Static Illusions: Why Moving Objects Can Appear to be Stationary

Piers Howe
Visual Attention Lab
Brigham and Women's Hospital
 
There are many “motion” illusions in which a stationary object appears 
to move. The converse, where a moving object appears to be stationary, 
is much rarer, yet potentially more important. In this talk I will 
discuss such “static” illusions, using the footsteps illusion as an 
example. It consists of a yellow bar that moves over a black-and-white, 
striped background. Although the bar moves at a constant rate, 
periodically it appears to come to a complete halt. Previously, this 
illusion has been explained in terms of the variations in contrast at 
the leading and trailing edges of the bar that occur as the bar 
traverses the striped background. Here, we show that this explanation 
is inadequate and instead propose an alternative. We show that this new 
theory can explain all the data on the footsteps illusion as well as the 
belly dancer and Wenceslas illusions. We conclude by presenting a new 
illusion, the kickback illusion, which, although geometrically similar 
to the footstep illusion, is mediated by a different mechanism, namely, 
reverse phi motion.