This movie was my fourth to get into the shortlist of top ten entries to the Best Illusion of the Year contest.

The illusion presents moving textures within static tessellating heads.  To check out tessellations, click on ‘tessellations’ near the bottom of the black menu bar to the left.  The outlines of the tessellating pattern of interlocking left and right-facing heads are stationary.  But one set of he heads appear to be moving.  In fact,   only the textures within the heads move.  The textures are identical in every head, except that they are rotated 180 degrees in one set of heads.

We might expect simply to see movement of the textures in opposed directions.  Instead most observers make a kind of figure/ground selection, seeing one or other set of heads as moving, against a static background presented by the other set.  Either head may be the one selected to move, and the selection can ‘flip’ spontaneously, or, with practice, by choice.   The brain seems to be trying to follow expectations about how objects and backgrounds behave, rather than the objective stimuli.

But then, at the end, we see the movie as a surface texture on moving surfaces.   We still see one just set of heads apparently moving.   But textures on surfaces don’t move about.   This is not what we might be expecting to see at all.  Whatever is going on is not straightforward.

The illusion is a variant on illusions of induced motion, reported by numerous western and islamic scholars in antiquity.   Ptolemy, writing in 2nd century AD Alexandria, described how a stationary boat floating in a moving stream of water could appear to be moving, whilst the water appeared stationary.

What is new in the movie here is that the illusion arises not from misperception of moving and static components, but of two rival moving components.