title: Perceived contrast explains asymmetries in visual-search tasks with shaded stimuli.
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.
authors.affiliation: Laboratorio de Visión, IUTC & Facultad de Psicología, Universidad Complutense de Madrid, Madrid, Spain. jchacon@psi.ucm.es
authors.forename: José
authors.initials: J
authors.lastname: Chacón
mesh_terms: D000328:Adult; D000368:Aged; D015350:Contrast Sensitivity; D003867:Depth Perception; D005556:Form Perception; D006801:Humans; D008027:Light; D008875:Middle Aged; D010775:Photic Stimulation / Q000379:methods; D011601:Psychophysics; D013028:Space Perception