Contrast salience across three-dimensional chromoluminance space.

We have extended previously studies of relative contrast salience [Switkes & Crognale, 1999. Comparison of color and luminance contrast: apples versus oranges? Vision Res. 39 (10), 1823-1831] to chromaticities intermediate to cardinal chromatic axes. We find that: (i) observers can reliably match the perceptual contrast of gratings differing in chromoluminance irrespective of whether they lie along canonical axes or intermediate axes; (ii) the relative perceptual contrast of gratings in the isoluminant plane correlates well with a metric based on an ideal observer but added luminance results in a perceptual contrast lower than would be predicted by this model; (iii) contrast matches in the isoluminant plane can be modeled by simple combinations of the cone contrasts for the LM and S pathways; (iv) unipolar, non-opponent, mechanisms may subserve the perception of contrast; and (v) the relative salience of suprathreshold chromatic gratings varies in accord with contrast thresholds but the low spatial frequency increase in luminance contrast threshold is not manifest in a reduced suprathreshold salience for luminance gratings.