Search Asymmetries in the Detection of Shape Orientation: The Role of Implicit Axes of Elongation and Symmetry

Two-dimensional (2-D) shape identification is sometimes described in terms of access to structural descriptions based on the extraction of principal axes of elongation and symmetry. In four experiments we examined the effects of implicit axes of elongation and symmetry on visual search for shape orientation, on the basis of previous observations of search asymmetries in the detection of the orientation of line segments. We found search asymmetries in the detection of vertical versus oblique line segments and in 2-D shapes, studied earlier by Quinlan and Humphreys (1993 Perception 22 1343 – 1364): search times for a vertical target among oblique distractors (rotated 18° from the vertical) are higher than those for an oblique target among vertical distractors.

We found that the search asymmetry in the detection of shape orientation is larger when the shape contains an unambiguous axis of elongation or symmetry. Search asymmetries disappear when the principal axis of the shape is oriented horizontally, or when the shape contains no unambiguous axis of elongation or symmetry. Overall search times tend to decrease when the axis of elongation is aligned with an axis of symmetry. These results suggest that the detection of implicit axes plays a role in the perception of object shape and orientation.