A piece of local, natural contour may evoke the perception of figure. We investigated what characteristics of natural contours provide the perception of figure. First, to examine what geometric factors could be the characteristics in the perception of contour, we performed similarity judgment among local, natural contours. Specifically, we tested whether geometric factors, such as convexity, closure and symmetry, explain perceptual similarity among the contours. We defined geometrically the three factors: for instance, convexity is defined as the logarithm of the curvature of the inscribed circle that passes through a point of a contour. We collected 50 patches from nearly 10,000 of local patches trimmed from natural images (the Berkeley Segmentation Dataset) so as to establish the equal distribution of the three factors. We performed psychophysical experiments to measure the similarity among the patches. MDS analysis showed the agreement between the arrangement of perceptual similarity and that of geometric factors, indicating that convexity, closure and symmetry are the characteristics used in the perception of contour. Second, we examined psychophysically whether convexity, closure and symmetry provide a clue in the perception of the direction of figure (DOF). A number of studies have reported the contribution of each factor to figure/ground segregation. In the present study, we focus on the combination of these factors, as they usually coexist in natural contours. We performed psychophysical experiments to judge the DOF at points along contours, and analyzed the dependence of the perception on the three factors. MLRA indicated that DOF is biased toward closure in comparison with convexity and symmetry. The result suggests that closure is dominant in the perception of DOF among mixtures of convexity, closure and symmetry as in natural contours.
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