Abstract
Vertical and horizontal orientations are perceived more precisely than oblique orientations (the oblique effect). Visual perception of object orientation is thought to be mediated by salient axes of symmetry and elongation (Boutsen & Marendaz, 2001). We expect that in objects with multiple salient axes, observers will select the axis affording the best orientation discrimination. This will result in lower JNDs in object orientation when one or more of the salient axes are either vertical or horizontal, compared to salient axes that are oblique.
We designed three experiments, using dot-stimuli to prevent interference of local and global contours; a two-dot ‘line’ stimulus, a three-dot ‘triangle’ stimulus and a four-dot ‘square’ stimulus, with one, three and four salient axes, respectively. Observers were presented with two temporally separated stimulus orientations and were instructed to judge the orientation of the second stimulus compared to the first. We measured JNDs for various object orientations using interleaved staircases.
The results suggest that observers use different object axes, depending on the axial orientation. This results in the predicted lower JNDs for the oblique stimulus orientations where the salient axis is vertical or horizontal. We conclude that observers are capable of selecting the most informative axes when judging visual object orientations.