Or, you know, by looking at the original picture. Regardless, the faces do change size relative to each other, with some heads being larger. That might be a significant factor.
I remember one researcher was studying why people recognize why charactures of famous people are more recognizable than accurate drawings. His theory was that the brain recognized faces by building a characture in the memory and then when they see a face the brain tries to match charactures. If that’s accurate then this might be a result of that effect.
I recall seeing an earlier version of this illusion, which I believe was created for the original 2011 study. It uses photographs of average faces, where expression, orientation, scale, and lighting are all kept constant. The only thing that varies between photos is the proportions of each face.
… Here we go, the researcher’s web page has the video I recall.
I have only one working eye and I see the effect quite plainly.
Article here
How do I keep my eyes on the cross and still see the pictures?
You’re supposed to look at the cross, not actually touch it with your eye…
