A concave mask or mould of a human face that looks like a normal convex face when viewed from a distance of about 150 centimetres or more and seems to turn to follow the viewer's movements. The illusion arises from a top-down processing assumption made by the visual system that objects are likely to be globally convex. The illusion is weaker for inverted faces and for other objects such as potatoes, disappearing in both cases at about 350 centimetres distance, when bottom-up processing based on stereopsis overcomes the top-down global convexity assumption. Illumination from below enhances the illusion by duplicating the shadows that would be cast on a normal convex face lit from above (see chiaroscuro), but the effect is seen even without special lighting, suggesting that the global convexity assumption is stronger than the assumption of lighting from above. A dramatic demonstration can be constructed from a mask obtained from a novelty store, by painting the hollow inside roughly to match the colours on the outside and attaching a length of stiff wire or other material so that the mask can be rotated. The illusion was first reported in 1916 by the US lighting engineer and psychologist Matthew Luckiesh (1883–1967) in his book Light and Shade and Their Applications, where he commented that ‘the mould actually appeared to be a relief lighted from below’ (p. 120) and that ‘the [hollow face] was not suspected of being a mould and not a relief because the relief is by far the most common in objects of this character’ (p. 122). Compare rotating head illusion.