Detection of shape orientation depends on salient axes of symmetry and elongation: Evidence from visual search

Three experiments investigated the role of the global spatial structure of two-dimensional (2-D) shapes in terms of symmetry and elongation on visual search for shape orientation. Experiment 1 demonstrated the often reported orientation search asymmetry (i.e., a faster detection of a tilted target among vertical distracters than the reverse) for the global orientation of 2-D polygons that possess a salient, "principal" axis of symmetry or elongation. Moreover, the search asymmetry depended on the orientation of the principal axis, rather than on the orientation of local contours. Further exploration of this effect with polygons (Experiment 2) showed that the search asymmetry for global orientation occurred for shapes containing an axis of symmetry; elongation, on the other hand, did not seem to be crucial. Finally, Experiment 3 demonstrated orientation search asymmetries with shapes composed of curved rather than straight contours: Here, the search asymmetry occurred as a function of the orientation of both axes of symmetry and elongation. Overall, search for global orientation was less efficient than search for local orientation. The results suggest that the perception of the global orientation of shapes is mediated by axis-based descriptions in terms of perceptually salient axes of symmetry and elongation.