One should concentrate on what is changing. My guess is that if edge colors are changing in concert you'll experience one motion phenomena and if the are changing alternatively you will find another motion phenomena. It has to do with the brain not being prepared to resolve time of edge change with color more or less like inserting sound in silent space can generate a sense of sound continuity.
WE are good at processing time, image moving and sound moving, if they are normally occurring in an environment where we generally habit.
For me the image seemed to be moving upward even though it wasn't moving at all.