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Using oriented Gabor patches, we found that for nearly cardinal target orientations, oblique flankers' orientations induced more interference than did cardinal flankers' orientations. This pattern was observed both at the local and global levels of flankers' orientation. With respect to the global orientation (flankers' global arrangement around the target), there was no difference between the effects of the two cardinal orientations, and both induced the same amount of interference. With respect to the local orientation (Gabors' orientation), in accordance with previous reports, a difference was found between the effects of the two cardinal orientations--a considerable amount of interference with flankers equal in orientation to the target (although less than with the oblique flankers) and almost no interference from flankers orthogonal to the target. Crowding was also affected by an anisotropy based on the target-fixation axis (radial, tangential, and diagonal) and by the flankers' relations. The magnitude of these latter factors was small relative to that of the former ones. The multiple factors that affected crowding, as well as the similarities and the differences between the effects at the two orientation levels, suggest that crowding is determined by multiple sources of interference operating at several levels of representation.

The visual system can integrate discrete units of information to construct a coherent description of the input it receives. Little is known about the processes of grouping and their implementation in the visual system. Previously we described a configural effect in which the global arrangement (degree of co-circularity) of Gabor flankers affected the degree of crowding with a Gabor target that they surrounded. Here we tested possible mechanisms by which the configural effect might operate in crowding. We ruled out simple explanations based on the effect of basic units constructing these configurations (pairs of opposing Gabors). Our results support an explanation for crowding that is based on grouping processes between flankers. They also suggest that not all flankers necessarily directly affect the target. Flankers might group together and interact with the target as a single element. Finally, using a computational model of crowding based on compulsory grouping (following Gestalt principles) and segmentation, we define the relative contribution of different pair relations of grouped elements to crowding.

The influence of configuration on visual crowding was tested. Eight Gabor patches surrounding a central one were arranged in a way that created several global configurations differing by their internal arrangements (smooth contour vs. random), while still preserving pairwise relationships between the target and flankers. Orientation discrimination and contrast detection of the central Gabor were measured. These measurements revealed differences in the magnitude of crowding produced by the different configurations, especially on the discrimination task. The crowding effect was stronger when random configurations were used and was reduced considerably when a smooth one was used. These results showed the typical dependence of crowding on eccentricity and target-flanker separation, which was independent of the configural effect. Controlling flankers' local orientation allowed addressing the nature of the effect. It was found to be sensitive to spatial relations and did not represent a simple averaging of local orientation estimates. Our results show that crowding operates at a level where configuration information has already been extracted. We relate all this to the object-based nature of perception.