Abstract
Lightness perception has been mainly studied in the achromatic domain. We started to explore how lightness perception works with achromatic and chromatic stimuli when they are perceived together. The general question we started to answer, exploring also the chromatic domain, concerns how lightness perception works when coloured and grey surfaces are presented at the same time. Following the concepts proposed by the anchoring theory (Gilchrist et al. 1999) we examined at first if the anchoring and the scaling rules worked as the same as with grey surfaces alone. We replicated the five squares Cataliotti and Gilchrist's experiment by using the conditions of low articulation and Gelb light. We substituted in each condition an achromatic surface with a chromatic one of the same luminance. We found that scaling is not precise and the lightness of the considered surfaces is compressed, but we also found that scaling is affected differentially compared to the achromatic condition, when the yellow, replacing the white, is present in the visual scene. In another experiment, subjects have been asked to judge a black surface in three different conditions: 1) classic Gelb (only the black surface is present), 2) grey surfaces (classic staircase Gelb effect), and 3) coloured surfaces (chromatic staircase) of the same luminance of the grey ones. We found a significant difference among the 3 conditions. Other experiments have been driven showing the importance of better understanding the relation which occurs between colour and luminance variations in lightness perception.