What you see in the movie is two striped disks moving bodily in one direction. Stripes are painted, stuck to the disks. On static background (upper field), what you see is what you get. But on flickering background (lower field), stripes now move much faster than the disk, overtaking it! Stripes are no longer stuck to the disks but drifting within a circular window. Illusion is stronger in peripheral vision; but you can see some of it even when you’re tracking the disks with your eyes.
Patterns don’t matter – random dots and sinusoidal gratings also give the same effect.
You can measure the strength of the illusion in this movie. Within each window, grating moves at 25%, 50%, 75%, 100% of the window speed (from top to bottom). Since on a flickering background (right), it makes the gratings move much faster than their windows, they look quite different from their corresponding windows on the left (control conditions). For most observers, the gratings appear locked to the windows labelled 25% or 50%.
While gray contour around the window enhances the illusion, you can still see it without the contour (at least in some cases).
We think this is due to reversed-phi illusion – windows lag behind because of reversed-phi, making gratings appear relatively faster.
To see reversed-phi motion, target (in this case, window) has to change its polarity, i.e. being lighter or darker than its background. So, if we replace the background flicker with color flicker with minimal luminance change, the illusion disappears.
For the same reason, if we replace the gray contour with black contour, the illusion disappears.
this only applies to WITHOUT-gray-ring stimuli; when background flicker has very low contrast, the illusion disappears, again. But lowering contrast of the grating does almost the opposite, enhances the illusion.
[notes for us: yes, this should only apply to without-ring condition, assuming reversed phi explains this illusion. But even with ring, low contrast background seems to reduce the illusion very much! How can this happen?]