Identifying Mechanisms and Boundary Conditions

Multiple attributes of a visual array are often more efficiently processed when they are attributes of a single object than when they are attributes of different objects—a pattern reflecting the limitations of object attention. This study used psychophysical methods to evaluate the object attention limitations in the report of attributes (orientation and phase) computed early in visual analysis for spatially separated objects. These limitations had large effects on dual-object report thresholds when different judgments were required for the two objects (orientation for one object and phase for the other), but the effects were small or nonexistent when the same judgment was made about both objects. Judgment consistency reduced or eliminated the expression of object attention deficits. Thus, the deficits in dual-object report reflect both division of attention over objects and the calculation of independent reference or judgment operations. Dual-object deficits, when they occurred, were substantial in displays with external noise masks. Smaller effects were observed in clear displays, even when difficulty was equated by stimulus contrast. Thus, the primary consequence of object attention is the exclusion of external noise, or mask suppression, and enhancement of the stimulus in clear displays is a secondary consequence. If subjects must report two aspects of a brief visual display, performance should depend on whether these aspects concern the same or different objects. Reporting two aspects of one object should be no more difficult than reporting only one because focal attention is paid to the object as a whole. In contrast, reporting aspects of two different objects should be less successful, reflecting competition between these objects for focal attention. Duncan (1984, p. 501) Since Duncan’s early investigation of limitations in the report of multiple aspects or attributes of separate objects, object attention has been considered a primary explanatory principle in the distribution of attention. A fundamental limitation in the distribution of attention over multiple objects is evinced by the observed disadvantages in dualobject report; several features of the same object are encoded efficiently, but features of different objects are encoded inefficiently (Duncan, 1984; see also Isenberg, Nissen, & Marchak, 1990; Vincent & Regan, 1995). Two judgments that concern the same object can be made simultaneously with little or no loss of accuracy compared with a single judgment about that object. However, two judgments about separate objects exhibit losses compared with single or dual judgments about a single object. This dual-object deficit has been demonstrated for many pairs of features, including brightness and orientation (Duncan, 1984); displacement and orientation (Duncan, 1993b); “where” and “what” (Duncan, 1993a); pairs of surface properties such as color, brightness, and texture; and pairs of boundary properties such as length and location (Duncan & Nimmo-Smith, 1996). Other elaborations of object attention have focused on the relationship between attributes that are parts of a hierarchically defined perceptual object (Egly, Driver, & Rafal, 1994; Moore, Yantis, & Vaughan, 1998) or object part (Vecera, Behrmann, & Filapek, 2001; Vecera, Behrmann, & McGoldrick, 2000). Indeed, object attention has evolved as an organizing principle for cortical mechanisms of visual object representation and visual attention (e.g., Desimone, 1998; Kastner & Ungerleider, 2000). The current study examined the roles of consistency of judgment frames and of masking in object attention effects in the perception of basic visual features. Object attention was evaluated using full psychometric analysis across a wide range of performance levels. JUDGMENT FRAMES IN OBJECT ATTENTION Duncan (1984, 1993b, 1998) has argued that object attention limitations are paramount—that object attention deficits do not depend on whether the same or different perceptual analyzers or codes are used (Duncan, 1984, 1993a, 1993b; Duncan & Nimmo-Smith, 1996). Object attention explanations of dual-report accuracy have historically been contrasted with alternative explanations of dual-task performance, including explanations based on capacity limitations within feature processors when they are required for both tasks (Allport, 1971; Wickens, 1971; Wing & Allport, 1972) and cross talk, competition, or confusion between responses (Navon & Miller, 1987). Both of these alternatives to the object attention position predict that the existence and size of dual-report deficits should depend on the nature and content of the reported judgments. In the prior literature (Duncan, 1993a, 1993b; Duncan & Nimmo-Smith, 1996), tests of the importance of the type of judgment have been incomplete. In this study, we explicitly tested the importance of judgment identity in deficits of dual-object report by contrasting conditions in which judgments based on the same feature were made for two objects and conditions in which different feature dimensions were judged for the two objects. This study differs from prior investigations in that the same-feature conditions were identical in reference frame, decision criteria, and response mapping. Both same-feature and different-feature dual-object report conditions were contrasted with single-object report conditions. NOISE EXCLUSION IN OBJECT ATTENTION Recent theories of spatial attention have suggested that attention has a special role in noise or mask suppression (Dosher & Lu, 2000a, 2000b; Enns & Di Lollo, 1997; Lu & Dosher, 1998, 2000; Shiu & Pashler, 1994). Previous investigations of dual-object report deficits generally used brief displays and high-contrast postmasks (e.g., Duncan, 1993a, 1993b) to reduce performance to measurable levels. This choice may have important theoretical implications. Are dual-object report deficits restricted to masked, or high-noise, situations and therefore a Address correspondence to Barbara Anne Dosher, Department of Cognitive Sciences, 3151 SSP, University of California, Irvine, CA 92697-5100; e-mail: