Sensitivity To Three-Dimensional Orientation in Visual Search

Previous theories of early vision have assumed that visual search is based on simple two-dimensional aspects of an image, such as the orientation of edges and lines. It is shown here that search can also be based on three-dimensional orientation of objects in the corresponding scene, provided that these objects are simple convex blocks. Direct comparison shows that image-based and scene-based orientation are similar in their ability to facilitate search. These findings support the hypothesis that scene-based properties are represented at preattentive levels in early vision. Visual search is a powerful tool for investigating the representations and processes at the earliest stages of human vision. In this task, observers try to determine as rapidly as possible whether a given target item is present or absent in a display. If the time to detect the target is relatively independent of the number of other items present, the display is considered to contain a distinctive visual feature. Features found in this way (e.g. orientation, color, motion) are taken to be the primitive elements of the visual systems. The most comprehensive theories of visual search (Beck, 1982; Julesz, 1984; Treisman, 1986) hypothesize the existence of two visual subsystems. A preattentive system detects features in parallel across the visual field. Spatial relations between features are not registered at this stage. These can only be determined by an attentive system that inspects serially each collection of features in the image. Recent findings, however, have argued for more sophisticated preattentive processes. For example, numerous reports show features to be context-sensitive (Callaghan, 1989; Enns, 1986; Nothdurft, 1985). Others show that spatial conjunctions of features permit rapid search under some conditions (McLeod, Driver, & Crisp, 1988; Treisman, 1988; Wolfe, Franzel, & Cave, 1988). These findings suggest that spatial information can be used at the preattentive stage. Recent studies also suggest that the features are more complex than previously thought. For example, rapid search is possible for items defined by differences in binocular disparity (Nakayama & Silverman, 1986), raising the possibility that stereoscopic depth may be determined preattentively. Indeed, it appears that the features do not simply describe two-dimensional aspects of the image, but also describe attributes of the three-dimensional scene that gave rise to the image. Ramachandran (1988) has shown that the convexity/concavity of surfaces permits spontaneous texture segregation, and Enns and Rensink (1990) have found that search for shaded polygons is rapid when these items can be interpreted as three-dimensional blocks. Although the relevant scene-based properties present at preattentive levels have not yet been completely mapped out, likely candidates include lighting direction, surface reflectance, and three-dimensional orientation. Correspondence should be addressed to Ronald A. Rensink or James T. Enns, Department of Psychology, University of British Columbia. , 2136 West Mall, Vancouver BC V6T 1Z4, Canada. Email: rensink@psych.ubc. ca; jenns@psych.ubc. ca. In this paper we systematically examine one of these candidates: three-dimensional orientation. Rapid search can be based on the orientation of items in the image plane (Beck, 1982; Julesz, 1984; Treisman, 1986), and it is natural to ask whether the same holds for orientation in the three-dimensional scene. To answer this question, we conducted a series of visual search experiments using the simple black-and-white items shown in Figures 1 to 3. Many of the target and distractor items were projections of rectangular objects differing in the three-dimensional orientation of their principal axes. These items always contained the same set of imagebased lines and polygons. Therefore, if rapid search were possible, it would depend necessarily on the spatial relations that capture the three-dimensional orientation of the corresponding objects.