The Hollow-Face illusion is an optical illusion in which the perception of a concave mask of a face as a normal convex face, which appears to follow you as you move past it.
Says Richard Gregory, "The strong visual bias of favouring seeing a hollow mask as a normal convex face (figure 1), is evidence for the power of top-down knowledge for vision (Gregory 1970). (Barlow (1997) takes a more ‘reductionist’ view preferring to think of this in terms of redundancies of bottom-up signals from the eyes. I would limit this to very general features, such as properties of’ edge-signalling giving contrast effects, rather than phenomena attached to particular objects or particular classes of objects, such as faces.) 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. (There is a weaker general tendency for any object to be seen as convex, probably because most objects are convex. The effect is weaker when the mask is placed upside down, strongest for a typical face. If the mask is rotated, or the observer moves, it appears to rotate in the opposite to normal direction, at twice the speed; because distances are reversed motion parallax becomes effectively reversed."
- "Knowledge in perception and illusion" by Richard Gregory, Phil. Trans. R. Soc. Lond. B (1997) 352, 1121–1128 - authorative introduction to optical illusions.
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