Hollow-Face Illusion

The hollow-face illusion is an optical phenomenon in which a concave mask of a human face, one that has actually been pushed inward rather than protruding outward, is nonetheless perceived by viewers as an ordinary, normally convex face. The vision scientist Richard Gregory explained the effect by observing that the strong visual bias favoring the perception of a hollow mask as an ordinary convex face is evidence for the power of top-down knowledge in vision, meaning the brain's learned expectation that faces bulge outward is strong enough to override competing visual information, including shading, shadows, and even the depth information the brain would normally receive from binocular, two-eyed vision. A hollow face also produces an unusual gaze effect: its eyes appear to move faster than the viewer's own head does, seeming to look straight ahead when the viewer is directly in front of the mask but swiveling to an extreme angle when the viewer stands only moderately off to the side. The illusion weakens noticeably when the mask is lit from below, which reverses the shading cues the brain normally relies on to interpret a face's shape, and research has found that people with schizophrenia and people on the autism spectrum experience the illusion less strongly than other viewers, a finding taken as evidence that the illusion demonstrates a real dissociation between how the brain perceives a scene and how it controls physical action in response to it.