(Video Link) The rapidly changing faces in this video stimulate your brain to morph them into aliens. As you play the video, defocus your brain and stare only at the "+" in the middle.
The images are all undistorted photos of familiar Hollywood actors. Apparently our visual brain rapidly and automatically tries to resolve the two different faces so that they fit together into a single whole. In so doing, strange exaggerations take place that look like extreme caricatures.
I haven't read a scientist's analysis of this effect, but my own hunch is that the brain is trying to reconcile the right and left visual field of each eye, as well as the images coming stereoscopically from the right and left eye. You can prove this by watching the progression with one eye closed. It still works.
It also strikes me that rather than leveling the differences between the two faces, the brain exaggerates the differences. I wonder, too, if our brain's facial recognition software functions differently on the peripheral retina compared to the way it works around the center of focus. Part of the effect may come from the afterimage of one set of faces switching to another set. Maybe someone who studies this can clear it up for us.
If this effect doesn't work for you, don't worry. Everybody's eyes and brains are different.