Visual search asymmetry for viewing direction

Abstract

In visual search experiments, we examined the existence of a search asymmetry for the direction with which three-dimensional objects are viewed. It was found that an upward-tilted target object among downward-tilted distracting objects was detected faster than when the orientation of target and distractors was reversed. This indicates that the early visual process regards objects tilted downward with respect to the observer as the situation that is more likely to be encountered. That is, the system is set up to expect to see the tops of these objects. We also found a visual field anisotropy, in that the asymmetry was more pronounced in the lower visual field. These findings are consistent with the idea that the tops of objects are usually situated in the lower visual field and less often in the upper field. Examination ofthe conditions under which the asymmetry and the anisotropy occur demonstrated the importance of the three-dimensional nature of the stimulus objects. Early visual processing thus makes use of heuristics that take into account specific relationships between the relative locations in space of the observer and 3-D objects. 211One of the primary tasks that the visual system has to accomplish when it tries to make sense ofa particular view ofthe world is to discriminate and identify objects. To accomplish this, the visual process first has to segregate visual surfaces in the three-dimensional (3-D) world. Since it is often not initially known where in the field of view the interesting or important objects are going to be, this initial process needs to take place across the whole visual field. It is also of importance that this first analysis occurs as rapidly as possible. These requirements put a lot of demands on the system. In order to facilitate the task, the primary process could be guided by later processes in a topdown fashion; these processes, including memory, might be better able to locate the kinds ofobjects and events that might be of importance (Wolfe, Cave, & Franzel, 1989). In addition to this relatively short-term information, there are basic facts about the world in which we live that can restrict the possibilities or set clear probabilities, so that the task could be simplified. It seems that the visual system has evolved in exactly such a way as to have incorporated certain "basic truths" about the world. That is to say, certain assumptions can be made about the world that are usually true, and that do not have to be rediscovered over and over again (utilitarian theory of perception; Ramachandran, 1989).

Asymmetries in visual search: An introduction

“…It is easier to detect the bump as opposed to the absence of a bump. Many experiments show that 3-D depth cues operate as basic features in visual search (Enns & Rensink, 1990Previc & Blume, 1993;Sun & Perona, 1996;von Grünau & Dubé, 1994). What makes the Kristjánsson and Tse paper interesting is a series of experiments that show that the asymmetry of Figure 2 is not a by-product of some sort of 3-D processing.…”

Target-tilt and vertical-hemifield asymmetries in free-scan search for 3-D targets

“…reversed (von Grünau & Dubé, 1994). However, search times for downward-tilted cubes among upward-tilted distractor cubes lit from below actually improve when the latter are placed along a simulated "floor" (Sun & Perona, 1996), which suggests that the advantage of the upwardtilted cubes in the lower visual field is not due solely to the simulated downward view-angle of the subject.…”

“…Several studies have also examined the nature of visual search to pictorial three-dimensional (3-D) targets defined by their orientation in space (Enns & Rensink, 1990, 1991Humphreys, Keulers, & Donnelly, 1994;Sun & Perona, 1996;von Grünau & Dubé, 1994). Although some researchers believe that a robust 3-D appearance leads to faster and more efficient search for solid targets than for 2-D figures, evidence of a 3-D advantage remains somewhat equivocal.…”

“…For example, von Grünau and Dubé (1994) showed that upward-tilted targets embedded among downwardtilted distractors are found more quickly in the lower visual field than in the upper visual field, whereas no significant vertical hemifield differences were reported in searching for downward-tilted targets. Von Grünau and Dubé also reported overall faster reaction times (RTs) in the lower visual field, although this trend actually proved significant only in Experiment 4.2 of their study.…”

Visual search asymmetries in motion and optic flow fields

“…For example, in color search, it is easier to find a magenta target among red distractors than it is to find a red target among magenta distractors (Treisman & Gormican, 1988). Such asymmetries have been found with a variety of basic features, such as gap detection, line convergence, shape (Treisman & Gormican, 1988), orientation (Foster & Ward, 1991), orientation in depth (Von Grünau & Dubé, 1994), and so forth. In the motion domain, Ivry and Cohen (1992) have shown an asymmetry between fast and slow moving stimuli, with subjects' finding a fast moving target among slow distractors much more efficiently than they found a slow target among fast distractors.…”