Measurements of geometric illusions, illusory contours and stereo-depth at luminance and colour contrast.

We investigated four geometric optical illusions (Zöllner, Müller-Lyer, Ponzo and Delboeuf), plus illusory contour/border induction (Kanizsa) and depth in random-dot stereograms (Julesz). Two different display conditions were compared: equiluminance with chromaticity contrast and heteroluminance without chromaticity contrast. The main results are as follows. (1) The strength of the four geometric optical illusions is the same under both display conditions. The Zöllner illusion reaches its maximum and levels off at a luminance contrast of about 80%; it disappears at luminance contrasts of less than 15%. (2) No illusory contours are perceived in equiluminant Kanizsa figures. The minimum luminance contrast for illusory contour induction in the Kanizsa square is on average 1.8%, for illusory border induction in the abutting grating illusion it is 5.3%. (3) Random-dot stereograms were found to induce depth equally well in both display modes. The disparity threshold for perceiving depth in isochromatic random-dot stereograms levels off at a luminance contrast of 30%. With larger disparities, depth is perceived down to about 10% contrast. The findings suggest that geometric optical illusions of parallelness (orientation), length and size are mediated by the parvocellular system; furthermore, that stereoscopic depth is mediated both by the magnocellular and the parvocellular systems; and that illusory contours are mediated by the magnocellular system.