For ECVP 2016
 The “witch ring” Illusion: experimental stimuli

This is a post to show animations to accompany a poster, which I and my colleague Priscilla Heard presented at ECVP 2016.  The poster reported experiments related to the witch ring illusion.  The movies below show illusory effects of sideways movement in streaming patterns of dots.

This first movie shows how the static track along which a single file of dots are travelling appears to move sideways when the single track is embedded in a fan-shaped pattern of tracks.


How do added depth cues affect the illusion?

The following three movies show how the illusion of sideways movement can be reduced if effects of perspective depth are added to the patterns of streaming dots. Each movie shows on the left the version of the movie shown above, for comparison with the movies with added perspective, shown on the right.  In the first movie both dot speed and dot size diminish as they would if the centre of the pattern was in the distance. In the second movie only dot speed changes;  in the third with only size changes.

At right above, both dot speed and dot size changes.  The sideways movement effect is replaced by a depth effect.

At right above, only dot velocity changes.  Velocities in different parts of an image are such powerful depth cues that even without dot size change, the sideways movement illusion is replaced by a depth effect.

To the right above, only dot size changes.  Without velocity change, there is less reduction of the sideways movement effect.

We are still puzzling out why the brain sees a sideways  movement effect in the absence of the size and velocity depth cues.