Feature conjunctions in texture segregation - colour and luminance contrast influence saliency and proximity grouping differently

To examine whether perceptual grouping on the basis of orientation can be performed simultaneously with or only subsequently to grouping according to colour or luminance, we tested whether subjects are able to segregate arrays of texture elements that differ from surrounding elements by conjunctions of either (i) colour and orientation, or (ii) luminance contrast and orientation, or (iii) luminance contrast polarity and orientation. Subjects were able to use conjunctions between luminance and orientation for segregation but not conjunctions between colour or contrast polarity and orientation. Our results suggest that (i) in agreement with earlier findings, there seem to exist no specific conjunction detectors for colour and orientation or contrast polarity and orientation, and (ii) when orientation defined textures are to be distinguished by virtue of differences in luminance, colour, or contrast polarity, luminance provides a much stronger cue than colour or contrast polarity for saliency-based orientation grouping.