1. © Van Lier & Vergeer, 2006 Visual Illusion Contest 2006 Rob van Lier & Mark Vergeer NICI, Radboud University Nijmegen, The Netherlands Catching Patches

2. © Van Lier & Vergeer, 2006 A fuzzy grid...

3. © Van Lier & Vergeer, 2006 A fuzzy grid. Dark background  no patches

4. © Van Lier & Vergeer, 2006 With luminance edges however...

5. © Van Lier & Vergeer, 2006 A fuzzy grid. Dark background, light edges  dark patches

6. © Van Lier & Vergeer, 2006 A fuzzy grid. Dark background, dark edges  dark patches (weak )

7. © Van Lier & Vergeer, 2006 A fuzzy grid. Light background  no patches

8. © Van Lier & Vergeer, 2006 A fuzzy grid. Light background, dark edges  light patches

9. © Van Lier & Vergeer, 2006 A fuzzy grid. Light background, light edges  light patches (weak)

10. © Van Lier & Vergeer, 2006 Another grid, but now with thicker fuzzy bars...

11. © Van Lier & Vergeer, 2006 A fuzzy grid. Dark background  no patches

12. © Van Lier & Vergeer, 2006 A fuzzy grid. Dark background, light edges  dark patches

13. © Van Lier & Vergeer, 2006 A fuzzy grid. Dark background, dark edges  dark patches (weak )

14. © Van Lier & Vergeer, 2006 A fuzzy grid. Light background  no patches

15. © Van Lier & Vergeer, 2006 A fuzzy grid. Light background, dark edges  light patches

16. © Van Lier & Vergeer, 2006 A fuzzy grid. Light background, light edges  light patches (weak)

17. © Van Lier & Vergeer, 2006 Patches follow random changes of the edges at the crossings...

18. © Van Lier & Vergeer, 2006 Random edges at intersections – 1

19. © Van Lier & Vergeer, 2006 Random edges at intersections – 2

20. © Van Lier & Vergeer, 2006..and using a ‘Geier pattern‘...

21. © Van Lier & Vergeer, 2006 A Geier-modified Hermann grid – no patches

22. © Van Lier & Vergeer, 2006 The return of the patches...

23. © Van Lier & Vergeer, 2006..catched between the edges!

24. The patches that appear at the crossings in the Hermann grid are known to disappear when the straight contours in the grid are distorted, e.g. by curvature (Geier, 2004). Here, we present an illusion based on Hermann- grid like gratings in which the contours are quite randomly distorted. These distortions guarantee a severe reduction or complete disappearance of the visibility of the patches. Starting with these gratings we show that the patches at the crossings return when luminance edges are introduced and extended at the intersections. The ‘returned’ patches have the same relative lightness properties as they would have in a regular Herman grid (dark patches when the crossing bands are relatively light, and light patches when the crossing bands are relatively dark). In addition, the polarity of the perceived lightness difference does not depend on the lightness of the edges (i.e., whether they are dark or light). A remarkable effect here is that at the crossings the whole area between the edges is perceived to have a different lightness, irrespective of the shape of that area (i.e., whether the edges bend inward or outward etc.). Measurements on perceived lightness differences confirmed the above observations (and also for observations on various other edge manipulations). A manuscript is currently in preparation (Van Lier & Vergeer). © Van Lier & Vergeer, 2006

25. The end Thanks!