THE COMPUTATION OF CONTOUR INFORMATION IN COMPLEX OBJECTS

Perceptual organisation, and especially the computation of contour inf ormation, has been the object of considerable interest in the last few years. In the first part of the paper we review recent accounts on th e mechanisms involved in the processing of contour. In the second part we report an experiment designed to examine (1) how physical paramete rs such as spatial proximity and collinearity of elements affect the i ntegration of global contour in objects and (2) whether the activation of stored representations of objects facilities the computation of co ntour. Incomplete forms varying in the spacing and the alignment of li ne segments on their contour were used as stimuli in a matching task. Subjects were asked to decide which of two laterally displayed figures matched a reference form presented previously. The matching target an d the distractor were physically identical but differed in their orien tation. In one condition the reference object was always an outline dr awing of an object. In a second condition the reference object was eit her a complete object or a more or less identifiable incomplete form. Little variation in performance was found for forms having continuous and discontinuous contour up to a spacing of 5 pixels (10.8 min) betwe en elements. Response times and errors increased abruptly beyond this limit. This effect occurred in the two conditions of reference stimulu s, suggesting that the computation of contour information is more affe cted by physical constraints at early processes than by high-level pro cesses involving activation of stored structural representations of ob jects.