The Computation of Contour Information in Complex Objects

Abstract

Perceptual organisation, and especially the computation of contour information, has been the object of considerable interest in the last few years. In the first part of the paper we review recent accounts on the mechanisms involved in the processing of contour. In the second part we report an experiment designed to examine (1) how physical parameters such as spatial proximity and collinearity of elements affect the integration of global contour in objects and (2) whether the activation of stored representations of objects facilitates the computation of contour. Incomplete forms varying in the spacing and the alignment of line 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 and the distractor were physically identical but differed in their orientation. In one condition the reference object was always an outline drawing of an object. In a second condition the reference object was either 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) between elements. Response times and errors increased abruptly beyond this limit. This effect occurred in the two conditions of reference stimulus, suggesting that the computation of contour information is more affected by physical constraints at early processes than by high-level processes involving activation of stored structural representations of objects.

Is object recognition automatic?

“…This lack ofa difference between targets and fillers in the congruent (i.e., canonical) condition may seem inconsistent, at first blush, with a predictable objectsuperiority effect (Weisstein & Harris, 1974) in the results of the present paradigm. In fact, this type of effect is normally found when the perceptual task requires processing ofa feature embedded in an object, and not when the task requires processing of an object as a whole (Boucart, Delord, & Giersch, 1994). Although several accounts have been proposed to explain the consistent asymmetry between facilitation and interference in Stroop tasks (e.g., Cohen, Dumbar, & McClelland, 1990;Glaser & Glaser, 1989;Lindsay & Jacoby, 1994), the case we build in the forthcoming section will be restricted to the fact that an interaction was found, and that this interaction reflects, at some level, the influence of automatically activated stored representations of real-world objects on the speed ofprocessing one feature of their perceptual appearance.…”

A New Pharmacological Tool to Investigate Integration Processes

“…This hypothesis was tested by using fragmented pictures composed of single elements whose spacing and collinearity were manipulated. Indeed, these two properties are known to affect the integration of contour information (Beck, Rosenfeld, & Ivry, 1989;Boucart et al, 1994;Zucker & Davis, 1988) and were not controlled in Snodgrass incomplete pictures. Second, lorazepam might have impaired the use of stored object representations.…”

“…Hence, the only parameter varying with the number of elements composing the contour of the square was the proximity of the elements. Proximity is one of the properties affecting grouping (Boucart et al, 1994;Zucker & Davis, 1988). We expected that the integration of the target with the distractors would increase as the number of distractors increases and that the visibility of the target would therefore decrease.…”

Now you see it, now you don't: The context dependent nature of category-effects in visual object recognition

“…Presenting the stimuli as fragmented forms is expected to increase the burden on shape configuration. This expectation is based on the well-established finding that it is more difficult to form elaborate shape representations from contours that are fragmented than from contours that are not (see e.g., Boucart, Delord, & Giersch, 1994;Boucart & Humphreys, 1992a, b). In comparison, processing of silhouettes is likely to place less demand on shape configuration than both fragmented forms and full line drawings because silhouettes reveal comparably fewer salient parts.…”