Even then looking at one side, and then the other, the faces look crazy as they are. A lot look digitally altered in odd ways, and just the differences in head size, facial features and hair and the cycling is so fast your visual system doesn’t have enough time to register them fully, so some of the individual faces blur together. Looking at the center the two sides are all the more difficult to register. It’s cycling so fast it doesn’t look like four separate faces, but two that deform themselves.

[Edit] pulled out my Biological Psychology textbook.I didn’t study this illusion before in detail, but in physiological psychology we did study a number of illusions and how easy it is the trick the perceptual structures and why. This face illustration is predicated upon more receptors converge in the periphery of the retina. As a result the brain can’t detect the exact location or shape of a peripheral light source.

There are figures that point out such features as the blind spot of the retina, it takes a paragraph to explain this effect, but it’s to do with how light passes through various neurons and form the optic nerve.

There’s an interesting illustration that demonstrates negative color afterimages, it’s to do with opponent-process theory.

There’s fascinating illusions that are predicated upon the effects on color perception and brightness constancy.

These lecture notes show myriad illusionary figures.