The Hollow-Face illusion (aka Hollow-Mask illusion) is an optical illusion in which the perception of a concave mask of a face appears as a normal convex face.
While a convex face will appear to look in a single direction, and a flat face such as the "Lord Kitchener Wants You" poster can appear to follow the moving viewer, a hollow face can appear to move its eyes faster than the viewer: looking forward when the viewer is directly ahead, but looking at an extreme angle when the viewer is only at a moderate angle.
This bias of seeing faces as convex is so strong it counters competing monocular depth cues, such as shading and shadows, and also very considerable unambiguous information from the two eyes signalling stereoscopically that the object is hollow. Lighting a concave face from below to reverse the shading cues making them closer to those of a convex face lit from above can reinforce the illusion.
The idea was that the fast flicking (rather like flicking a small insect off the face) would engage the vision-for-action networks in the dorsal stream – and thus would be directed to the actual rather than the perceived position of the target. The results were clear. Despite the presence of a robust illusion in which people perceived the hollow face as if it were a normal protruding face, the flicking movements they made were accurately directed to the real, not the illusory location of the target. This result suggests that the bottom-up cues that drive the flicking response are distinct from the top-down cues that drive the Hollow-Face illusion.
- Authoritative introduction to optical illusions by Richard Gregory.
- Hollow mask illusion fails to fool schizophrenia patients
- Hollow Face Illusion at VisualFunHouse
- "The Hollow Face Illusion" at Grand Illusions
- Dragon illusion
- Richard Gregory
- Dragon Illusion Printable cut-out dragon illusion in PDF.
- Templates for the Dragon Illusion in different colors