In the animation below, you can see a ring of four faces gradually morphing between Leighton Meester and Natalie Portman. If you keep your eyes fixated on the green cross in the center, you will notice that the morphing is quite difficult to see—but only when the faces are moving. If you match the movement with your eyes, or if the faces stop moving, you can clearly see the morphing happening.
This illusion was presented twice, in different guises, in the finale of last year's vision sciences illusion contest. Once by Jordan Suchow and George Alvarez, who ended up winning the contest. And once by Rob van Lier and Arnon Koning. So this illusion may not be entirely original, but it is certainly the most aesthetically pleasing rendition out there!
So what's going on here? Previously, I explained this phenomenon in terms of retinal motion. When the faces move, they slide across your retina (the light sensitive part of your eye), at least if you keep your eyes still. So the same face will be 'seen' successively by different parts of your retina. And because the organization of the retina is roughly preserved in visual areas of the brain, the same face will be successively processed by slightly different (sub)areas of your brain. According to the 'retinal motion' explanation, these different brain areas do not communicate effectively enough for changes to be detected. At least not if they are small and gradual.
But, as it turns ...