The effect of contour closure on shape perception.

We studied psychophysically whether 'contour closure' enhances the accuracy of shape perception. Stimulus configurations (presented on a blank background) always consisted of identical pattern elements, but the positions of the local elements were varied: the global contour shape either contained closure or not. In the first two stimulus conditions (Closure), the oriented pattern elements (Gabor patches) formed a 'closed' rectangular shape composed of either four long lines or four corners. In the third condition (No closure), the global shape was composed of the four corners, but they were outward oriented, and hence they did not form the outline of a closed contour. We measured the precision of shape perception using a discrimination task in which observers judged the aspect ratio of the outline shape i.e. whether the rectangular shape was tall or wide. We found that: (i) shape discrimination was better (more precise) for Closed contours than for Non-closed contours, i.e. the aspect ratio discrimination thresholds were lower for the Closed than Non-closed configurations. The improved performance could not be explained by differences in visibility of the local elements in the two conditions. (ii) For closed contours, shape discrimination was more precise when the local elements were aligned with the global shape, than when the local elements were orthogonal to it.