Visual search for dimensionally redundant pop-out targets: Evidence for parallel-coactive processing of dimensions

In many workplaces, the operator’s task involves frequent and efficient scanning of displays with high information density, and fast detection of and reaction to target objects may be vital (e.g., Wickens, 1992). Therefore, target objects that need to be inspected urgently should be made highly salient in order to summon attention. There are several means of manipulating the saliency of display objects. One efficient means is to create feature contrast between the target and the background (distractor) objects (e.g., Nothdurft, 1991, 1993; Wolfe, 1994). The finding that objects that differ from a more or less homogeneous background in a salient feature are detected rapidly, and independently of the number of distractors, has been referred to as pop-out (e.g., Treisman, 1988). Most theories of visual search assume that feature contrast is computed by early, preattentive mechanisms (see below). The present study investigated whether the combination of two such contrasts within a single target stimulus would permit faster and more accurate task performance, in comparison with the detectionof targets defined by feature contrast in one dimension only. Surprisingly, there is as yet relatively little systematic knowledge concerning the effect of combining two (or more) feature dimensions in such a redundant way on visual search performance. The most detailed investigation of redundant target processing to date has been presented by Mordkoff and his colleagues (e.g., Mordkoff, Miller, & Roch, 1996; Mordkoff & Yantis, 1993; Mordkoff, Yantis, & Egeth, 1990), who have also provided very thorough analyses of methodological problems involved in distinguishing between various models designed to explain redundancygains (see below). However, the number of objects in their displays was generally small (e.g., one or two in the study of Mordkoff & Yantis, 1993), and the task required target identification rather than mere detection (e.g., identification of the target-defining features, involving matching of the actual stimulus presented with memorized information defining possible targets, in Experiment 1 of Mordkoff & Yantis, 1993). Thus, it cannot be taken for granted that their findings generalize to the detection of any odd-oneout object in a search display exhibitingfeature contrast in one or two dimensionsto a large number of densely spaced, homogeneous background objects. The present experiments were designed to address this issue, with the particular feature dimensions examined being color and orientation (i.e., form). The following sections will review relevant theories of visual search in order to derive predicThis work was supported by Deutsche Forschungsgemeinschaft Grants HE 1132/5-1 and Schr 375/8-1. We thank Asher Cohen and one anonymous reviewer for their valuable comments on a draft of this article. Correspondence concerning this article should be addressed to J. Krummenacher, Institut für Psychologie,Rheinisch-Westfälische Technische Hochschule Aachen, Jägerstr. 17/19, D-52056 Aachen, Germany (e-mail: joseph.krummenacher@post.rwth-aachen.de). Visual search for dimensionally redundant pop-out targets: Evidence for parallel-coactive processing of dimensions