What can you see? A thin black line runs between two rows of black squares that are slightly shifted against each other (as shown on the far right). In the upper part of the picture the line tilts slightly to the right. In the lower part of the picture, however, the line tilts to the left. In fact it is a straight vertical line. This illusion was first described 1897 in a journal article by the German-American psychologist Hugo Münsterberg (1863-1916). That’s why it has the name Münsterberg illusion. The effect can be seen even if the line only has grey tones (as shown on the left). Most explanations are based on the interplay of differences in contrast and the perception of geometric units.

What can you do? In the program with the sliders you can change the thickness of the separating line (llne), the size of the squares (side_square), and the distance between the squares (gap_squares). Moving the mouse you can shift the left row of squares vertitally. Thus you can try to reproduce the above original. You can fix a result by pressing the [space bar]. With a mouse click you can shift the squares again. At which distances of the squares is the subjectively perceived inclination of the figure strongest for you? What is the effect of widening the dividing line?

With color_line or color_squares the graphic elements can be brightened up. What influence does this have on the perceived effect?

Related topics: Checkerboards, Café wall illusion

References:
Kitaoka, A. (2007). Tilt illusions after Oyama (1960): A Review. Japanese Psychological Research 49, No. 1, 7-19. Availabe as download.
Kreiner, W.A. (2016). Die Münsterberg-Täuschung. Ulm: Universität Ulm. Available as download.