Reverse Phi

Reverse phi is a fabulous effect discovered by Stuart Anstis and Brian Rogers where a stimulus moves to the right and reverses contrast. The result is to see motion to the left, all well predicted by the Fourier components of the stimulus.

What I add here is a compelling version of reverse phi with counterrotating inner and outer rings. When fixating the central dot, the outer ring appears to rotate counterclockwise and the inner ring clockwise. Look directly at the moving bars themselves though and you will see that they are actually moving clockwise in the outer ring and counterclockwise in the inner ring.

Reverse phi rings