Depth-Inversion "Easillusions" and "Hardillusions": Differences for Scenes and Faces

Abstract

Introduction: Two depth-inversion (DI) illusions, where viewers perceive depth structure opposite to the stimulus's physical depth, are hollow masks and reverspectives [Wade & Hughes, 1999]. For faces, one explanation is that face-specific 3D stored knowledge and a general convexity bias overcome data-driven depth cues to produce DI. For reverspectives, stored general perspective rules (e.g., that retinal trapezoids are rectangles slanted in physical space with their long retinal edge closer to viewer) may account for the DI [Gregory, Phil. Trans. R. Soc. B, 2005]. We call such easily obtained DI illusions "Easillusions". Rationale for present study: We investigated whether humans can downplay stored knowledge and rules to obtain DI "hardillusions" for stimuli in which stored knowledge and rules oppose, rather than favor, DI [Papathomas et al. ECVP 2015]. Examples of such stimuli are normal, convex 3-D faces and "proper-perspectives", in which the retinal trapezoids are consistent with the depth of the physical surfaces. However, our 2015 study included only unpainted masks and fragments of reverse-perspectives. Methods: This study included a complete reverspective and realistically painted convex masks. Stimuli were both easillusions (hollow mask, reverspective) and hardillusions (convex mask, proper-perspective). Also, all stimuli were either realistically painted or unpainted. We assessed how long it took 16 subjects to obtain DI. Results: Painted reverspectives tended to produce DI faster than unpainted ones, whereas painted proper perspectives were much slower than unpainted ones. For faces, the difference for obtaining DI between painted and unpainted faces was much smaller both for the hollow and convex masks. The only stimuli that no subject was able to get DI were painted convex masks. Conclusions: Perspective painted cues played a much stronger role than facial painted cues, providing additional evidence that facial 3D geometry plays a larger role than scene 3D geometry.

Meeting abstract presented at VSS 2017