , the change in discrimination thresholds induced by the surround followed a simple law that only depended on the perceptual distance between the surround and each of the two compared stimuli. In the other experiment, subjects were asked to match the color of two stimuli surrounded by two different chromaticities . Again, in the perceptual coordinates the induction effect produced by surrounds followed a simple, symmetric law. We conclude that the individually-tailored notion of perceptual distance reveals the symmetry of the laws governing perceptual computations .
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The perceived color of a stimulus depends not only on its spectral properties, but also on those of its surround. For instance, a patch that looks gray on an achromatic surround appears reddish when surrounded by green, and greenish when surrounded by red. Previous studies showed that the effect of the surround is repulsive: It enhances the perceptual difference between stimulus and surround. Here, we performed psychophysical experiments to quantify the repulsion. To report the results, a notion of distance in color space was required. We therefore
Our sensory systems transform external signals into neural activity, thereby producing percepts. We are endowed with an intuitive notion of similarity between percepts, that need not reflect the proximity of the physical properties of the corresponding external stimuli. The quantitative characterization of the geometry of percepts is therefore an endeavour that must be accomplished behaviorally. Here we characterized the geometry of color space using discrimination and matching experiments. We
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in color space that captured the perceptual abilities of each observer. To define the metric, we determined the
defined in terms of the
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between a stimulus and its surroundrequired by each subject to detect the stimulus. Next, observers performed discrimination experiments between two spatially localized stimuli presented on a surround of a different chromaticity . The surround color affected the discrimination thresholds. Quite remarkably, when these thresholds were expressed in the color coordinatesdefined before
required for each observer to differentiate a stimulus from its surround. Next, we showed that this perceptual metric was particularly adequate to describe two additional experiments, since it revealed the natural symmetry of perceptual computations. In one of the experiments, observers were required to discriminate two stimuli surrounded by a chromaticity that differed from that of the tested stimuli. In the perceptual coordinates
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thresholds
discrimination thresholds induced by the surround
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perceptual
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each of
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Perceptual coordinates, hence, reveal the symmetry of the repulsion effect. This finding was confirmed and modeled with a third experiment, in which
In the other experiment,
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. Again, in the perceptual coordinates the induction effect produced by surrounds followed a simple, symmetric law. We conclude that the individually-tailored notion of perceptual distance reveals the symmetry of the laws governing perceptual computations