Chevreul illusion is a luminance staircase comprising homogeneous grey steps whose border neighbouring a lighter step appears to be darker. The generally accepted explanations for this illusion are based on the concept of centre-surround antagonism. In our viewpoint, the predictions of these explanations contradict actual perception, as our psychophysical experiment (subjects=20) shows.

In order to test our concept, we inserted the luminance staircase in a luminance ramp as background. Afterwards, we girdled the previous image with another background ramp. The variations of these images were presented on a computer screen. Subjects could modify the gradient of the outer ramp, until they perceived that each step of the inner staircase became homogeneous.

Results: (i) If the direction of the luminance change of the background ramp and the inner staircase is different, the illusion decreases or ceases, whereas it increases in the opposite case. (ii) Another ramp encompassing the image including the staircase and its first background ramp has no impact on the perception of the inner luminance staircase. Consequently, edges behave as segment boundaries, and the impact of the background ramp infiltrates only through one segment boundary. To explain these results, we present a computational model, and its computer simulation.