Psychology

We examined the capabilities of preattentive processing by asking subjects to report the relative location of two visual targets, each placed in a separate group of stimuli. In the first experiment, the two targets differed from their distractors on the same feature dimension (shape or color) or on different dimensions, one distinguished by its form and the other by its color. Reaction times (RTs) suggest that the two targets were processed simultaneously (i.e., in parallel). Indeed, the overall processing time was as long when two feature dimensions were involved as the longest processing time for either dimension alone. These results imply that the preattentive processing of several stimuli can occur simultaneously along at least two different feature dimensions. In the second and third experiments, when the target in one group of stimuli was used as the distractors in the other group, RTs increased significantly over RTs in conditions in which the distinctive feature of the targets was unique in the display. In none of these conditions did RTs vary with the number of distractors. These results indicate that although both targets are processed simultaneously when the distinctive features of the targets are unique in the display, two parallel searches (one for each target) must be performed sequentially when there is an overlap between the target in one group of stimuli and distractors in the other. The notion of location as a separate feature dimension is also discussed.

By using a visual search task, this study examined the encoding of orientation and size for stimuli defined in five different surface media: luminance, color, texture, relative motion, and binocular disparity. Results indicated a spatially parallel analysis of size and orientation features for all surface media, with the possible exception of binocular disparity. The data also revealed a search rate asymmetry in the orientation task for all media: Parallel or shallow search functions were obtained for oblique targets in vertical distractors, whereas steeper serial search functions were obtained for vertical targets in oblique distractors. No consistent asymmetry was found for the large and small targets in the size task. There seemed to be common principles of coding in all these different media, suggesting either a single analysis of shape features applied to a common representation or multiple analyses, one for each surface medium, with each extracting a similar set of features. The shared coding principles may facilitate the use of redundancy across media to reduce ambiguities in the locations and shapes of contours in the visual scene.

This article examines how the human visual system represents the shapes of 3-dimensional (3D) objects. One long-standing hypothesis is that object shapes are represented in terms of volumetric component parts and their spatial configuration. This hypothesis is examined in 3 experiments using a whole-part matching paradigm in which participants match object parts to whole novel 3D object shapes. Experiments 1 and 2, consistent with volumetric image segmentation, show that whole-part matching is faster for volumetric component parts than for either open or closed nonvolumetric regions of edge contour. However, the results of Experiment 3 show that an equivalent advantage is found for bounded regions of edge contour that correspond to object surfaces. The results are interpreted in terms of a surface-based model of 3D shape representation, which proposes edge-bounded 2-dimensional polygons as basic primitives of surface shape.