Orientation sensitivity and texture segmentation in patterns with different line orientation.

Orientation sensitivity (OS) and the ability of human subjects to discriminate structured areas of different texture orientation (DOT) were investigated using line arrays of varying line length. In general, OS was mediated by shorter lines than was DOT; lines can be distinguished by their orientation before they give the impression of a texture border between adjacent regions with lines differing in orientation. This difference was found to hold over a range of retinal eccentricities from 5 degrees nasal to 30 degrees temporal. Decreasing visual acuity, associated with increasing distance from the fovea, cannot, however, account for the higher threshold of DOT, even taking into account that DOT requires a larger area for analysis than OS. When angle of orientation between adjacent texture areas was varied instead of line length, DOT thresholds at different retinal locations were reached at similar values. The difference between OS and DOT, found consistently at all retinal positions, suggests that they are mediated by distinct neural mechanisms.