Answer: The binocular depth inversion illusion is a visual perception illusion where a concave image looks convex.
The binocular depth inversion illusion is commonly observed when a person looks at the inside of a mask. Here, the structures of the face, such as the nose and the lips, appear as being convex rather than concave, even if we are aware that it is the inside of the mask. It is often demonstrated by putting the mask on a rotating turntable. The inside of the mask, as it turns away from the viewer, will appear to be outward and rotating in the opposite direction. This is sometimes also called the "hollow face" illusion.
The inversion illusion is largely shaped by experience and memory. When we see lip shapes or noses, almost 100% of the time these shapes are “outward.” So when we see those shapes again, even if they are inverted, our expectations tell us that the shapes are outward.
The binocular depth inversion illusion is not usually perceived by younger children. In theory, this is because young children do not have as many experiences with seeing faces compared to adults. They are better able to perceive the concavity of the mask since their perception is not altered by experience.
Other populations are less likely to experience this visual perception illusion are people who are intoxicated with psychedelics, and people with various psychiatric disorders, such as psychosis.
The evidence that the illusion is shaped by experience is demonstrated by the fact that other hollow objects, when rotated, usually are perceived as being concave.