Contrast of the borders determines stratification of transparent figures.

Figure/ground segregation of both opaque and transparent objects involves their mutual stratification, fixed or reversible, in different depth planes. The essential factor deciding the stratification of transparent figures seems to be colour contrast; but surface colour contrast can be artificially altered through the Craik-O'Brien-Cornsweet illusion with a consequent effect on the surface stratification. We show that two outlined overlapping rectangles appear differently stratified as a function of the contrast of their margins with the background. In a first experiment 2 subjects measured the contrast threshold for inverting the depth position of two rectangles, either with achromatic or with equiluminant chromatic edges and background. In the second experiment 22 subjects decided the depth position of the same rectangles and its perceptual evidence as a function of the contrast of the margins with the background. Results from both experiments show that margins of high contrast displace the adjacent areas in different planes while low contrast put them on the same plane.