It’s hard to judge depth with only one eye. When you see the masks with one eye closed and from some distance away, your brain assumes that both masks protrude outward, as ordinary faces do.
But why does the concave mask seem to follow as you move? This illusion is your brain’s attempt to make sense of two conflicting sources of information. First, your visual system notices that the nose of the concave mask moves less than the rest of the face when you move your head. This information suggests that the nose is the most distant feature on the face (and the face is therefore concave), because to a moving observer, distant objects appear to move less than nearby objects. (Think of how highway signs whiz by when you are in car, while the distant scenery seems to barely move at all). But based on its experience in the world, your brain can’t accept the existence of a concave face. So, it concludes that the hollow mask is instead an ordinary protruding face that turns to watch you walk past, its nose following your gaze and therefore having little apparent motion.
The intensity and angle of illumination can influence the effectiveness of the illusion. Areas of shadow and bright reflection can either enhance or detract from the illusion, depending on their location.
Disneyland’s Haunted Mansion uses hollow masks to create eerie illusions of faces that seem to rotate and follow you as you move. The placement of the masks and the lighting are carefully adjusted to maximize the illusion.
Researchers in visual perception have traditionally concluded that, in the absence of other cues, we normally assume objects are illuminated from above (based on the fact that the sun is such a basic source of illumination). We therefore perceive objects with shadows at the bottom as convex and those with shadows at the top as concave. Nonetheless, we tend to perceive all human faces as convex—even hollow masks that have shadows at the top. It would seem that our perception of faces is an exception to our usual interpretation of shape based on light and shadow, perhaps because of the special importance faces hold for us. However, neuropsychologist V. S. Ramachandran has found that this propensity to view hollow masks as projecting outward actually extends to many complex inward-curving objects. His work generalizes this tendency, rather than attributing it to a unique perception of the human face by the brain.