What to do and see
Look steadily at one of the three black crosses in the middle. Note that the red balls follow a curvy path.
Now you follow a red ball with your eyes: It no longer follows a curved path! This becomes even clearer when you toggle the Lines checkbox (or press space). In fact, all balls always move along a straight line.
A strong effect for me. There are many parameters (speed, color, size, angle) that can be adjusted for your own experiments. Hidden controls:
Spacebar hides/shows the lines, key “b” reverses view order of balls/lines.
Comments
First the name: It is a tongue-in-cheek play on the German town Mainz, where the European Conference on Visual Perception (ECVP) took place in 2025. There Ian M. Thornton, Anna Riga, Sunčica Zdravković and Dejan Todorović first demonstrated this illusion.
Second: how does it work? This is not fully clear at this time. The authors argue convincingly that afterimages play a role: When a ball traverses part of a line, it “releases” an afterimage. If the ball trajectory is offset from the white curved lines (use the offset slider), the illusions vanishes.
Sources
Thornton IM, Riga A, Zdravković S, Todorović D (2025) OSF Preprint.