Flank facilitation and contour integration: different sites.

Observers' ability to integrate features into extended contours, and to exploit the flanking structure to facilitate contrast detection (flank facilitation), exhibit a similar dependence on element spacing and orientation. Here, we investigate whether this reflects the operation of a common cortical mechanism by comparing performance for both tasks under monocular, binocular, dichoptic, and stereoscopic viewing conditions. Our results clearly implicate different cortical sites for flank-facilitated detection and contour integration; the former is a purely monocular phenomenon and must therefore occur at the earliest stages of cortical processing. In contrast, contour integration is a binocular process and occurs after the encoding of relative disparity, suggesting substantial extra-striate involvement. We conclude that the sites, and therefore the mechanisms, underlying these two seemingly related psychophysical phenomena are different.