At alternation rates around 1Hz, it is easy to pair colour and form correctly

 

At alternation rates around 5Hz, the perceptual alternations between the two spirals give way to a perceptual superposition of clockwise and anti-clockwise spirals. It is now impossible to pair the colours and forms correctly.

 

In the sum (or at alternation rates in excess of 20Hz) there is not time to extract global form before the next frame. Consequently it is impossible to distinguish alternation between opposing spirals from alteration between concentric and radial patterns.