Some of the neurons in our visual system are specialized for face detection. Despite these dedicated neural circuits—perhaps because of them—our brain's face processing system uses short cuts that can lead to perceptual misinterpretations and illusions. One such example is our visual system's assumption that all faces are convex. Because the nose protrudes forward, and the face never sinks into the head, the brain’s circuits have evolved to take that fact for granted. Even when it’s physically wrong.
One case in point can be found in Antoni Gaudí's masterpiece, the Sagrada Familia church, in Barcelona. During our visit two weeks ago we found a sculptural decoration by Josep Maria Subirachs, on the Passion façade of the church, that features scenes from Jesus' crucifixion according to the New Testament. Veronica, the woman said to have used her veil to wipe Jesus's brow during his cross-carrying ascent to Golgotha, displays a large piece of cloth permanently embedded with Jesus's face, though in actuality it is sculpted as a hollow face. Moving side-to-side in front of the art brings the illusion to life. The face looks like a normal three-dimensional protruding face, not different from the faces of the other characters in the sculptural composition. When moving, however, the face appears to rotate towards you. It always looks directly at you, no matter your position. And coolest of all, it looks at everybody in all positions all at once. Divine indeed.
The illusion occurs because an actual protruding convex face would rotate away from you as you moved away, and towards you as you moved towards it. In a concave face, the shading does exactly the same thing, but since our brains are built to process only the shading on convex faces, everything seems to rotate backwards and the hollow face appears to rotate towards you as you move away and with you as you move towards it. Creepy, and god-like!