This study examines how two factors affect target selection: the contiguity of the target with the surrounding surface and certainty about target location. Previous studies indicate that a target among distractors is easier to find when the search items are on the same surface rather than different surfaces. In contrast, our recent study indicates that when the target is in a known location, sensitivity to the target is higher when it is clearly separated from the surrounding surface. Here we examine the effects of both surface contiguity and uncertainty about target location on contrast discrimination. Observers were asked to detect a contrast change on a grating target that was either segmented or contiguous with the surround grating and occurred either at a known or unknown location. Thresholds for contrast discrimination depended critically on both segmentation and location uncertainty. When the contrast change appeared at a known location isolated from the background, segmentation aided the selection of the target location, but when the contrast change occurred at an unknown location on a contiguous background, grouping of the surface as a single entity aided the detection of the target location as a discontinuity from the surface.
Keywords: location uncertainty; surface organization; target selection; texture segmentation.