Perceptual spaces and their symmetries: The geometry of color space

Abstract

Our sensory systems transform external signals into neural activity, therebyproducing percepts. We are endowed with an intuitive notion of similaritybetween percepts, that need not reflect the proximity of the physicalproperties of the corresponding external stimuli. The quantitativecharacterization of the geometry of percepts is therefore an endeavour thatmust be accomplished behaviorally. Here we characterized the geometry of colorspace using discrimination and matching experiments. We proposed anindividually tailored metric defined in terms of the minimal chromaticdifference required for each observer to differentiate a stimulus from itssurround. Next, we showed that this perceptual metric was particularly adequateto describe two additional experiments, since it revealed the natural symmetryof perceptual computations. In one of the experiments, observers were requiredto discriminate two stimuli surrounded by a chromaticity that differed fromthat of the tested stimuli. In the perceptual coordinates, the change indiscrimination thresholds induced by the surround followed a simple law thatonly depended on the perceptual distance between the surround and each of thetwo compared stimuli. In the other experiment, subjects were asked to match thecolor of two stimuli surrounded by two different chromaticities. Again, in theperceptual coordinates the induction effect produced by surrounds followed asimple, symmetric law. We conclude that the individually-tailored notion ofperceptual distance reveals the symmetry of the laws governing perceptualcomputations.