Hollow-Face illusion

From New World Encyclopedia
This face of Björn Borg appears convex, (pushed out) but is actually concave (pushed in).

The Hollow-Face illusion is an optical illusion in which the perception of a concave mask of a face appears as a normal convex face. This illusion reveals the significant role of prior experience, and thus hypotheses or expectations, in the perceptual process. Psychologists and other scientists have found the hollow face illusion a valuable tool to examine the relationships between perception and knowledge.

This particular illusion also has many applications in the world of art and illusory magic. The astonishing transformations that concave masks appear to go through when rotated (or the viewer moves) provide great fun and enjoyment. Such artworks reveal humankind's endless fascination with the creative and unusual. These instances can also help us realize that our own perceptions may be limited or different from those of another person viewing the same thing, but from a different angle.


The hollow face illusion was first brought to the public's attention by Richard Gregory, who published it in Illusion in Nature and Art in 1973.


A paper hollow face dragon, shown from four angles across the 90 degree effective viewing area.
The back of the paper dragon.

While a convex face can 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. Thus, changing the viewing angle of a hollow face can dramatically change the apparent orientation of the face itself. Where a two dimensional figure can appear to follow the viewers movements, the hollow face actually appears to swivel.

The hollow face illusion works best with monocular vision; filming with a camera or closing one eye to remove stereoscopic depth cues greatly enhances the illusion.

Another example of the Hollow-Face illusion is found in a popular folded paper cutout of a dog or dragon. This dragon's head seems to follow the viewer's eyes everywhere (even up or down), when lighting, perspective and/or stereoscopic cues are not strong enough to tell its face is actually hollow. Keen observers will note that the head doesn't actually follow them, but appears to turn twice as fast around its center than they do themselves.


Human beings have a great amount of bias towards seeing faces as convex. This bias is so strong that it counters competing monocular depth cues such as shading and shadows, as well as considerable stereoscopic depth cues. The effect of the hollow face illusion is the weakest when the face is viewed upside down, and strongest when in the most commonly viewed, right side up orientation.[1] 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.

Nevertheless, as Gregory has noted, the illusion persists under less than optimal conditions, albeit less strongly, supporting his thesis that "top-down knowledge can dominate bottom-up signals."[2] In other words, that perception involves hypotheses, based on learning and past experience, which allow us to interpret what we see, and these hypotheses can overpower the actual sensory data that are being received, causing erroneous perceptions—illusions.

An example of a hollow-face illusion outside of a bank in Barcelona.


It is interesting to note that viewers see the hollow face as concave even though they consciously know that it is hollow. Psychologists and other scientists can use the perception of illusions such as the hollow face illusion to examine the relationships between perception and knowledge, as well as study the way the brain perceives such illusions. Researchers have found the hollow face illusion a valuable tool in researching the role of prior knowledge in visual perception, for example by having people reach out to touch the nose or cheek of a hollow face:

We show that prior knowledge about the general shape of faces can interact with the information the person acquires from vision at the time of the action. Without the knowledge that they are reaching to a face and what the typical shape of a face is, participants would perform the reaching movements differently. This stresses the importance of object recognition even for very simple motor tasks and shows that humans perform these actions in a more complex way than, for example, typical applications in robotics which do not take into account prior knowledge about objects.[3]

This particular illusion also has many applications in the world of art and illusory magic. The astonishing transformations that concave masks appear to go through when rotated (or the viewer moves) provide endless fascination and novelty to the curious.


  1. Richard Gregory, "Knowledge in perception and illusion" University of Bristol, 1997. Retrieved January 2, 2008.
  2. Richard Gregory, Eye and Brain, (Princeton, NJ: Princeton University Press, 1997, ISBN 0691048371) p. 208.
  3. B. Hartung, P.R. Schrater, H.H. Bülthoff, D. Kersten, and V. H. Franz. 2005. Is prior knowledge of object geometry used in visually guided reaching? Journal of Vision, 5(6):2. June 10, 2005. 504-514. Retrieved January 3, 2008.

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