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ILLUSIONS - "Stepping feet” Motion Illusion
What to do
With Grating± you can toggle the grating appearance animation. Do it at the right moment, and the grating stays on or off.
The sliders at top and bottom control the luminance of their respective ‘foot’. When you make the yellow foot brighter than the light bars of the grating, it does not lag behind any more. When you make it very dark, it moves together with the blue foot. The sliders (added 2010) allow you to experiment with the explanations below, I still prefer the level 2.
There are more buttons with (hopefully) obvious functions.
Stuart Anstis first demonstrated this illusion in 2003.
Press the "Color–” button. Now it becomes obvious that the edges of the light ‘foot’ merge with the light bars, and are only visible when they traverse the dark bars. So half of the time there really is no motion cue, and perception goes into default, i.e. no motion. For the dark foot the same holds, only at alternate times.
With reduced contrast of the grating (button "Contrast –”) isoluminance of edges and grating is no longer present, so the effect disappears.
Level-1-explanation plus: The edge information is only contained in colour, and the (magnocellular) motion system cannot see it.
The effect still persists when there is a slight contrast between foot and grating (can be achieved with the sliders). Anstis (2004) attributes this to slowing down of motion under conditions of reduced contrast (Thompson 1982), and goes much deeper into the subject.
Just when I thought this illusion can be easily understood, there appears this complicated paper Howe et al. (2007), seemingly disproving my above thoughts and suggesting a very intricate explanation. Recently, I included this paper in a seminar and we tried to follow its evidence and reasoning, and couldn't. In consequence, my favoured explanation regresses to levels 1+2 above.