title: What's in a face? The role of depth undulations in three-dimensional depth-inversion illusions.
abstract: Upright hollow human faces produce among the strongest depth-inversion illusions (DIIs), but why? We considered the role of depth undulations by comparing four types of hollow objects: an ellipsoid, a human mask, and two symmetric 'Martian'masks, which wavered in depth like the human mask but which lacked face-like features. Illusion strength was quantified either as the critical viewing distance at which the 3-D percept switched between convex and concave (experiment 1) or as the proportion of time ('predominance') that observers experienced DII from a fixed intermediate viewing distance (experiment 2). Critical distances were smallest--and hence the illusion was strongest--for the upright human mask; the remaining objects produced undifferentiated critical distance values. The predominance results were more fine-grained: illusions were experienced most often for the upright human mask, least often for the hollow ellipsoid, and to an intermediate extent for the Martian and upside-down human masks. These results suggest: (1) an upside-down human mask and a surface with nonface features undulating in depth are equivalent for the purposes of generating DIIs; (2) depth undulations contribute to DII; and (3) such undulations are most effective when structured into an upright human face.
authors.forename: Vanja M
authors.initials: VM
authors.lastname: Vlajnic
authors.forename: Thomas V
authors.initials: TV
authors.lastname: Papathomas
authors.forename: Brian P
authors.initials: BP
authors.lastname: Keane
authors.forename: Anna
authors.initials: A
authors.lastname: Zalokostas
authors.forename: Steven M
authors.initials: SM
authors.lastname: Silverstein
mesh_terms: D000293:Adolescent; D003867:Depth Perception; D004192:Discrimination, Psychological; D004215:Distance Perception; D005145:Face; D005260:Female; D006801:Humans; D008297:Male; D009903:Optical Illusions; D009949:Orientation; D010364:Pattern Recognition, Visual; D011601:Psychophysics; D055815:Young Adult