Cross-orientation summation in texture segregation

Human texture vision has been modeled as a filter-rectify-filter (FRF) process, in which '2nd-order' filters detect changes in the rectified outputs of luminance-based '1st-order' filters. This study tested the validity of the two basic assumptions of the standard FRF model, namely (a) that the 2nd-order filters are sensitive to spatial modulations in both contrast and orientation, and (b) that the 2nd-order filters are tuned to different 1st-order orientations. In the first experiment, we tested subthreshold summation between two orthogonal carrier orientations in detection of a texture region, which was defined by contrast modulations across regions in the two carrier orientations, while systematically varying the relative change magnitudes between the two orientations. The results showed that the detection thresholds were determined by spatial difference in the contrast integrated over the two orientations. Orientation difference did act as a segregation cue, but only when there was no differences in carrier contrast. This suggests that two mechanisms are involved in texture segregation; one that detects changes in luminance contrast and another that detects changes in orientation. To further analyze the latter mechanism, a second experiment measured cross-orientation summation in the detection of purely orientation-defined textures, using stimuli that were density modulations of two orientations presented among randomly-orientated distractors. Again, the relative modulation magnitudes between the two orientations was systematically varied. The results are consistent with the notions that (a) the dominant orientation is extracted from the 1st-order outputs before the 2nd-order process, and that (b) the 2nd-order, spatial comparison process integrates those dominant signals over different orientations.