  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.
