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Chacón J, 2004, "Perceived contrast explains asymmetries in visual-search tasks with shaded stimuli" Perception 33(12) 1499 – 1509
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Perceived contrast explains asymmetries in visual-search tasks with shaded stimuli

José Chacón

Received 24 November 2003, in revised form 27 March 2004; published online 10 November 2004

Abstract. Shaded stimuli have traditionally been used in the context of three-dimensional (3-D) shape perception. Many studies have shown a persistent asymmetry in that a circle filled with a shaded gradient that is dark at the top and bright at the bottom (top-dark circle) is much easier to locate among top-bright circles than in the opposite arrangement (a top-bright circle among top-dark circles). The immediate 3-D interpretation of top-dark and top-bright circles as hollows and protuberances, respectively, and the asymmetry just described have been explained in terms of 3-D percepts. The work described here challenges this view: the results of the first experiment show that top-dark circles are perceived as having 10% higher contrast than top-bright circles of the same physical contrast. Experiment 2 replicates classical visual-search experiments but adding a new condition where target and distractors were subjectively equated in contrast. For five of six subjects, the ubiquitous asymmetry disappears in this condition.

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