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Abstract:
A hollow face mask tends to appear convex even if the mask is rotating, viewed binocularly [Gregory, 1970 The Intelligent Eye (London: Weidenfeld and Nicolson)], or illuminated from above. The standard explanation for this illusion is that the visual system has a strong preference for familiar shapes (faces) over unfamiliar ones (hollow masks). This illusion confounds two preferences, however: the first is for a familiar shape, the second is for a globally convex shape. Here we demonstrate that the latter preference can be significant, using unfamiliar stochastically corrugated smooth surfaces that are viewed statically and monocularly under perspective projection. Each surface is illuminated by a collimated light source that is either above or below the line of sight, and each surface is either globally convex or globally concave. Nine naïve observers judged the local qualitative shape at 320 isolated points on 64 such surfaces. The percentages of correct scores were 57 vs 38 for globally convex vs concave, and 70 vs 23 for light source from above vs one from below. Both differences are significant ( p < 0.01). We conclude that a preference for global convexity plays a significant role in local-shape perception, even in the presence of other significant shape cues.
