Processing of linear elements in stereopsis: effects of positional and orientational distinctiveness

The stereoscopic processing of small linear elements is probed through the comparative analysis of stereograms containing needles or crosses, differing in the local spatial arrangement and orientation of the elements, and the presence or absence of slant. Depending upon the details of the textural design, depth analysis may proceed faster with crosses than with needles, or the reverse. It proceeds faster with vertical than with horizontal needles, except in the case of unslanted regularly-spaced needles. On the whole the data suggest that the elements to be matched in a stereogram are first processed along a common pathway, in which positional regularity has a detrimental effect. In the presence of small linear elements, orientation-tuned neurons would be recruited and their participation would lead either to an inhibition effect when the elements are all similarly oriented, or to a facilitation effect when there is sufficient orientational diversity among the elements. Here, slant plays an indirect role, by widening the orientation spectrum in otherwise regularly oriented textures. Positional irregularity is useful to suppress false matches, while orientational diversity helps to stabilize the perceived surfaces.