Context influences contour integration

Much research over the last decade has examined how the brain links local activity within primary visual cortex to signal the presence of extended global structure. Here we bring together two themes within this area by addressing how the immediate context that features arise in influences how they are integrated into contours. Specifically, observers were required to detect and discriminate the shape of contours that were surrounded by elements with a fixed orientation offset compared to contour elements. By comparing performance with contours made of elements oriented either near parallel ("snakes") or near perpendicular ("ladders") to the contour orientation, we were able to isolate the effect of orientation contrast on observers' ability to perform our task with near-collinear contour structure. We report both substantial facilitation of contour integration in the presence of near-perpendicular surrounds and inhibition in the presence of near-parallel surrounds. These results are consistent with known orientation dependence of suppressive surround interactions in the primary visual cortex and suggest that the "rules of association" for contour integration must incorporate the influence of local orientation context. Specifically we show that our results are consistent with contour integration relying on an opponent-orientation energy response from a bank of first-stage oriented filters.