In the animation above you see “pigeons” craning their neck while scurrying along, and worms shrinking and extending while worming their way. Looks a little silly…
As you probably already guessed: the movement is, in reality, quite smooth. Try deselecting the “Grid” checkbox: now all scurrying gives way to fluid movement.
It is interesting to play with the colour of the figures (initially blue). With bright colours the scuttling is less apparent, and there is a (rather light) grey level where the scuttling and motion in general is minimal. The illusion becomes stronger if you don’t look directly at it. Some presets are available in the color pop-up.
This was Jun Ono’s, Akiyasu Tomoeda’s, and Kokichi Sugihara’s contribution to the 2014 “Best illusion of the year” contest. As these authors write, it is a version of the “kickback illusion”, which is closely related to the Stepping Feet illusion – see my explanation there.
A simple concrete explanation: Start with a “worm”, imagine it had the same colour than the dark grid stripes. Thus, when its front end crosses the dark stripe, this edge has no luminance step – thus cannot trigger our motion detectors. The length of a worm is chosen so that when its front is on a dark stripe, its back will be on a light stripe. Consequently, when the front end is not visible, its back end will be obvious, and its length seemingly changes: the worm seems to shrink. As soon as the front end leaves the dark stripe, it extends again.
For the pigeons the same explanation holds. Their heads are one stripe ahead of the body, thus the nicking motion ensues.
A commenter interestingly suggested this could be explained by the Moiré effect (here and here). So I added the color selection pop-up, initially set to
No color sets the shape to a dark gray, the “pigeon neck” effect still obtains. Both ´High contrast…´ options let the effect vanish. Why: because the luminance of the shape is about the middle between the grid bars, leaving sufficient contrast to drive the motion detectors whether the border is on the dark or white grid lines. This argues against the Moiré explanation.
Ono J, Tomoeda A, Sugihara K (2014) “Pigeon-Neck illusion” entry to the 2014 illusion contest
Howe PDL, Thompson PG Anstis SM, Sagreiya H, Livingstone MS (2006) Explaining the footsteps, belly dancer, Wenceslas, and kickback illusions. JOV 6,12:5