Colors on grey: Chevreul's laws of contrast and depth

Birgitta Dresp (1) , Adam Reeves (2)

1 CNRS - Centre National de la Recherche Scientifique
2 Northeastern University [Boston]

Chevreul (1839) predicted that a color on a grey background would be the least likely to produce mutual interactions altering the appearance of either (law of true color), and that its luminance contrast directly determines its likelihood to be seen as figure against the grey background (law of contrast). We show that such "true" colors produce unsuspected simultaneous brightness induction effects on their grey backgrounds, where assimilation and apparent contrast may occur in one and the same configuration. We examined the possible link between these induction effects and the perceived relative depth of colored and achromatic inducers as a function of their luminance contrast. In a first task, we measured probabilities of contrast, assimilation, and no effect in a three-alternative forced-choice procedure (background appears brighter on "left", on "right", or the "same"). Sets of 20 red, green, blue, yellow, and light grey, square-shaped inducers were placed on light and dark grey background fields. Inducers of a sole color, darker on one side of the display and brighter on the other, were shown on spatially separated backgrounds and on single fields of a given intensity. Background intensity varied between displays, generating five different levels of inducer contrast for each color, presented in random order. Experiments were run under three environmental conditions: dark-adaptation, daylight, and rod-saturation after exposure to bright light. While these had no significant influence on induction effects produced by colored inducers, we found that achromatic inducers produced significantly stronger contrast effects after dark-adaptation, and significantly stronger assimilation in daylight conditions. The luminance contrast of colored inducers did not influence the induction effects. Grouping two backgrounds into a single one was found to significantly increase probabilities of apparent contrast. Under the same conditions, we measured apparent depth effects in terms of probabilities of the inducers to be perceived as nearer to the observer on the left or right, or at the same distance. These were determined by luminance contrast only, with a marked asymmetry between negative and positive contrast signs, and with significantly higher probabilities of the brighter inducers to be seen as nearer, regardless of adaptation level or background grouping. It is concluded that the induction effects and the depth effects originate from different mechanisms. Having excluded all other cues to depth, we found that Chevreul's law of contrast holds for strong luminance contrasts of positive sign.

Domaines: Sciences cognitives

Birgitta Dresp, Adam Reeves. Colors on grey: Chevreul's laws of contrast and depth. 2014. ⟨hal-01066604⟩