Relative shear disparities and the perception of surface inclination.

A dichoptic display in which the images are cyclorotated in opposite directions does not appear inclined. This suggests that perceived inclination depends on the difference between horizontal-shear and vertical-shear disparity. Large random-dot stereoscopic displays were presented with various types of shear disparity. Perceived inclination was the same magnitude for horizontal and vertical shear disparities. Opposed horizontal and vertical shear produced greater inclination than a single-axis shear. Same-sign vertical and horizontal shear (rotation) produced no inclination. These results support the relative-shear hypothesis. Cyclovergence was measured and was insufficient to account for these effects. We conclude that perceived inclination depends on the difference between horizontal- and vertical-shear disparities. Perceived inclination was not based on vertical disparity within small displays or within large displays with a zero-disparity surround. Relative-shear disparities are therefore extracted globally rather than locally.