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Ramakrishna Chakravarthi, Denis Pelli; What role does contour integration play in crowding?. Journal of Vision 2008;8(6):968. doi: 10.1167/8.6.968.
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Linking features together is called ‘contour integration’ when it helps object recognition and ‘crowding’ when it hurts it. But, so far, the two processes have been studied independently. Good continuation, usually assessed by contour integration tasks, is considered an essential property that forges strong links between features. As a consequence, one might expect that closely placed elements with aligned orientations might crowd each other to a greater degree than non-aligned elements. To test this prediction, we measured orientation discrimination thresholds for a target bar as a function of the goodness of continuation between it and two flankers. Targets and flankers were oriented bars 1 deg in length, 1.5 deg apart, presented at 10 deg eccentricity in the lower visual field. In experiment 1, in accordance with the above hypothesis, collinear flankers raised threshold 4.5 times more (on average) than orthogonal flankers. However, this result can also be explained by flanker similarity in orientation. Experiment 2 tested this latter possibility by presenting flankers with similar or dissimilar orientations as the target but which did not form a contour with it. Despite absence of good continuation, parallel flankers raised threshold 4x more than flankers with different orientations (that is, the target was crowded as much as with collinear flankers). Finally, experiment 3 directly pitted the two hypotheses against each other by presenting a single flanker that either shared orientation with the target or had good continuation with it but not both. Results indicate that crowding is most effective with a parallel (and hence similar) flanker and is weak or non-existent with a flanker that forms a contour. We conclude that elements bound together by contour integration do not crowd each other any more than unbound elements (beyond the effect of flanker similarity). Thus contour integration processes are transparent to crowding processes.
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