Stroboscopic alternative motion (SAM)

From Michael’s “Visual Phenomena & Optical Illusions”

What to see

On the right there are 2 blue dots which change position. Decide for yourself: are they moving (hopping) horizontally, vertically, or (counter-) clockwise?

What to do

If you see them moving horizontally, use the button “vertical bar”. This erects a barrier, and your percept will likely switch to the condition of vertical motion. Right? You can force the percept to horizontal or vertical using the bars. Once you get the hang of it, you may be able switch it mentally, without the black bars. You even may be able to perceive a rotation, clockwise or counter-clockwise.

Comment

What is the “real motion” of the discs? None; there are actually 4 discs staying put, just their visibility is switched. The grouping to horizontal, vertical or rotatory occurs in our brain.

This belongs to the class of multistable figures and countless research papers have been written on it, starting with von Schiller 1933. The figure is also known as “motion quartet” and, unfortunately, recently a new name “ambiguous quartet” was ascribed (Carter et al. 2008, interestingly in a tactile version).

Sources

von Schiller P (1933) Stoboskopische Alternativbewegungen [Stroboscopic alternative motion]. Psychologische Forschung 17:179–214

Demo of “clustered bistable quartets” (and more) on Peter Schiller's site

Ramachandran VS, Anstis SM (1986) The perception of apparent motion. Scientific American 254:102–109