Spatial properties of shear disparity processing.

We investigated whether vertical-shear disparity was extracted from the whole visual field or from a more local area and how global estimates of vertical disparity are derived. We also investigated the role of cyclovergence in processing shear disparity. Random-dot stereoscopic displays in various configurations were presented with horizontal-shear disparity, vertical-shear disparity or same-sign horizontal- and vertical-shear (rotation) disparity. Vertical-shear disparity introduced into only the right half of a 60 deg-wide display produced perceived inclination of the whole display when the center of shear was on the fovea, but did not produce inclination, either of the whole display or of a local area when the centre of shear was in an eccentric retinal position. A display containing dots with vertical-shear disparity mixed with dots with zero-disparity produced one inclined surface. Horizontal-shear disparity always produced inclination confined to the local area of disparity. Rotation disparity produced no inclination when introduced into the whole display, but when introduced with zero-disparity dots. It produced an inclined plane distinct from the plane defined by the zero-disparity dots. These results could be attributed to cyclovergence, which we therefore eliminated in our last experiment. We conclude that the perception of surface inclination is based on the difference between local horizontal-shear disparity and global vertical-shear disparity averaged over the whole visual field.