The paper expands an earlier study [Nothdurft, H.C. (1993). The role of features in preattentive vision: comparison of orientation, motion, and color cues. Vision Research 33, 1937-1958] into the visual dimensions luminance and depth. Two perceptual phenomena were tested; (1) the ability of observers to detect a single salient target that differs from neighboring items (measurements on salience), and (2) the ability of observers to group salient items to larger figures (measurements on grouping). In each experiment, feature variations were restricted to the studied dimension, here luminance or disparity. Grouping processes were also studied with a monocular depth cue, depth from shadow. All experiments support the earlier notion that salience occurs from feature differences, not features themselves. Even for grouping, feature differences (which make a target salient) are more important than feature identities, at least in short stimulus presentations. Target salience: Like with orientation, motion and color, small differences in luminance or disparity are sufficient to make a target stand out from an array of identical items. But when background items themselves vary, target differences must be increased to be detected. With large background variations, even a strong feature contrast may not be sufficient to make the target stand out. Grouping: In brief inspections, observers group targets for salience, not for similarity. In the present experiments, however, certain modifications were necessary to make categorically different targets (bright – dark; near – far) equal salient. Apparent depth-from-shadow improved the grouping over that of rotated items with no such depth impression, but a similar difference was found when luminance gradients were replaced by luminance steps that do not resemble shadow or depth.

Published online: 15-Dec-2015

Citation: Nothdurft, H.C. (2015). Feature contrast in salience and grouping: luminance and disparity. VPL-reports, 3, 1-32, www.vpl-reports.de/3/