Near-bilateral symmetry impedes symmetry discrimination

Bilateral symmetry is a salient object property. Will perceived bilateral symmetry impede our ability to discriminate shapes that are slightly asymmetric? Faces are naturally suitable to address this question, since they are asymmetric in varying degrees but largely symmetric in frontal views. We used 3-D scans of real faces to construct natural-looking synthetic faces with varying degrees of asymmetry. Two images of a synthetic frontal-view face were displayed side by side. The subjects decided which one was more symmetric. Discrimination sensitivity increased with the average degree of asymmetry of the two faces, when their difference was kept constant ( p < 0.0001, n = 16). This suggests that perceived symmetry impedes symmetry discrimination when asymmetry is introduced by interpolating between an asymmetric natural face and the bilaterally symmetric face of the same person (as opposed to adding uncorrelated random dots to a symmetric dot pattern). This result held for both upright and inverted faces, suggesting that it is not due to a familiarity effect. Nor is the result due to any artifact of the scale of asymmetry in the interpolation dimension. This is because the asymmetry distortion we used produces a linear displacement of feature points in 3-D space; and in the image space, the Euclidean distance of pixel values between the left and right halves of a face was found to decrease with the increasing degree of asymmetry of the test faces. We conclude that symmetry perception impedes symmetry discrimination