The figure on this site was taken from:
Hamburger, K., Prior, H., Sarris, V.
& Spillmann, L. (2006). Filling-in with colour: Different modes
of surface completion. Vision
Research, 46, 1129-1138.
Fixate the central point strictly for
period of time (e.g., 20 seconds)
without blinking or any eye movements.
You should perceive filling-in after a
few seconds, meaning that the central disk
will start to appear greenish or the whole pattern
will appear in gray.