“Stepping feet” Motion Illusion

From Michael’s “Optical Illusions & Visual Phenomena”

What to do & observe

Fixate off-centre, but observe the blue and yellow patches (‘feet’). Whenever the grid is visible, the feet seem to step out of phase, while in reality their movement is always parallel. Like tiny feet going tip-tap-tip-tap…

Comments

Stuart Anstis first demonstrated this illusion in 2003. Latest pertinent paper: Howe et al., 2006

Level-1-explanation:

Press the “colour off” 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-2-explanation:

Level-1-explanation plus: The edge information is only contained in colour, and the (magnocellular) motion system cannot see it.

Level-3-explanation:

The effect still persists when there is a slight contrast between foot and grating (not shown here). Anstis (2004) attributes this to slowing down of motion under conditions of reduced contrast (Thompson 1982), and goes much deeper into the subject.

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.

Level-2007-explanation:

Just when I thought this illusion can be rather intuitively understood, there appears this complicated paper Howe et al. (2007), disproving my above thoughts and suggesting a very intricate explanation. Recently, I included this paper in a seminar and we tried to follow its reasoning and evidence. In consequence, my 2008 explanation regresses to level 1 & 2 above.

Sources

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