Observe the movement of blue and yellow “feet”. The feet seem to step alternately, like tiny feet going tip-tap-tip-tap… This is more pronounced if you do not look directly at the feet, but between them.
In reality, their movement is always simultaneous. I find this phenomenon particularly cute.
When deselecting the Hi contrast checkbox, the grating contrast is reduced, demonstrating that the feet are not stepping out of phase.
There is a shape pop-up, defaulting to Step; check out what happens when setting to Wiggle (an addition by Akiyoshi Kitaoka).
The stepper bars/foot sets the number of bars per foot. This is preset to 4; the effect occurs also at all other even values. For odd values, e.g. 5, the motion also becomes odd: instead of stepping alternately, the feet seem to move like worms, extending and shortening. [This option added in 2012.]
The sliders at the top and bottom control the luminance of their respective feet. When you make the yellow foot brighter than the light bars of the grating, it does not lag behind anymore. When you make it very dark, it moves together with the blue foot. The sliders (added in 2010) allow you to experiment with the explanations below; I still prefer the “level 2” explanation.
The pop-up menu below Hi contrast allows you to select other colour combinations, namely red-green and gray shades. For red-green, the contrast of the grating is reduced a little, making the dark lighter, but it still needs a very dark red (=brown) to equate luminance.
There are more buttons with (hopefully) obvious functions.
Stuart Anstis first demonstrated this illusion in 2003.
Level-1-explanation:
Color checkbox. 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.Level-2-explanation:
Level-3-explanation:
Level-2007-explanation:
The demonstration above was inspired by Stuart’s version. Thanks to Wolfgang Wesemann for first drawing my attention to this phenomenon, and Wolfgang Beyer for bright ideas.
Kitaoka A, Anstis S (2021) A review of the footsteps illusion. Journal of Illusion 2021, 2: 5612
Anstis SM (2003) Moving objects appear to slow down at low contrasts. Neural Netw 16:933–938
Anstis SM (2004) Factors affecting footsteps: contrast can change the apparent speed, amplitude and direction of motion. Vision Res 44:2171–2178
Thompson P (1982) Perceived rate of movement depends on contrast. Vision Res 22:377–380
Howe PDL, Thompson PG, Anstis SM, Sagreiya H, Livingstone MJ (2006) Explaining the footsteps, belly dancer, Wenceslas, and kickback illusions. Journal of Vision 6, 1396–1405