Dimension-Based Visual Attention Modulates Dual

Journal of Experimental Psychology: Human Perception and Performance 2000, Vol. 26, No. 4, 1332-1351

Copyright 2000 by the American Psychological Association, Inc. 0096-1523/00/$5.00 DOI: 10.1037//0096-1523.26.4.1332

Dimension-Based Visual Attention Modulates Dual-Judgment Accuracy in Duncan's (1984) One- Versus Two-Object Report Paradigm Hermann J. Mtiller

Rebecca B. O'Grady

Universit~t Leipzig and University of London

University of London

Four experiments, adapting the object-judgment paradigm developed by J. Duncan (1984), examined the relationship between object-based and domain-based mechanisms of visual attention. The experiments demonstrated a cross-domain cost, in terms of accuracy, when observers made dual color-form judgments to one or two overlapping objects presented briefly, relative to within-domain, dual-color and dual-form judgments. This domain-based selection effect was additive to an object-based effect, a cost of making dual judgments to separate objects, as reported by J. Duncan (1984). The pattern of object- and domain-based effects points to a capacity limitation in how multidimensional features are bound into a coherent object representation, consistent with the dimension-weighting account of H. J. MOiler, D. Heller, and J. Ziegler (1995), which postulates that there is a limit to the total selection weight available to be allocated to an object's dimensions.

Visual attention refers to the selection of information from the environment for limited-capacity visual processing and the control of visually guided behavior. Over several decades of visual attention research, three main types of visual selection theory have developed: space-based, domain-based, and object-based. Spacebased theories propose that attention is oriented to spatial regions or locations of the visual field. The mechanisms underlying spacebased attention have been described in terms of a spotlight (e.g., Posner, 1980; Posner, Snyder, & Davidson, 1980), a zoom lens (e.g., Eriksen & Eriksen, 1974; Eriksen & Hoffman, 1973; Eriksen & Yeh, 1985), a gradient (e.g., Downing, 1988; LaBerge & Brown, 1989), and a gradient-filter analogy (Cheal, Lyon, & Gottlob, 1994). Domain- or attribute-based theories propose that selection is limited by the nature of the required discriminations between different stimulus attributes, more precisely, between categories or domains of attributes. One domain-based theory is the analyzer theory deriving from the work of Treisman (1969) and Allport (1971, 1980). They assumed independent systems of analyzers, each processing a particular domain of stimulus attributes (e.g., form, color). According to analyzer theory, two simultaneous discriminations relying on the same analyzer will give rise to mutual interference. Object-based theories assume that selection operates on perceptually delineated and integrated object represen-

Hermann J. MOiler, Institut fiir Allgemeine Psychologic, Universi~t Leipzig, Leipzig, Germany, and Birkbeck College, University of London, London, United Kingdom; Rebecca B. O'Grady, Birkbeck College, University of London, London, United Kingdom. This research was supported by Deutsche Forschungsgemeinschaft Grants He 1132/5-1 and Schr 375/8-1 and by an Economic Social Research Council studentship. We thank Kyle Cave and two anonymous reviewers for their valuable comments on a draft of this article. Correspondence concerning this article should be addressed to Hermann J. MOiler, Institut for Allgemeine Psychologic, Universi~t Leipzig, Seeburgstr. 14/20, 19--04103 Leipzig, Germany. Electronic mail may be sent to h.mueller @psychologie.uni-leipzig.de. 1332

tations (e.g., Baylis & Driver, 1993; Duncan, 1984). Neisser (1967) proposed that visual scenes are segmented, initially, into groups (i.e., objects) on the basis of the operation of the Gestalt principles of perceptual organization; visual attention is then focused on separate objects sequentially for further processing (see also Kahneman & Henik, 1977, 1981; Kramer & Jacobson, 1991; Treisman, 1983; Treisman, Kahneman, & Burkell, 1983). Much research over the last two decades has been concerned with space-based and object-based selection, and there has recently been a tendency for space-based accounts to be subsumed under, or integrated with, object-based theories (e.g., Kramer, Weber, & Watson, 1997; Vecera, 1994; Vecera & Farah, 1994). In contrast, domain-based selection has received little interest following Duncan's (1984) criticism of the analyzer theory. However, recent studies of simple visual search for singleton (odd-one-out) feature targets under conditions in which the target-defining dimension varied across trials has led to an alternative account to analyzer theory, a dimension-weighting account (Found & MUller, 1996; Mtiller, Heller, & Ziegler, 1995). The present study was designed to examine this account in more complex, object-attribute judgment tasks, using variants of Duncan's (1984) original paradigm. Thus, the intent of the present experiments was twofold: to investigate the existence of domain-based selection and to examine its relationship with object-based selection. The most important finding was that domain-based selection effects could be shown to coexist with object-based effects. Because the present experiments used variants of Duncan's (1984) paradigm, it is useful to review his evidence for objectbased selection and to consider it in relation to other evidence concerning domain-based selection effects obtained with the same or very similar paradigms. Object-Based Selection Duncan's (1984) study yielded empirical evidence for objectbased selection while controlling for space- and domain-based aspects of selection (by presenting stimuli within a central area of

DOMAIN-BASED VISUAL SELECTION 1° of visual angle and requiting within-domain discriminations only). In Duncan's Experiment 1, observers were presented briefly with two superimposed two-dimensional (2D) objects: a box (rectangle, vertically oriented) and a line. Each object was discriminable by two independent attributes: The box could be large or small and have a gap in its left or fight side; the line could be of dashed or dotted texture and be tilted slightly toward the left or the right. Observers had to make single and dual judgments concerning attributes of one object only or dual judgments concerning attributes of both objects. Duncan observed that when judgments were directed to one object only, accuracy was as high for dual discriminations (box size and gap; line texture and tilt) as for single discriminations (box size; box gap; line texture; line tilt). However, accuracy was reduced when dual judgments were directed to separate objects (box size and line texture; box size and line tilt; box gap and line texture; box gap and line tilt). In other words, there was an accuracy cost for dual judgments (relative to single-judgment accuracy) only when attention was divided between separate objects. Because both objects were presented at the same (2D) spatial location, and because the attributes to be judged all concerned aspects of the objects' forms, the two-object cost could only arise because of a limitation in the number of object representations to which attention can be directed at the same time. Although Duncan's (1984) study has been widely accepted as supporting object-based selection, it has been argued, by Vecera and Farah (1994) and Kramer et al. (1997), that space-based selection was not ruled out conclusively (see also Cave & Kosslyn, 1989). Vecera and Farah advocated a definition of a true object, likened to Marr's (1982) spatially invariant 3D model representation, as distinct from a mere collection of place tokens (locations) grouped into an object-like array, likened to Marc's spatially variant primal sketch representation. Selection in Duncan's study may not have been truly object based but rather based on a mere 2D object array. Vecera and Farah investigated this issue in a series of experiments (using Duncan-type stimuli) from which they concluded that selection may be based on either type of representation, depending on the task to be performed (luminanceincrement detection vs. object-attribute judgments). In contrast, Kramer et al. (1997), who reinvestigated the findings of Vecera and Farah, argued that the initial selection of objects is essentially location based, in terms of the grouped-array notion, whatever the task. Domain-Based Selection Dimension-based (or discrimination-based) selection was proposed by Treisman (1969) and Allport (1971; analyzer theory). According to this theory, interference between concurrent discriminations might arise only when they involve the same analyzer. Allport presented his observers with tachistoscopic displays consisting of three colored outline shapes, each containing a small black numeral. In the divided-attention conditions, observers had to report (a) shapes and colors, (b) numerals and colors, or (c) shapes and numerals. According to analyzer theory, performance was expected to be poor in the shape and numeral condition (c) because both the shape and numeral stimuli required form analysis. The results conformed with the prediction: Shape and color accuracy was as good as in a single-attribute control condition (reporting color only or shape only), and shape and numeral accuracy was poor. There was also some loss in accuracy for numeral and color

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reports, which was not predicted by analyzer theory, though the loss was much less marked than that for shape and numeral reports. Duncan (1984) argued that the overall pattern of results (no accuracy loss with shape and color, but losses with shape and numeral and with numeral and color) was consistent with object-based selection, according to which there should be an accuracy loss only if attributes of separate objects are to be reported; however, the finding that the loss in the critical condition differentiating discrimination- and object-based selection (numeral and color) was only small could be accommodated by the analyzer theory. Duncan (1984) concluded that "as things stand, evidence for the analyzer theory does not seem especially strong" (p. 503). According to the Treisman-Allport account (Allport, 1971; Treisman, 1969), it is possible for, say, the shape-analyzer system to work on one object and the color-analyzer system to work on another object concurrently. Such independent processing is denied by Duncan's (1993) object-based selection theory, according to which "an object's different attributes may be processed by separate subsystems, but selective attention is then a co-ordinated state in which the outputs of multiple subsystems, describing these various separate attributes, are made available together for the control of behaviour" (p. 425). To decide more conclusively between dimension-based and object-based accounts, Duncan (1993) and Duncan and Nimmo-Smith (1996) examined a larger range of dimensions than had been used previously: shape, size, orientation, spatial frequency (contour dimensions), and color and brightness (surface dimensions). Duncan (1993) used a variant of his 1984 paradigm in which two objects were presented spatially apart (rather than overlapping). In Experiment 1, the objects were two letters, one presented to the left, C or G, and one to the fight, E or F. The letters could also be either small or large. Observers reported either (a) the two attributes, shape and size, of a single letter, (b) the same attribute of both letters (i.e., either shape or size), (c) two different attributes of the two letters (e.g., shape of left letter and size of right letter), or (d) both attributes for both letters (i.e., shape and size of left letter and shape and size of right letter). Accuracy was highest in (a), which required dual judgments to a single object (74.9%), and reduced but not different in (b) through (d), which required dual judgments to separate objects (69.5% on average). Importantly, there was no significant difference between (b) and (c) (69.7% vs. 67.9%). According to the analyzer theory, judging the same attribute of both letters (b) should have been less accurate than judging a different attribute for each letter (c). In a second experiment, Duncan (1993) presented two grating patches (consisting of lines) that could vary in orientation (horizontal or vertical), length, and spatial frequency (the experiment consisted of three subexperiments comparing orientation and length, orientation and spatial frequency, and length and spatial frequency). There were three conditions of interest: (a) single-attribute judgment to one object, (b) dual same-attribute judgments to both objects, and (c) dual different-attribnte judgments to both objects. The overall accuracies were 86.4%, 72.5%, and 75.9% for (a), (b), and (c), respectively, the only significant difference being that between oneobject (single-attribute) and two-object (dual-attribute) judgments. The lack of a difference between the two two-object dualjudgment conditions is at variance with the analyzer theory but is consistent with Duncan's object-based selection account. However, one could doubt the independence of stimulus attributes such as shape, size, orientation, and spatial frequency,

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which are all contour or boundary dimensions. To deal with this limitation, Duncan and Nimmo-Smith (1996) conducted a further study in which they examined dual judgments to boundary (e.g., length) and surface (e.g., color) properties of two separate objects, relative to single judgments. Importantly, Duncan and NimmoSmith found a dual-judgment deficit even with concurrent discriminations of color and boundary attributes, which is at odds with analyzer theory. In summary, Duncan's (1984) conclusion still holds that there is little support for dimension-based selection along the lines proposed by Treisman (1969) and Allport (1971). Generally, the critical variable in Duncan's paradigm is not the similarity of dimensions or, respectively, attributes to be judged concurrently but rather whether or not dual judgments are to be directed to one or to two objects. Rationale and Overview of the Present Study However, recent evidence from studies of visual search for singleton (odd-one-out) feature targets under conditions in which the target-defining dimension varied across trials (cross-dimension search) has led to an alternative account to analyzer theory, a dimension-weighting account (Found & Miiller, 1996; Miiller et al., 1995). According to this account, there is a limit to the total attentional weight available to be allocated at any one time to the various dimensions of the target object, with potential targetdefining dimensions (i.e., dimensions in which the target might differ from nontarget objects) being assigned weight in accordance with their instructed importance and their variability across trials. The greater the weight allocated to a particular dimension, the faster the presence of a target defined in that dimension can be discerned. By this account, dimensional weighting should not only modulate target detectability in odd-one-out feature search tasks. Rather, there should also be a limitation in how multidimensional features are bound into a coherent object representation. Concerning dual-attribute judgments directed to one or two objects, this account would predict that reports of attributes from separate dimensions or domains are more difficult under short exposure conditions than are reports of attributes of the same dimension or domain. Thus, for example, if the color dimension is weighted, color processing is enhanced for all objects, and because the dimensional weight is limited, processing of other object attributes such as form is impaired. Two points are noteworthy. First, this pattern of effects is exactly opposite to that predicted by the analyzer theory (Allport, 1971; Treisman, 1969). Second, dimension-based selection along the lines described might operate in addition to object-based selection. However, it would be incompatible with a strong theory of object-based selection, according to which the outputs of multiple subsystems, describing various separate attributes, are made available together by a coordinated selection process (Duncan, 1993). One could object that a test of the predictions derived from the dimension-weighting account using a variant of Duncan's (1984) paradigm is not needed because they are inconsistent with Duncan's data (Duncan, 1984, 1993; Duncan & Nimmo-Smith, 1996). However, Duncan (1984, 1993) examined report of attributes that all related to object contour in one way or another (boundary dimensions), preventing a strong test of dimension-based selection effects. Further, although Duncan and Nimmo-Smith (1996) used attributes from separate domains (boundary and surface), they

presented the two objects to be judged at separate, parafoveal display locations, making it difficult to rule out space-based attentional effects unequivocally (this also applies to Duncan, 1993). Thus, although there is little evidence supporting dimension-based selection operating in the Duncan paradigm, dimension-based selection is not definitively ruled out by the existing data. Therefore, the present experiments were designed to provide a stronger and unequivocal test of the idea that dimension-based selection effects operate not only in simple odd-one-out feature-detection tasks but also in complex object-judgment tasks requiring the binding of multidimensional features into a coherent object representation. Experiments 1-3 were designed to examine the relationship between object-based and dimension-based selection, using a variant of Duncan's (1984) paradigm. The modifications involved the use of other (overlapping) stimuli, a changed response procedure that did not require different responses to different objects to be associated with various fingers of the two hands, and a withinsubject design. Furthermore, and most importantly, the modified paradigm required reports of attributes from separate domains (form and color). Experiment 1 compared judgments of attributes of one object (cross-domain judgments) with judgments of attributes of two objects (within- and cross-domain judgments). The results showed dimension-based selection effects, which were operating in addition to object-based effects. Experiment 2 compared within-domain judgments with cross-domain judgments directed to one object, with only a single object (rather than two overlapping objects) being presented. The results revealed dimension-based selection effects to be operating within a single object. Experiment 3 examined whether the domain- and objectbased effects established in the preceding experiments are independent (i.e., whether the null hypothesis of additivity could be maintained). The results were consistent with independence (i.e., additivity) of the two types of selection effect. Experiment 4 was designed to resolve a discrepancy between the results of Experiments 1-3 and the findings of Duncan and Nimmo-Smith (1996). The results replicated the pattern of effects observed in Experiments 1-3 (in particular, Experiment 1).

Experiment 1 The purpose of Experiment 1 was to ascertain whether there are reliable effects of domain-based selection in addition to objectbased selection. Domain-based selection effects would be predicted by both Allport's (1971; see also Treisman, 1969) analyzer theory and the dimension-weighting account recently proposed by MOiler et al. (1995; Found & MOiler, 1996). According to the analyzer theory, dual within-domain judgments are expected to incur an accuracy loss relative to dual cross-domain judgments because the former compete for the same domain-specific analyzers. Conversely, according to the dimension-weighting account, dual within-domain judgments should show accuracy superior to that for dual cross-domain judgments because there is a limit to the attentional weight that can be allocated at any one time to the various dimensions on which an object is defined. Thus, in addition to attempting to ascertain the existence of domain-based selection, Experiment 1 was also intended to discriminate between the two theories that emphasize the importance of domain boundaries for visual selection.

DOMAIN-BASED VISUAL SELECTION In Duncan's (1984) Experiment 1 and in a pilot study to the present experiments, 1 the attributes to be judged belonged to the same domain, form. For example, in the pilot study, observers made judgments to the sizes and textures o f one or two overlapping box objects (one horizontal and one vertical box). The texture judgment (dashed vs. dotted) concerned the elements that made up the boundary contour of an object, rather than surface texture (the objects had no surface texture filling in the space enclosed by the boundary contour). The size judgment (small vs. large) was ultimately dependent on the length of longer boundary lines making up the object rectangle, which were themselves made up of the collinearity-grouped texture elements. Consequently, the discrimination of the size and texture attributes would have involved similar coding processes. In contrast, in the present Experiment 1, attributes could belong to either one domain, form (line texture = text), or the other, color (line hue = hue), or both. With dual judgments directed to one object, two attributes of different domains were to be reported (cross-domain condition: hue & text). With dual judgments directed to two objects, either attributes of the same domain (within-domain condition: hue & hue or text & text) or attributes from different domains (cross-domain condition: hue & text) were to be reported for each of the separate objects. Although, neurophysiologically, the color and form pathways are not well separated (e.g., see Zeki, 1993, who described a color pathway and a form pathway linked to color), there is psychophysical evidence for the (at least relative) independence of the color and form domains. For example, Boucart and Humphreys (1992, 1994) reported findings suggesting that access to semantic object information was unrestricted during attention to global orientation, size, and global shape of objects but was prevented during attention to color and luminance of objects. Boucart and Humphreys took the filtering out of form information during luminance and color judgments to imply independent processing of color and form and of luminance and form. Furthermore, for example, colorbased grouping processes behave differently from form-based grouping processes (e.g., Found & Mtiller, 1996; Nothdurft, 1993; Wolfe, Chun, & Friedman-Hill, 1995), and color-based and formbased saliency signals may combine to co-activate a common response-relevant representation (e.g., Krummenacher, Mtiller, & Heller, 2000).

Method Observers. Four observers, 1 woman and 3 men, aged 18-31 years, participated in Experiment 1. All observers had normal vision, including normal color vision. They were paid £4.00 per hour. Apparatus. The experiment was run in a completely darkened laboratory. Stimuli were presented on a high-resolution color CRT monitor driven at a frame rate of 60 Hz by a Silicon Graphics Iris Indigo Unix workstation. Observers responded on a trial using the computer mouse, and they initiated the next block of trials by pressing the Enter key on the computer keyboard. Observers viewed the monitor from a distance of 80 cm, with viewing distance maintained through the use of a chin rest. The brightness of the monitor was adjusted to a comfortable level and remained unchanged throughout the experiment. Stimuli. On each experimental trial, the following sequence of stimuli occurred on a black monitor background (see Figure 1 ). A trial started with the presentation of a fixation square in the screen center, outlining the region within which the target (and masking) stimuli would be displayed subsequently (the square was composed of lines 1 pixel in width and 84 pixels in length [corresponding to 1.89° of visual angle] and was centered on the screen center-point coordinates; the lines were of a pale white hue

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[Silicon Graphics R-G-B color map: 50, 50, 50]). The square was presented for 400 ms to give observers sufficient time to focus their spatial attention on the display region. The removal of the fixation square was followed by a 1,020-ms interval with the screen blank. Then the target stimuli, two colored horizontal and vertical overlapping boxes, each varying in texture (dashed or dotted) and hue (red or yellow), were presented briefly (see Target exposure durations section below) for a predetermined exposure duration in the center of the monitor. With both dashed and dotted textures, the lines were made up of approximately the same number of pixels. In the dotted case, these pixels were spaced more evenly along the contour; in the dashed case, they were more grouped. The size of each of the boxes was 0.33 ° of visual angle for the shorter side and 0.62 ° of visual angle for the longer side. After termination of the target display, there was another blank interval of 17 ms, which was then followed by a contour mask. The composite contour mask consisted of two large intersecting boxes (vertical and horizontal) of the same size as the target box stimuli. The lines of the masking boxes were rendered solid, and their hues were matched for luminance (see next paragraph). This composite contour mask was presented at the exact pixel coordinates of the preceding target stimuli. To avoid a hue-himinance confound (color luminance covarying with color wavelength), we used luminance-matched hues. The matching procedure was as follows. First, physical isoluminance was attained for redand yellow-filled circles of radius 0.64 °. Second, subjective isoluminance was attained for the observers, using the actual target stimuli. Observers adjusted the luminance of the yellow stimulus to match that of the red stimulus in eight trials. The single-trial settings were then averaged to define isoluminance for a given observer. Because the individual observers' means were in good agreement (largest difference < 2.0 cd/m2), the luminance of the yellow stimulus was fixed at the group mean for all observers. The target hues introduced in Experiment 1 were red (color map: 200, 0, 0) and yellow (color map: 190, 120, 0), and the mask hue was orange (color map: 190, 60, 0). The physical luminances were 8.7 and 12.0 cd/m2 for the red and yellow target stimuli, respectively, and 8.4 cd/m 2 for the mask stimulus. Simultaneously with the onset of the mask, a mouse pointer was presented for response in the lower part of the monitor, where a set of four response alternatives (click panels) was to appear after 1,033 ms. The four click panels, which were outlined by a pale white square (size: 6.75 °, line width: 2 pixels) centered 4.35 ° below the mask, corresponded to the two values of each of the target attribute(s) to be judged by the observer. In the two single-judgment conditions (hue only or texture only), the click panels each displayed only the object that the observer was instructed to judge (e.g., the vertical box) or a correspondingly oriented (e.g., vertical) line. When box hue was to be judged, two click panels showed a red box and two showed a yellow box (mouse-clicking on either of two duplicate panels registered the same response), with line texture rendered solid to reinforce response to the instructed attribute. When box texture was to be judged,

The pilot study was conducted to test whether object-based selection effects along the lines of those found by Duncan would also be obtained with the modified paradigm used in the present experiments. That paradigm included, notably, the use of other (overlapping) stimuli that were designed to avoid a possible spatial-frequency channel account of Duncart's (1984) two-object costs (cf. Watt, 1988); a changed response procedure that did not require different responses to different objects to be associated with various fingers of the two hands; and a within-subject design. Concerning the changed response procedure, instead of having to give two successive responses on a four-button response pad (e.g., a box-size response followed by a line-texture response), observers gave a single integrated response by mouse-clicking one of four alternative choice panels on the computer monitor, each panel depicting a possible combination of the attribute(s) to be judged. The pilot study essentially replicated Duncan's finding in that all observers showed a greater dual-judgment cost for two-object relative to one-object decisions.

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Figure 1. Illustration of displays presented in Experiment 1, with the judgment-relevant displays [top left of (a) and (b)] consisting of a red dashed vertical box and a yellow dotted horizontal box. The orange postdisplay masks and the response panels are shown, respectively, at the top right and the bottom of (a) and (b). Red, yellow, and orange are indicated by dark, light, and intermediate shadings, respectively; the response panels are drawn in a scale different from the one used for the stimuli shown in the top left of (a) and (b). The condition illustrated in (a) required single judgments of hue to the horizontal box; the condition illustrated in (b) required dual one-object judgments of vertical box hue and texture. In the response panels for both (a) and (b), red box options are on the left and yellow box options are on the right.

two click panels displayed a dashed line and two displayed a dotted line to represent outline texture; the lines were white (color map: 100, 100, 100) in hue (to avoid contamination of the texture judgments by the hue of the target box) and were presented at an orientation corresponding to that of the target box (horizontal or vertical). In dual-judgment one-object conditions, observers were presented with four click panels each displaying one of the four possible combinations of box hue and outline texture. For example, when the vertical box was to be judged, each panel would depict a vertical box on the left and a vertical line on the right; one panel would

contain a red box and a dashed line, one a red box and a dotted line, one a yellow box and a dashed line, and one a yellow box and a dotted line. In dual-judgment two-object conditions, observers were presented with four click panels each displaying one of the four possible combinations of the hue of one box and the outline texture of the other box. For example, when the vertical box was to be judged in terms of hue and the horizontal box in terms of texture, each panel would depict a vertical box on the left and a horizontal line on the right; one panel would contain a red (vertical) box and a dashed (white, horizontal) line, one a red box and a dotted line, one

DOMAIN-BASED VISUAL SELECTION a yellow box and a dashed line, and one a yellow box and a dotted line. In all conditions, observers responded by selecting and mouse-clicking the appropriate panel. The mask and response panels stayed on the screen until a mouse click was recorded. After the mask and the set of response alternatives were removed, a dim gray feedback symbol (+ or - ) appeared centrally for 500 ms, followed by a blank interval of 533 ms. The use of click-box response panels was considered preferable (i.e., more user-friendly, more immediate, and less error prone) to keypress responses involving complex stimulus-response mapping rules (each stimulus attribute being mapped to the index or middle finger of the left or right hand). In Duncan's (1984) study, which used a between-subjects design, each condition involved a different set of mapping rules. Duncan's keypress reaction procedure required that observers respond to one dimension first (using the left hand) and to the other dimension second (using the right hand; for single judgments, one hand only was used). The assignment of a dimension to a hand was fixed throughout the experiment. For a given hand, the leR key (Key 1) referred to one value of the dimension to be judged and the right key (Key 2) to the other value (e.g., if box size was to be judged, Key 1 referred to small and Key 2 to large). This complex response procedure was deemed unsuitable for the present experiment, which used a within-subject design; that is, each observer would have had to learn a new mapping with each new condition. Target exposure durations. Prior to the experiment, all observers performed in single-judgment conditions only, with appropriate adjustments to the target exposure duration made so as to match the single-hue and single-texture judgment accuracies across observers. At the end of singlejudgment practice, the observers achieved an average target exposure duration of 50 ms, at which their single-judgment accuracies were hueH = .938, hueV = .907, textH = .955, and textV = .945 (where hue and text refer to hue and texture judgments and H and V to judgments made to the horizontal and vertical objects). All observers then practiced the task under their starting dual-judgment two-object condition (at the end of practice, their dual-judgment two-object accuracy averaged .784). Further fine adjustments of the target exposure durations were made during this practice session. The final exposure durations reached by the observers and introduced in the main experiment ranged between 17 and 33 ms, with a mean of 25 ms. Design. All observers performed in all experimental conditions: the single-judgment control condition, requiring a single response to a prespecified attribute of one target object; the dual-judgment one-object condition, requiring a joint response to two attributes of one target object; and the dual-judgment two-object condition, requiring a joint response to two prespecified attributes each belonging to a different target object. Each observer undertook judgments of all single attributes and pairs of attributes. In dual-judgment two-object conditions, the four possible pairs of attributes from different objects were vertical (hue) plus horizontal (texture), horizontal (hue) plus vertical (texture), vertical (hue) plus horizontal (hue), and vertical (texture) plus horizontal (texture). These attribute pairs are referred to as hueV & textH, hueH & textV, hueV & buell, and textV & textH, respectively. In the dual-judgment one-object condition, the two possible pairs of attributes to be judged were vertical (hue) plus vertical (texture) and horizontal (hue) plus horizontal (texture), which are referred to as hue & text V and hue & text H. In single-judgment (one-object) conditions, the single attributes to be judged were vertical (hue), vertical (texture), horizontal (hue), and horizontal (texture). These attributes are referred to as hueV, textureV, hueH, and textureH, respectively. Thus, there were four single-attribute judgment conditions, two dual-attribute one-object judgment conditions, and four dual-attribute two-object judgment conditions, that is, 10 judgment conditions in total. One hundred twenty-eight judgments were recorded for each of the 10 judgment conditions. Procedure. Each observer served in an initial practice session of between 1 and 2 hours on Day 1, followed by four experimental sessions performed during the same week. Experimental sessions presented three blocked judgment conditions: 128 single-attribute judgments, 64 dualattribute une-object judgments, and 128 dual-attribute two-object judg-

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ments. Judgment conditions were separated by a break of 10 rain. Each condition started with an unrecorded practice block of 32 trials followed by either two blocks (dual-attribute one-object judgments) or four blocks (single-attribnte judgments and dual-attribute two-object judgments) of 32 recorded trials, with short breaks between blocks. Each session started with either a dual-attribute one-object or a dual-attribute two-object judgment condition, followed by a single-attribute judgment condition, followed by either a dual-attribute two-object or a dual-attribute one-object judgment condition. The order of dual-attribute one-object and two-object judgment conditions was counterbalanced across observers and sessions. At the beginning of each judgment condition, observers were informed of the exact object attribnte(s) to be judged throughout the next 64 (+ 32 practice) or 128 (+32 practice) trials. For example, one observer started Session 1 with dual-attribute one-object judgments of hue & text V (64 trims) and then performed singie-attribute judgments of textH (128 trials) followed by dual-attribnte two-object judgments of hueV & textH (128 trials); in Session 2, this observer started with dual-attribute two-object judgments of textV & textH (128 trials) and then performed single-attribute judgments of hue V (128 trials) followed by dual-attribnte one-object judgments of hue & textH (64 trials); and so forth for Sessions 3 and 4. In terms of the exact attributes to be judged, each condition (e.g., dual-attribute two-object conditions: hueV & textH, hueH & textV, hueV & buell, and textV & textH) was judged once by each observer, except for the dual-attribnte one-object conditions, which were judged twice (in nonconsecutive sessions). However, the order of conditions was different for each observer such that a given condition was performed first by one observer and second, third, and fourth by the other observers, thus counterbalancing any condition-specific practice effects across observers. The prior practice sessions consisted of a succession of blocks of 64 trials under single-judgment conditions, each block using a fixed target exposure duration. The master sequence of exposure durations used was 2 5 0 - 2 0 0 - 1 6 7 - 1 5 0 - 1 1 7 - 1 0 0 - 8 3 - 6 7 - 5 0 ms (Duncan's [1984] corresponding master sequence was 2 2 0 - 1 8 0 - 1 4 0 - 1 0 0 - 8 0 - 6 0 - 4 0 ms). If an observer performed at ceiling in a given block, the next lower exposure duration in the master sequence was skipped. If an observer's average singie-judgment accuracy during a session (day) of the main experiment exceeded 95%, the exposure duration was adjusted downward by 17 ms for any subsequent sessions. At the beginning of each judgment condition, observers were informed of the exact object attribute(s) to be judged next. In single-judgment conditions, the importance of attending only to the one attribute to be judged was emphasized; in dual-judgment conditions, it was stated that the two attributes were equally important. Observers were reminded carefully to fixate at the start of each trial and to concentrate on the accuracy of response rather than the speed (observers were informed that their reaction times would not be recorded). Analysis. Two alternative measures of dual-judgment accuracy were derived from the raw data: the average probability of one judgment being correct, which is the accuracy measure reported by Duncan (1984, 1993; Duncan & Nimmo-Smith, 1996), and the joint probability of both (dual) judgments being correct. Arguably, on a strong "late" object-based selection account such as that advocated by Duncan (e.g., 1996), the joint probability of both (dual) judgments being correct is the most appropriate performance measure. According to Duncan's position, the different attributes of an object are selected together. It would follow, then, that when one of two judgments is incorrect, the other correct judgment is likely to be a lucky guess. Such lucky guesses would tend to inflate the measure average probability of one of the two judgments being correct more than the measure joint probability of both judgments being correct. Note that because observers gave a single, integrated response relating to both attributes to be judged in dual-judgment conditions, it was impossible to analyze their response accuracy according to which attribute was judged first or second. Recall that Duncan had observed a dual-judgment twoobject cost, in terms of the average probability of one judgment being correct, only for the second-reported attribute. Because the integrated

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MOLLER AND O'GRADY

response required in the present experiment essentially averaged across the first- and second-reported attributes, one would expect any dual-judgment two-object cost to be less than in Duncan's (second-reported attribute) data.

Results The results are summarized in Table 1. The upper half of Table 1 presents, separately for each observer and for the average observer, the proportion of correct responses for each of the four main judgment conditions: single-attribute, dual-attribute oneobject (cross-domain), dual-attribute two-objects (within-domain), and dual-attribute two-objects (cross-domain). For all dualattribute judgments, the average probability of one judgment being correct is given, which is the accuracy measure reported by Duncan (1984, 1993; Duncan & Nimmo-Smith, 1996), along with the dual-attribute judgment cost relative to the single-attribute judgment condition. The lower half of Table 1 presents the joint probability of both (dual) judgments being correct, along with the joint probability correct expected on the assumption that dual judgments are based on independent single judgments. 2 Only the joint dual-judgment data (lower half of Table 1) were analyzed statistically, using a priori t tests. The average probabilities of one judgment being correct (upper half of Table 1) showed a very similar pattern of effects, though, not surprisingly, they were reduced in magnitude. Without exception, whenever an observer showed a performance difference in the joint dual-judgment data, he or she also showed a difference in the average probabilities of one judgment being correct. Note that values given in brackets in the rest of the article refer to average probabilities of one judgment being correct. When observers directed dual judgments to two separate objects, their accuracy was significantly greater when they judged

Table 1

Judgment Accuracy for the Single-Attribute, Dual-Attribute OneObject (Cross-Domain), Dual-Attribute Two-Object (WithinDomain), and Dual-Attribute Two-Object (Cross-Domain) Judgment Conditions of Experiment 1

Observer

Single

Dual, one-object, across

O1 02 03 04

.928 .954 .904 .960

.938 (-.010) .944 (.010) .892 (.012) .943 (.017)

.916 (.012) .951 (.003) .921 (-.017) .935 (.025)

.892 (.036) .918 (.036) .809 (.095) .903 (.057)

M

.937

.929 (.008)

.931 (.006)

.881 (.056)

O1 02 03 04

.886 (.860) .892 (.909) .800 (.816) .907 (.920)

.889 (.861) .913 (.910) .861 (.817) .898 (.922)

.763 (.859) .854 (.883) .661 (.816) .826 (.919)

M

.871 (.876)

.890 (.878)

.776 (.869)

attributes from the same domain (within-domain) than when they reported attributes from different domains (cross-domain): .890 [.931 ] versus .776 [.881 ]; all [all] observers showed the difference, t(3) = 3.56, p < .040. 3 Interestingly, there was little difference in accuracy between dual cross-domain judgments directed to one object and dual within-domain judgments directed to two objects (within-domain): .871 [.929] versus .890 [.931], t(3) = 1.24, ns, which seems to be at odds with the notion of object-based attention. However, when the reported domains are equated between dual judgments to one and to two objects (cross-domain judgments), a two-object cost was manifest: .871 [.929] versus .776 [.881]; all [all] observers showed the effect, t(3) = 4.20, p < .025. Detailed examination of the data according to the particular attributes judged (hue or texture) revealed that both dual twoobject judgments of hue (within: hue & hue) and dual two-object judgments of texture (within: text & text) were more accurate than dual two-object judgments of hue and texture (across: hue & text): .902 and .879 versus .776. Thus, the effect of domain-based selection (advantage for within-domain judgments to two objects) is not confounded by a differential difficulty of making hue and texture discriminations. In addition, the evidence for object-based selection remains: For dual judgments involving both hue and texture (cross-domain judgments), performance was greater when the judgments were directed to one object rather than two objects (see above). This pattern of effects argues that domain-based selection is operating in addition to object-based selection. One further issue of interest concerns whether the dualjudgment accuracies in the various conditions of Experiment 1 were based on independent or interdependent decisions. Objectbased constraints on selection, as proposed by Duncan (1984), ought to produce negatively interdependent decisions such that, when Object A is selected, judgments of Object B's attributes should be impaired systematically. Likewise, domain-based constraints as proposed by Miiller et al. (1995) also ought to lead to negatively interdependent decisions such that, when an attribute of Domain A is being judged, judgments of Domain B's attribute should be impaired. To examine this issue, we derived the expected joint probabilities of both (dual) judgments being correct

Dual, two-object Within

Across

Note. For dual-attribute judgments, the average probability of one judgment being correct is given in the top half of the table, along with the cost relative to the single-attribute judgment condition (in parentheses); the joint probability of both judgments being correct is given in the bottom half, along with the joint probability correct expected on the assumption that dual judgments are based on independent single judgments (in parentheses).

2 For each observer in each experiment, the expected joint probabilities for each judged attribute pair were calculated on the basis of the single probabilities for the relevant attributes, and the expected joint probabilities were then averaged across all attribute pairs judged. Calculation of the expected joint probabilities on the basis of the single probabilities reported in the upper halves of Tables 1, 2, 3, and 4 (i.e., the single probabilities averaged across the individual attributes) would have been incorrect (because the sum of products is almost always unequal to the product of sums). The fact that the expected joint probabilities were calculated on the basis of single probabilities per individual attribute is the reason why the expected probabilities may differ slightly between the various dualjudgment conditions of Experiments 1, 2, 3, and 4. 3 Because observers gave a single, integrated response relating to both attributes to be judged in dual-judgment conditions, it was impossible to analyze their response accuracy according to which attribute was judged first or second. Recall that Duncan had observed a dual-judgment twoobject cost, in terms of the average probability of one judgment being correct, only for the second-reported attribute. Because the integrated response required in the present experiment essentially averaged across the first- and second-reported attributes, one would expect any dual-judgment two-object cost to be less than in Duncan's (second-reported attribute) data.

DOMAIN-BASED VISUAL SELECTION from the observed probabilities of the single judgments being correct on the basis of the assumption that dual judgments are made independently. As can be seen from a comparison of the observed joint probabilities of both (dual) judgments being correct with the expected probabilities (see lower half of Table 1), there was clear evidence for negatively interdependent decisions only for two-object cross-domain judgments, t(3) = - 3 . 6 2 , p < .04. In the other two dual-judgment conditions, there was little difference between the observed and expected joint probabilities correct. Discussion

In summary, the accuracy advantage for dual judgments to the same object over dual judgments to two separate objects (which was evident for each of the 4 observers) provides further support for object-based selection theories. Importantly, the data also provide unequivocal evidence of domain-based selection. Dual twoobject judgments to the same attribute (within) were more accurate overall than dual two-object judgments to different attributes (across). The difference between dual within- and dual crossdomain judgments cannot be attributed to differential accuracy for one of the dual discriminations, of texture or hue, required. This result strongly supports the idea that a domain-based selection process was operating in addition to an object-based selection process. The two attributes of texture and hue judged in Experiment 1 relate to different domains: form and color. Hence, the discrimination of the hue and texture attributes would have involved different coding mechanisms. Domain-based effects on judgment accuracy were predicted both by Allport's (1971) analyzer theory and the dimension-weighting account of Mtiller et al. (1995; Found & Mtiller, 1996). However, the two theories make different predictions as to the direction of the effects. Although Allport predicts that within-domain costs should be greater than cross-domain costs, Mtiller and his colleagues predict the opposite. The finding in Experiment 1 that the cross-domain cost was greater than the within-domain cost agrees with the dimension-weighting account. Experiment 2 In Experiment 1, dual judgments directed to one and the same object involved only cross-domain attributes. It is possible that greater cross-domain than within-domain costs are observed only when dual judgments are made to two objects; that is, domainbased selection may operate only when dual judgments are made to competing objects. Experiment 2 was designed to examine the generalization of domain-based selection to conditions in which only one object was to be judged. Restated, the purpose of Experiment 2 was to ascertain whether there are reliable effects of domain-based selection operating within one object. In Experiment 2, each observer attended to only one object at a time, a vertical or a horizontal box, while making single and dual judgments to attributes within or across domains. To examine within-domain dual judgments to one object, it was necessary to add two new attributes to those used in Experiment 1. The newly introduced attributes were size and saturation (sat) so that there were two separate attributes within the form domain (size and texture) and two separate attributes within the color domain (hue and saturation), which permitted independent within-domain judgments to be made to one and the same object.

1339

In a pilot experiment to Experiment 2, both objects, the vertical and the horizontal box, were always displayed as stimuli, even though different observers were instructed to attend to only one object out of the two presented. 4 The results showed lower accuracy for dual cross-domain judgments than for dual within-domain judgments (but see Footnote 4). However, although observers directed judgments to only one prespecified object, it is possible that the accuracy of dual cross-domain judgments is reduced only when there are two objects in view, which requires the foregrounding of one of the two objects as the target. Experiment 2 was therefore designed to examine whether domain-based selection effects would be evident even when the target display contained only a single object. In other words, only the relevant object to be judged by a given observer was displayed as the target stimulus.

Method Observers. Four observers, 2 women and 2 men, aged 22-32 years, participated in Experiment 2. Three of the observers had participated in the pilot experiment to Experiment 2; the other observer had taken part in Experiment 1. Payment was £4.00 per hour. Stimuli. On each trial, observers were presented with only one (vertical or horizontal) box stimulus. The form and color attributes of the box stimuli were varied systematically. The boxes were either large or small (large, same size as in Experiment 1; small, 0.50° for the longer side), and their outline texture was either dotted or dashed (as in Experiment 1). Further, the boxes were either red or yellow in hue, with a high or low level of saturation. The mask consisted of superimposed large and small, vertical and horizontal boxes, of a hue and saturation in between the saturated and less saturated reds and yellows of the target boxes. To avoid hue and luminance confounds, as well as saturation and luminance confounds, we followed the same procedure as in Experiment 1 to achieve subjective isoluminance of both hues at both saturation levels. The color map coordinates attained were [220, 0, 0] and [220, 85, 85] for the saturated and less saturated reds, respectively, as before, and [190, 190, 0] and [140, 140, 120] for the saturated and less saturated yellows, respectively. The mask color map coordinates were also adjusted slightly to produce a subjective combination of the individual hues. The physical luminances for each of these colors were 8.6, 8.4, 10.5, and 8.9 cd/m2 for the red, less saturated red, yellow, and less saturated yellow colors, respectively, and 8.8 ccFm2 for the mask. The texture discrimination (dashed vs. dotted) required in Experiment 2 was made somewhat harder than that in the previous experiments because the single texture judgments of 1 of the observers had tended to approach ceiling accuracy in Experiment 1. We made the discrimination more difficult by modifying the dotted texture to more closely resemble the dashed texture. We matched the two types of textured boxes in terms of overall luminance by matching the number of pixels used to form the dotted and dashed textures.

4 The pilot experiment to Experiment 2 showed a significant cost in cross-domain relative to within-domain dual-judgment accuracy even when only one object was to be judged (.690 [.816] vs. 734 [849]). This effect suggests that there is an additional cost for making judgments of attributes defined in different (form and color) domains (hue & size, hue & text, sat & size, sat & text) relative to judgments of attributes defined within the same domain (hue & sat, size & text)----not only when dual judgments refer to two separate objects (Experiment 1) but also when they refer to one and the same object. However, this finding should be treated with caution because the disadvantage for cross-domain relative to withindomain judgments was wholly due to dual judgments involving a report of saturation combined with a report from the form domain (size or texture), possibly confounding the evidence in favor of domain-based selection.

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MI~LLER AND O'GRADY

The response alternatives presented after termination of the target display by the contour mask referred to the box that was to be judged by the observer (i.e., the vertical or the horizontal box). Only the to-be-reported attribute(s) varied across alternative click panels. For example, in dual hue and saturation judgments to the vertical box, only the vertical box was displayed, and hue and saturation varied, whereas size and texture remained fixed. Thus, the four click panels displayed a saturated red box, a saturated yellow box, a less saturated red box, and a less saturated yellow box, with box size fixed at large and box outlines rendered solid. In general, when an attribute was fixed (i.e., when it was not to be reported), hue was red for 2 observers and yellow for 2 observers, saturation was high for all observers, size was large, and outline texture was solid. In judgments involving texture as a to-be-reported attribute, the click panels displayed a line representing the possible outline textures of the target box, as in Experiment 1 (the orientation of the line corresponded to that of the target's axis of elongation, i.e., vertical or horizontal). In single texture judgments, only the line was presented. In dual judgments involving texture, the line was presented in addition to a box. Target exposure durations. The starting target exposure durations were the final times reached by the observers in the pilot experiment to Experiment 2 (3 observers) or in Experiment 1 (1 observer). The initial exposure durations ranged from 33 to 150 ms (M = 104 ms). Over the course of the experiment (10 sessions or conditions), the target exposure durations were adjusted downward progressively to avoid ceiling effects. An adjustment was triggered if an observer's average single-judgment accuracy during a session exceeded 95%, in which case the exposure duration was reduced by 17 ms for any subsequent sessions. All observers reached a single frame of exposure (17 ms) by the ninth session (1 observer reached it by Session 4, 1 by Session 5, 1 by Session 7, and 1 by Session 9). Design. The target stimuli resembled those in Experiment 1, with additional attributes of size (form domain) and saturation (sat; color domain). There were 10 judgment conditions, all relating to one object only: 6 dual judgments--hue & sat, hue & text, hue & size, sat & text, sat & size, size & text--and 4 single judgments--hue, sat, size, text. Each observer performed all 10 conditions, with condition order counterbalanced across observers as well as possible. For example, 1 observer performed the 10 conditions in the following order: within-domain--hue, hue & sat, size, size & text; cross-domain--size & hue, hue & text, text, size & sat, sat & text, sat. Two observers started the experiment performing within-domain judgments; the other 2 observers started by performing cross-domain judgments. The insertion of single-judgment conditions among within- or cross-domain dual-judgment conditions was counterbalanced across observers. Two of the observers judged the vertical box, and 2 judged the horizontal box, throughout the experiment. Procedure. Each observer started immediately with the main experiment. In all other respects, the procedure was as in Experiment 2. Each observer performed all 10 experimental conditions (in separate sessions), each of which consisted of an unrecorded block of 32 warm-up trials followed by eight experimental blocks of 32 trials each.

Resul~ The results are summarized in Table 2. The upper half of Table 2 presents the proportion of correct responses for each of the three main judgment conditions: single-attribute, dual-attribute one-object (within-domain), and dual-attribute one-object (crossdomain). For all dual-attribute judgments, the average probability of one judgment being correct is given, along with the dualattribute judgment cost relative to the single-attribute judgment condition. The lower half of Table 2 presents the joint probability of both (dual) judgments being correct, along with the joint probability correct expected on the assumption that dual judgments are based on independent single judgments (see Footnote 2). We statistically analyzed only the joint dual-judgment data (lower half

Table 2 Judgment Accuracy f o r the Single-Attribute, Dual-Attribute OneObject (Within-Domain), and Dual-Attribute One-Object (Cross-Domain) Judgment Conditions o f Experiment 2 Dual, one-object Observer

Single

Within

Across

O1 02 03 04

.906 .915 .852 .876

.897 (.009) .910 (.005) .908 (-.056) .884 (-.008)

.854 (.052) .873 (.042) .826 (.026) .865 (.011)

M

.887

.900 (-.013)

.855 (.032)

O1 02 03 04

.827 (.819) .844 (.841) .854 (.720) .822 (.768)

.750 (.820) .756 (.831) .712 (.725) .778 (.765)

M

.837 (.787)

.749 (.785)


of Table 2) by using a priori t tests. The average probabilities of one judgment being correct (upper half of Table 2) showed a very similar pattern of effects (in text see values given in brackets). Without exception, whenever an observer showed a performance difference in the joint dual-judgment data, he or she also showed a difference in the average probabilities of one judgment being correct. Table 2 shows that when observers directed dual judgments to a single object that was the only object in the display, their accuracy was significantly greater when they reported attributes from the same domain (within-domain) than when they reported attributes from different domains (cross-domain): .837 [.900] versus .749 [.855]; all [all] observers showed the difference, t(3) = 4.31, p < .025. This pattern of results is similar to that observed in the pilot study to Experiment 2 (see Footnote 4), in which the target display contained two overlapping objects only one of which was to be judged. Scrutiny of the data according to the particular attributes judged (hue, saturation, size or texture) revealed that (in contrast to the pilot experiment to Experiment 2; see Footnote 4), the accuracy of dual cross-domain judgments involving saturation (sat & size and sat & text) now was comparable with that of dual cross-domain judgments involving hue (hue & size and hue & text): .760 and .718 versus .758 and .761. Therefore, the disadvantage for crossdomain judgments relative to within-domain judgments cannot be attributed to conditions combining saturation with size or texture reports. With accuracy of saturation judgments now more in line with the accuracy of hue, size, and texture judgments (.844 vs. .870, .885, and .949; single judgments only), the effect of domainbased selection (advantage for within-domain judgments) is replicated in the absence of a confound because of inadequate matching of the various attributes to be judged. Importantly, the effect is also evident when saturation judgments are not considered: dual cross-

DOMAIN-BASED VISUAL SELECTION domain h u e & text judgments show a cost relative to dual withindomain size & text judgments: .718 versus .854. One further issue examined concerns possible interdependencies between the component decisions in both the within- and crossdomain dual-judgment conditions of Experiment 2. See the lower half of Table 2 for a comparison of the observed joint probabilities of both (dual) judgments being correct with the expected probabilities. There was no consistent evidence for negatively interdependent decisions in the one-object cross-domain judgment condition. In the one-object within-domain judgment condition, all 4 observers showed larger observed than expected values--though t(3) = 1.64, ns--providing some evidence for positively interdependent decisions.

Discussion

In summary, the accuracy advantage in Experiment 2 for dual judgments made within the same domain over dual judgments made across two separate domains (which was evident for all 4 observers) provides added support for the existence of domainbased selection, even when judgments are directed to a single target object that is the only object in the display. The difference between within- and cross-domain judgments cannot be attributed to differential accuracies for particular attribute discriminations. Taken together with the results of Experiment 1, the results of Experiment 2 suggest that domain-based selection operates in addition to, and independently of, object-based selection, that is, it is manifest even when there is no need for object-based selection (which requires prior object segmentation and foregrounding of the target object) because there is only one object in view.

Experiment 3 Experiments 1 and 2 revealed superior accuracy for dual withindomain relative to dual cross-domain judgments---an effect indicative of domain-based selection. Experiment 1 also revealed superior accuracy for dual cross-domain judgments directed to one object relative to dual cross-domain judgments directed to two objects--an effect indicative of object-based selection. Furthermore, Experiment 2 revealed dual-judgment accuracy to be superior for within-domain relative to cross-domain judgments even though only one object was presented. Assuming that presentation of single object displays eliminates the need for object-based selection, the result of Experiment 2 suggests that domain-based selection can operate independently of object-based selection. Nevertheless, it remains possible that object-based and domainbased detection are nonindependent when there are two (overlapping) objects in view. This possibility is not ruled out by Experiments 1 and 2, which used different observers, different displays (one vs. two objects), and different attributes to be judged and therefore do not allow the magnitudes of the domain-based selection effects with one and with two objects to be compared directly. Therefore, to permit such a comparison, we carded out Experiment 3, in which the same observers were presented with two overlapping objects (as in Experiment 1) and directed dual withindomain and dual cross-domain judgments to one or two objects, with the same attributes to be judged in both conditions (attributes as in Experiment 2). If domain-based selection is dependent on object-based selection, the effect of domain-based selection would

1341

be expected to be greater when two objects are to be judged rather than just one object. Method Observers. Six new observers, all women, aged 19-30 years, participated in Experiment 3. All observers had normal vision, including normal color vision. Payment was £5.00 per hour. Stimuli. The stimuli were identical to those used in Experiment 2 except that both objects, a horizontal box and a vertical box, were presented simultaneously regardless of the judgment condition. Target exposure durations. Prior to the experiment, target exposure durations were set for all observers in one to two practice sessions in which they performed single-judgment conditions only. At the end of singlejudgment practice, the observers achieved target exposure durations ranging between 17 and 100 ms (M = 53 ms), which were then introduced into the main experiment. At these durations, the observers' single-judgment accuracies averaged .792 (hue = .877, sat = .711, size = .824, and text = .756). Over the course of the experiment (nine sessions), the target exposure durations were lowered progressively to avoid ceiling effects (except for the observer who had reached a target exposure duration of 17 ms at the end of practice). Design. All observers performed all experimental judgment conditions: single, dual one-object (within), dual one-object (across), dual twoobject (within), and dual two-object (across). Each observer undertook judgments of all single attributes and pairs of attributes, with the following restrictions: Under dual one-object judgment conditions, half the observers undertook judgments of the horizontal box only, whereas the other half undertook judgments of the vertical box only. Under dual two-object judgment conditions, in which the attributes were different, half the observers undertook judgments involving Attribute A of the horizontal box and Attribute B of the vertical box, whereas the other half undertook judgments involving Attribute B of the horizontal box and Attribute A of the vertical box. In single-judgment (one-object) conditions, the single amibutes to be judged were hueV, buell, s a t e satH, sizeV, sizeH, textV, and textH, respectively. In dual one-object (within) judgment conditions, the four possible pairs of attributes to be judged were hue & sat V, hue & sat H, size & text V, and size & text H (with V and H judgments counterbalanced across observers, hence giving two possible pairs of attributes to be judged per observer). In dual one-object (across) judgment conditions, the eight possible pairs of attributes to be judged were size & hue V, size & hue H, hue & text V, hue & text H, size & sat V, size & sat H, sat & text V, and sat & text H (also with V and H judgments counterbalanced across

observers, hence giving four possible pairs of attributes to be judged per observer). In dual two-object (within) judgment conditions, the eight possible pairs of attributes from different objects were hueV & satH, hueH & satV, sizeV & textH, sizeH & textV (different attributes), hueV & hueH, satV & satH, sizeV & sizeH, and textV & textH, respectively (with V and H judgments counterbalanced across observers for the different-attribute conditions, hence giving six possible pairs of attributes to be judged per observer). In dual two-object (across) judgment conditions, the eight possible pairs of attributes from different objects were hueV & sizeH, hueH & sizeV, hueV & textH, hueH & textV, satV & sizeH, satH & sizeV, satV & textH, and satH & textV (with V and H judgments counterbalanced across

observers, hence giving four possible pairs of attributes to be judged per observer). Thus, there were 24 judgment conditions in total. Two hundred fifty-six judgments were recorded for each of the 24 conditions. Procedure. Each observer served in an initial dual-judgment practice session of between 1 and 2 hr on Day 1, followed by eight experimental sessions performed during the next 2 weeks. The order of dual-attribute one-object (within), one-object (across), two-object (within), and twoobject (across) judgment conditions was counterbalanced across observers. Single-judgment conditions were interspersed arbitrarily. In all other respects, the procedure was as in Experiment 2. Each observer performed all 24 experimental conditions (in separate sessions), each of which con-

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M~LLER AND O'GRADY

sisted of an unrecorded block of 32 warm-up trials followed by eight experimental blocks of 32 trials.

Resul~ The results are summarized in Table 3. The upper half of Table 3 presents the proportion of correct responses for each of the five main judgment conditions: single-attribute, dual-attribute oneobject (within-domain), dual-attribute one-object (cross-domain), dual-attribute two-object (within-domain), and dual-attribute twoobject (cross-domain). For all dual-attribute judgments, the average probability of one judgment being correct is given, along with the dual-attribute judgment cost relative to the single-attribute judgment condition (in parentheses). The lower half of Table 3 presents the joint probability of both (dual) judgments being correct, along with the joint probability correct expected on the assumption that dual judgments are based on independent single judgments (see Footnote 2). We statistically analyzed only the joint dual-judgment data (lower half of Table 3) by using a priori t tests. The average probabilities of one judgment being correct (upper half of Table 3) show a very similar pattern of effects (in text see values in brackets). With only one exception, whenever an observer showed a performance difference in the joint dualjudgment data, she also showed a difference in the average probabilities of one judgment being correct. As can be seen from Table 3, the observers' dual-judgment accuracy was significantly greater when they reported attributes from the same domain (within-condition) than when they reported attributes from different domains (across-condition), both when they directed dual judgments to two separate objects--.730 [.851] v s . . 6 4 4 [.813]; all [5] observers showed the difference, t(5) = 4.06, p < .01--and when they directed dual judgments to

one object--.836 [.915] versus .774 [.888]; all [all] observers showed the difference, t(5) = 5.14, p < .005. Furthermore, the observers' dual-judgment accuracy was significantly greater when they responded to one object rather than to two objects, both when they judged different-domain attributes--.774 [.888] versus .644 [.813]; all [all] observers showed the difference, t(5) = 5.30, p < .005--and when they judged same-domain attributes--.836 [.915] versus .730 [.851]; all [all] observers showed the difference, t(5) = 5.07, p < .005. This pattern of effects replicates that observed in the previous experiments, providing evidence for both domain-based and object-based selection. To test whether the domain-based and object-based selection effects were interactive (nonindependent), rather than additive (independent), we subjected the data to an analysis of variance (ANOVA) with main terms for object condition (one vs. two objects) and domain condition (within vs. across). This ANOVA revealed both main effects to be significant: F(1, 5) = 41.21, MSE = 0.0021, p < .005, and F(1, 5) = 48.08, MSE = 0.0007, p < .005, respectively. However, the Object × Domain interaction was not significant, F(1, 5) < 1.0. Thus, the null hypothesis can be maintained that the object- and dimension-based limitations of visual selection are additive, independent, effects. Numerically, the effects of domain-based selection were .062 [.027] and .086 [.038] for the one- and two-object conditions, respectively, and the effects of object-based selection were .106 [.064] and .130 [.075] for within- and cross-domain judgments, respectively. Detailed examination of the data according to the particular attributes judged (hue, saturation, size, or texture) revealed that both dual two-object judgments of hue (within: hue & hue) and dual two-object judgments of texture (within: text & text) were more accurate than dual two-object judgments of hue and texture

Table 3

Judgment Accuracy for the Single-Attribute, Dual-Attribute One-Object (Within-Domain), DualAttribute One-Object (Cross-Domain), Dual-Attribute Two-Object (Within-Domain), and Dual-Attribute Two-Object (Cross-Domain) Judgment Conditions of Experiment 3 Dual, one-object

Dual, two-object

Observer

Single

Within

Across

Within

Across

O1 02 03 04 05 O6

.872 .861 .914 .934 .891 .867

.927 (-.055) .881 (-.020) .922 (-.008) .915 (.019) .918 (-.027) .929 (-.062)

.894 (-.022) .858 (.003) .910 (.004) .909 (.025) .878 (.013) .878 (-.011)

.835 (.037) .842 (.019) .879 (.035) .876 (.058) .842 (.049) .830 (.037)

.783 (.089) .780 (.081) .815 (.099) .882 (.052) .799 (.092) .816 (.051)

M

.892

.915 (-.023)

.888 (.004)

.851 (.041)

.813 (.079)

01 02 03 04 05 06

.864 (.811) .760 (.775) .854 (.826) .838 (.859) .848 (.767) .854 (.770)

.792 (.809) .723 (.774) .816 (.827) .802 (.897) .764 (.769) .748 (.774)

.709 (.765) .708 (.735) .783 (.828) .779 (.873) .710 (.799) .689 (.753)

.588 (.668) .599 (.724) .633 (.844) .765 (.891) .625 (.796) .652 (.754)

M

.836 (.801)

.774 (.808)

.730 (.792)

.644 (.780)


DOMAIN-BASED VISUAL SELECTION (across: hue & text): .842 and .770 versus .634. A similar pattern was revealed for dual judgments of hue (within: hue & hue) and of size (within: size & size) relative to hue and size (across: hue & size), .842 and .722 versus .667; for dual judgments of saturation (within: sat & sat) and of size (within: size & size) relative to saturation and size (across: sat & size), .696 and .722 versus .656; and for dual judgments of saturation (within: sat & sat) and of texture (within: text & text) relative to saturation and texture (across: sat & text), .696 and .770 versus .617. Similarly, dual one-object within-domainjudgments of color (within: hue & sat) and of form (within: size & text) were, or tended to be, more accurate than all four combinations of dual one-object crossdomain judgments (across: hue & size, hue & text, sat & size, sat & text): .873 and .805 versus .799, .785, .798, and .707, respectively. Thus, the effect of domain-based selection (advantage for within-domain judgments) was not confounded by a differential difficulty of making hue, saturation, size, and texture discriminations. One further issue examined concerns possible interdependencies between the component decisions in the various dual-judgment conditions of Experiment 3. See the lower half of Table 3 for a comparison of the observed joint probabilities of both (dual) judgments being correct with the expected probabilities. There was clear evidence for negatively interdependent decisions for twoobject cross-domain judgments, t(5) = -6.98, p < 0.001, and two-object within-domain judgments, t(5) = -5.95, p < 0.002, consistent with an object-based constraint on selection (Duncan, 1984). In the other two dual-judgment conditions, there were no consistent differences between the observed and expected joint probabilities correct. Discussion In summary, in Experiment 3, dual within-domain judgments were more accurate than dual cross-domain judgments, irrespective of the number of objects to which judgments were to be directed. At the same time, dual judgments directed to one object were more accurate than dual judgments directed to two objects, irrespective of whether the judgments related to same-domain or to different-domain attributes. This pattern of findings replicates the domain-based and object-based selection effects observed in Experiments 1 and 2 within a single experiment. Furthermore, statistical analysis revealed no evidence of an interaction between the object-based and domain-based effects in Experiment 3. This pattern of findings supports the idea that domain-based selection operates in addition to, and independently of, object-based selection. Experiment 4 In Experiments 1-3, domain-based selection was evidenced by the finding of an accuracy loss for cross-domain relative to withindomain dual judgments. However, Duncan and Nimmo-Smith (1996) recently reported results at variance with this finding. A possible reason for this discrepancy was examined in Experiment 4. Before developing the rationale of Experiment 4, we consider in some detail the method and results of Duncan and Nimmo-Smith (Experiment 1). Participants were presented with a horizontal line left or right of fixation and a vertical line above or below fixation. The horizontal

1343

line was either red or orange and the vertical line either green or blue, with line length either 0.8 ° or 0.4° . There were four experimental conditions, each consisting of three subconditions: single judgment directed to the horizontal line, single judgment to the vertical line, and dual judgments to both lines. Duncan and Nimmo-Smith (1996) analyzed judgment accuracy separately for each attribute (color or length) depending on whether judgments were to be directed to the same attribute or different attributes. They found a dual-judgment cost, relative to the single-judgment baseline, for color in color-color judgments (11.1%), for length in length-length judgments (14.2%), and for length in color-length judgments (15.7%), but not for color in color-length judgments (-0.2%). What is at variance with the present results is that Duncan and Nimmo-Smith failed to observe an added cost for dual different-attribute (cross-domain) judgments relative to dual sameattribute (within-domain)judgments. (In fact, there was no cost at all, relative to the single-judgment baseline, for color in colorlength judgments.) However, this pattern of results may be explained if one assumes that spatial attention driven by color cues was involved. Duncan and Nimmo-Smith (1996) presented their line stimuli at separate locations 1.5° from fixation. Although the two lines appeared simultaneously, it is possible that spatial attention was drawn to one or the other stimulus on the basis of the color of the line to be judged first. The color of the stimuli could act as a spatial cue because the hues of the left or right horizontal stimulus and the upper or lower vertical stimulus were very different: red-orange and blue-green, respectively. Color cues are among the most effective in attracting attention (e.g., Theeuwes, 1992; Yantis, 1993), and observers can set themselves to segment the field into task-relevant and irrelevant objects on the basis of their knowledge of the target and nontarget colors (e.g., Baylis & Driver, 1993; Egeth, Virzi, & Garbart, 1984; Kaptein, Theeuwes, & van der Heijden, 1995). Further, there is evidence that the color dimension consists of a number of separate subdimensions representing relatively narrowband categories such as red, green, and blue (e.g., Nothdurft, 1993; Wolfe et al., 1995). Thus, in the study of Duncan and Nimmo-Smith, observers might have set themselves for redorange or blue-green, respectively, so that one or the other (horizontal or vertical) stimulus defined by the relevant color attracted spatial attention. In this way, the precise color of a color target (e.g., red or orange) would have been accessible relatively directly, and even dual color-color judgments would have required only shifting of attention between categorically close color subdimensions (e.g., from red-orange to blue-green). In contrast, the length of a length target, in single length and dual color-length and length-length judgments, would have been available only indirectly by shifting from the color to the form (size) domain. In this account, one would expect color judgments to be superior to length judgments, particularly if two attributes are to be judged, and one would expect the loss in dual-judgment accuracy between the targets reported first and second to be small for color-color judgments but large for color-length judgments and length-length judgments. Duncan and Ninuno-Smith's (1996) data conform with these expectations. Finally, color judgments would evade the interference of concurrent length judgments, as found by Duncan and Nimmo-Smith, simply because of the priority assigned to the color analysis. In summary, the alternative, color-cue account developed here goes some way toward explaining the overall pattern of findings reported by Duncan and Nimmo-Smith without

1344

MOLLER AND O'GRADY

exhibiting a fundamental discrepancy with the explanation provided for the results o f the present experiments. If the color-cue account is correct, then a different pattern o f results, consistent with the previous experiments, should be obtained w h e n the use o f color cues is prevented. This consideration was the rationale o f Experiment 4, in w h i c h we prevented the use o f color cues by using the same colors ( r e d - o r a n g e ) for both the horizontal line and the vertical line. In all other major respects, Experiment 4 was essentially the same as Experiment 1 o f Duncan and N i m m o - S m i t h (1996). Responses were made to panels presenting copies o f the response options, as in the previous experiments, but two mouse-clicks were required under dual-judgment conditions to liken the response procedure more to that used by Duncan and N i m m o - S m i t h . Thus, it was possible to analyze observers' response accuracies for each attribute separately (and according to which attribute was j u d g e d first or second) in dualj u d g m e n t conditions.

Method Observers. The same 4 observers who had taken part in Experiment 2, participated in Experiment 4, along with 2 new observers (1 woman). They were paid £5.00 per hour. Stimuli. On each trial, the following sequence of stimuli was shown on a black monitor background (see Figure 2). A trial started with the presentation of a central fixation dot and four location markers, each composed of four dots, one to the left and one to the right of fixation and one above and one below fixation. Each dot was 6 pixels (0.08 ° of visual angle) in diameter and filled with a faint white hue. Each location marker was centered 1.53 ° from fixation, and its longer and shorter "sides" (distances from the center of one dot to the center of the other) were 1.20 ° and 1.00 °, respectively. The longer "side" was horizontal for left and right location markers (to accommodate the horizontal target line) and vertical for top and bottom location markers (to accommodate the vertical target line). The fixation dot and location markers were presented for 400 ms. The removal of the fixation display was followed by a 67-ms interval with a blank screen. Then the fixation display reappeared, together with the target stimuli: two lines, one oriented horizontally and one vertically, that were 2 pixels in width (the lines had the appearance of solid stimuli, in contrast to the lines used by Duncan and Nimmo-Smith, 1996, which had the appearance of lined-up dots; see their Figure 1). The target stimuli were presented briefly, for a predetermined exposure duration, the horizontal line in the center of the left or right location marker and the vertical line in the center of the top or bottom location marker. Each of the lines was either red or orange (color maps were [220, 85, 20] and [178, 112, 30], respectively; luminances were 8.5 and 8.6 cd/m2, respectively) and either long (0.52 ° for the horizontal line, 0.55 ° for the vertical line) or short (0.43 ° for either line). To compensate for the horizontal length-discrimination advantage observed in Duncan and Nimmo-Smith's study, the length-discrimination difficulty was slightly reduced for the vertical line (22% length difference) relative to that for the horizontal line (18% length difference). A mask, consisting of a 1.00 ° × 0.62 ° random pattern of dots (0.08 ° diameter) of the two target colors (red and orange) was presented immediately after the termination of the target display. Two copies of the pattern dot mask were presented simultaneously, each centered inside the location marker of a preceding target line and oriented horizontally if appearing in the left or right marker or vertically if appeadng in the top or bottom marker. The mask was presented for 500 ms. The mouse pointer and a set of response alternatives (click panels) were presented immediately after the mask disappeared, in the lower part of the monitor. Under single-judgment conditions, a set of two click panels was presented below the screen center, one panel above the other. The stimuli were centered in each of the two click panels and corresponded to the two possible response options under single-judgment conditions. For example,

if the color of the horizontal line was to be judged, the upper and lower click panels displayed a horizontal red line and a horizontal orange line, respectively (both lines were of medium length [0.48 ° ] to avoid any confusion due to association with the actual length of a given colored line in the target display). If the length of the horizontal line was to be judged, the upper click panel showed a horizontal long line and the lower click panel showed a horizontal short line (both lines were of medium color-color map [199, 98, 25]--to avoid any confusion due to association with the actual color of a given length line in the target display). Corresponding options existed for vertical-line color judgments and vertical-line length judgments. When color was being judged, the red option always appeared in the upper click panel and the orange option in the lower panel; when length was being judged, the long and short options always appeared in the upper and lower panels, respectively. The predictability of this arrangement, which held under both single- and dual-jodgment conditions, eased the response procedure for observers. Under dual-judgment conditions, a set of four click panels was presented. The response panels were grouped into two pairs, each pair being outlined and corresponding to the two values of each of the target attribute(s) to be judged by the observer. Two responses were to be made, the first always to the two possible response options in the left-side outline and the second always to the options in the tight-side outline. For example, if the color and length of the horizontal line were to be judged (not necessarily in that order), the upper and lower panels on one side displayed horizontal red and orange lines, respectively, and the upper and lower panels on the other side showed horizontal long and short lines, respectively (all lines in the color panels were of medium length; all lines in the length panels were of medium color). The order of discriminations required to be made first and second (i.e., the response options displayed on the left and right) was counterbalanced within each judgment condition. If the color of the horizontal line and the color of the vertical line were to be judged, the upper and lower panels on one side showed horizontal red and orange lines, respectively, and the upper and lower panels on the other side showed vertical red and orange lines, respectively (all lines were of medium length). If the length of the horizontal line and the length of the vertical line were to be judged, the upper and lower panels on one side displayed horizontal long and short lines, respectively, and the upper and lower panels on the other side showed vertical long and short lines, respectively (all lines were of medium color). Corresponding options existed for color-length judgments (e.g., color judgment to horizontal line and length judgment to vertical line). The response panels stayed on the screen until one mouse click (single-judgment conditions) or two clicks (dual-judgment conditions) were recorded. There was then a blank interval of 1,000 ms until the beginning of the next trial. Target exposure durations. For the 4 experienced observers, the target exposure duration was set to 17 ms (the final duration achieved by each observer in Experiment 2) for practice and main sessions. For the 2 new observers, exposure durations were set in a preexperimental session with single-judgment conditions only. At the end of this session, the new observers achieved target exposure durations of 34 and 68 ms, respectively. Design. All observers performed all experimental conditions: singlejudgment control condition, requiring report of a single, prespecified attribute of one target object; dual-judgment one-object (across) condition, requiting reports of different-domain attributes of one target object; dualjudgment two-object (within) condition, requiting reports of same-domain attributes of different target objects; and dual-judgment two-object (across) condition, requiting reports of different-domain attributes of different target objects. Each observer undertook judgments of all single attributes and pairs of attributes. In dual-judgment two-object (across) conditions, the two possible pairs of attributes from different objects were horizontal (hue) and vertical (length), and horizontal (length) and vertical (hue); these attribute pairs are referred to as hueH & lengthV and lengthH & hueV, respectively. In dual-judgment two-object (within) conditions, the two possible pairs of attributes from different objects were hueH & hueV and lengthH & lengthK In the dual-judgment one-object condition, the two

DOMAIN-BASED VISUAL SELECrlON

1345

Figure 2. Illusta'ation of a display presented in Experiment 4. The judgment-relevant stimuli, shown at the top (a), are a long red horizontal line at the right location and a short orange vertical line at the upper location. The postdisplay masks are shown at the bottom (b). (Red, orange, and faint white are indicated by dark, intermediate, and light shadings, respectively.)

possible pairs of attributes to be judged were hue & lengthH and hue & lengthV. In single-judgment (one-object) conditions, the single attributes to be judged were hueH, hueV, lengthH, and lengthV. Thus, there were four single-attribute judgment conditions, two dual-attribute one-object judgment conditions, and four dual-attribute two-object judgment conditions, that is, 10 judgment conditions in total; 128 judgments were recorded for each of the 10 judgment conditions. Procedure. Each observer served in an initial practice session of about 30 rain, which was followed by an experimental session of about 1 hr on Day 1 and a second 1-hr session on Day 2. Each experimental session

presented the 10 judgment conditions in blocked order: 64 trials for each of the four single-attribute judgments (hueH, hueV, lengthH, and lengthV, in that order), 64 for each of the two dual-attribute une-object judgments (hue & lengthH and hue & lengthV, in order), 64 for each of the two dualattribute two-object (within) judgments (hueH & hueV and lengthH & lengthV, in order), and 64 for each of the two dual-attribute two-object (across) judgments (hueH & lengthV and lengthH & hueV, in order). For each observer, the conditions were presented in the order just described in one of the sessions and in the reversed order in the other session, with session order counterbalanced across observers. Within each dual-

1346

MOLLER AND O'GRADY

judgment condition, and for each observer and session, the object attributes to be reported first and second were counterbalanced.

Results The results are summarized in Table 4. The upper half of Table 4 presents the proportion of correct responses for each of the four main judgment conditions: single-attribute, dual-attribute oneobject (cross-domain), dual-attribute two-object (within-domain), and dual-attribute two-object (cross-domain). Dual judgments to one and the same object necessarily referred to different domains (hue & length). In contrast, dual judgments to two objects could be partitioned into within-domain (hue & hue, length & length) and cross-domain reports (hue & length). For all dual-attribute judgments, the average probability of one judgment being correct is given, along with the dual-attribute judgment cost relative to the single-attribute judgment condition (in parentheses). The lower half of Table 4 presents the joint probability of both (dual) judgments being correct, along with the joint probability correct expected on the assumption that dual judgments are based on independent single judgments (see Footnote 2). We statistically analyzed only the joint dual-judgment data (lower half of Table 4) by using a priori t tests. The average probabilities of one judgment being correct (upper half of Table 4) show a very similar pattern of effects (in text see values given in brackets). With only one exception, whenever an observer showed a performance difference in the joint dual-judgment data, he or she also showed a difference in the average probabilities of one judgment being correct.

Table 4

Judgment Accuracy for the Single-Attribute, Dual-Attribute OneObject (Cross-Domain), Dual-Attribute Two-Object (WithinDomain), and Dual-Attribute Two-Object (Cross-Domain) Judgment Conditions of Experiment 4

Observer

Single

Dual, one-object, across

Dual, two-object

O1 02 03 04 05 06

.885 .979 .924 .940 .969 .963

.834 (.051) .959 (.020) .932 (-.008) .924 (.016) .946 (.023) .934 (.029)

.893 (-.008) .920 (.059) .889 (.035) .898 (.042) .903 (.066) .826 (.137)

.842 (.043) .846 (.133) .887 (.037) .844 (.096) .860 (.109) .818 (.145)

M

.943

.922 (.021)

.888 (.055)

.850 (.113)

O1 02 03 04 05 06

.735 (.781) .922 (.924) .872 (.855) .856 (.883) .899 (.939) .879 (.928)

.817 (.783) .843 (.958) .789 (.854) .813 (.883) .821 (.939) .696 (.928)

.715 (.782) .723 (.958) .785 (.853) .719 (.884) .739 (.939) .688 (.928)

M

.861 (.885)

.797 (.891)

.728 (.891)

Within

Across


As can be seen from Table 4, when observers directed dual judgments to two separate objects, their accuracy tended to be greater when they reported attributes from the same domain (within-condition) than when they reported attributes from different domains (across-condition): .797 [.888] versus .728 [.850]; all Jail] observers showed the difference, t(5) = 3.36, p < .02 [t(5) = 3.37, p < .02]. Furthermore, the observers' dual-judgment accuracy was significantly greater when they responded to different-domain attributes from one object as compared with different-domain attributes from two objects: .861 [.922] versus .728 [.850]; all 6 [5] observers showed the difference, t(5) = 4.74, p < .01 [t(5) = 3.76, p < .02]. However, there was no significant difference in dual-judgment accuracy between reports to one object and dual judgments directed to attributes from the same domain in each of two objects (within-condition): .861 [.922] versus .797 [.888], t(5) = 1.83, ns [t(5) = 1.52, ns]. This pattern of effects replicates that observed in Experiments 1-3, providing evidence for both object-based and domain-based selection. A detailed examination of the data according to the particular attributes judged (hue or length) revealed that dual two-object judgments of hue (within: hue & hue) and dual two-object judgments of length (within: length & length) were each more accurate than dual two-object judgments of hue and length (across: hue & length): .839 and .754 versus .728. In other words, the advantage for within-domain judgments directed to two objects relative to cross-domain judgments directed to two objects was not simply due to any differential difficulty of attribute judgments. The data of Experiment 4 can also be analyzed as Duncan and Nimmo-Smith (1996) did; they examined the accuracies for each attribute judgment separately. The corresponding data are listed in Table 5, which shows the pattern of effects to be very similar to that of the joint dual-judgment accuracies (see lower half of Table 4), although the differences between conditions are less marked. Importantly, there was an object-based cost only for two-object cross-domain relative to one-object cross-domain dual judgments (.850 vs..922; 5 observers show the cost), but not for two-object within-domain dual judgments (.888 vs..922). And there was a domain-based cost for two-object cross-domain relative to twoobject within-domain dual judgments (.850 vs..888; all observers show the cost). Furthermore, the object- and domain-based effects show evidence of a difference between color and length judgments, as well as between judgments directed to the horizontal and vertical objects. For color and for the horizontal object, the object- and domain-based selection effects are reduced (2 observers failed to exhibit an object-based effect, and 2 different observers failed to exhibit a domain-based effect); for length and for the vertical object, the effects are stronger and more consistent (1 observer failed to exhibit a domain-based effect, and a different observer failed to exhibit an object-based effect). An ANOVA with main terms for judgment condition, domain (color vs. length), and object (horizontal vs. vertical) revealed significant main effects of condition, F(3, 15) = 11.51, MSE = 0.0035, p < .001, and of object, F(1, 5) = 7.57, MSE = 0.0044, p < .05, as well as a significant Condition × Object interaction, F(3, 15) = 3.98, MSE = 0.0015, p < .03. There was a slight judgment accuracy advantage for horizontal objects (object main effect), consistent with the results of Duncan and Nimmo-Smith (1996), and this advantage was more marked for (one- and two-object) cross-domain judgments.

DOMAIN-BASEDVISUAL SELECTION Table 5

Mean Probability Correct for Each Attribute Judgment in Experiment 4 Two-object Attribute Color H V M Length H V M M

Single

One-object, across

Within

Across

.970 .916 .943

.961 (.009) .887 (.029) .924 (.019)

.938 (.032) .888 (.028) .913 (.030)

.918 (.052) .822 (.094) .870 (.073)

.944 .943 .944

.926 (.018) .912 (.031) .919 (.025)

.857 (.087) .870 (.073) .864 (.080)

.839 (.105) .819 (.124) .829 (.115)

.943

.922 (.021)

.888 (.055)

.850 (.093)

Note. The dual-attributejudgment cost relative to the single-attribute judgment conditionis shown in parentheses. H = horizontal;V = vertical.

One further issue examined concerns possible interdependencies between the component decisions in the various dual-judgment conditions of Experiment 4. See the lower half of Table 4 for a comparison of the observed joint probabilities of both (dual) judgments being correct with the expected probabilities. There was evidence for negatively interdependent decisions only for twoobject cross-domain judgments, t(5) = -5.08, p < .005, and two-object within-domainjudgments, t(5) = -2.70, p < .05. For the one-object cross-domain judgment condition, there were no consistent differences between the observed and expected joint probabilities correct.

Discussion Experiment 4 used a variant of Duncan and Nimmo-Smith's (1996) paradigm in which the possible color values of the horizontal and vertical objects were chosen to be identical (both red-orange, rather than one red-orange and one blue-green, respectively) to prevent observers from selecting one or the other object by setting themselves for a particular range of colors. The results confirmed domain-based selection to be operating in addition to, and independently of, object-based selection, in agreement with the results of the previous experiments (in particular, Experiment 1). There was evidence of object-based and domain-based selection effects both when the joint accuracies of dual-attribute judgments were analyzed and when the accuracies for each attribute judgment were examined separately. However, in the latter analysis, they tended to be less consistent and reduced in magnitude for color judgments (which would be in agreement with the findings of Duncan & Nimmo-Smith, 1996, who reported that color judgments escaped the two-object cost) and for judgments of the horizontal object (which is also consistent with the findings of Duncan & Nimmo-Smith). Nevertheless, the majority of observers showed both object-based and domain-based effects even with color judgments and judgments of the horizontal objects, which is in agreement with the results of the previous experiments. A general distinction between attribute domains that has often been drawn is that between boundary and surface domains (e.g., Grossberg, 1987). Attributes defmed by boundary contours (e.g., shape, orientation, outline texture) can be distinguished by an

1347

essentially edge-based characteristic, whereas surface-based attributes (e.g., hue, luminance, surface texture) refer to the characteristics filling in a surface bounded by such an outline. Differences between form and surface domains have been obtained empirically (e.g., Boucart & Humphreys, 1994), with color sometimes exhibiting unusual behavior (Found & Mtiller, 1996; Nothdurft, 1993; Wolfe et al., 1995). However, in contrast to Duncan and Nimmo-Smith (1996), the present findings suggest that the color domain does not behave in a fundamentally different manner from boundary domains. One cross-domain example that has been much discussed is that concerning the boundary attribute of orientation and the surface attribute of color. Orientation is generally accepted as a basic feature, even though orientation may be detected through surface media such as differences in luminance, hue, and texture (Bravo & Blake, 1990; Cavanagh, Arguin, & Treisman, 1990; Found & Mialler, 1997; Wolfe, 1994). These findings demonstrate that even at the level of "primitive" features within nonsurface domains, surface media may play a role. More generally, attempts to draw a fundamental distinction between boundary and surface domains may not be helpful in the present context. General Discussion

Summary of Results The present experiments were designed to examine performance in a variety of dual-judgment conditions relative to a singlejudgment baseline, presenting form- and color-defined objects adapted from those used by Duncan (1984). Experiment 1 examined both within- and cross-domain dual judgments to two overlapping objects, but only cross-domain dual judgments to one (of two overlapping) objects. Experiment 2 compared within- and cross-domain dual judgments to one object presented singly. Experiment 3 combined the judgment conditions of Experiments 1 and 2, though presenting two overlapping objects in all instances. Experiment 4 examined both within- and cross-domain dual judgments to two objects as well as cross-domain dual judgments to one object, using similar stimuli to those presented by Duncan and Nimmo-Smith (1996). The results (see Table 6 for a summary) showed, fLrSt, that dual within- and dual cross-domain judgments were made more accurately when attributes of only one object had to be reported relative to attributes of two objects (Experiments 1, 3, and 4). Second, dual-judgment accuracy was greater when attributes from the same domain had to be reported relative to attributes from separate domains (Experiments 1, 2, 3, and 4). Thus, the present experiments demonstrated domain-based selection operating in addition to object-based selection. Experirnents 1, 3, and 4 demonstrated domain-based selection operating across two objects, as well as the operation of object-based selection. Experiments 2 and 3 confLrmed domain-based selection processes within one and the same object, both when only one object and when two objects were in view. Quantitatively, the object-based and domain-based selection effects were similar, for example, in Experiment 3:. 118 [.070] and .074 [.033], respectively (joint probability of both judgments correct [average probabilities of one judgment correct]). This, together with the fact that the two effects can be observed within the same experiment (Experiments 1, 3, and 4) and without statistical interaction (Experiment 3), suggests independence of the underlying processes.

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Table 6

Mean Judgment Accuracy for All Judgment Conditions in Experiments 1-4 Dual, one-object Experiment Single 1 2 3 4 1 2 3 4

Within

.937 .887 .900 (-.013) .892 .915 (-.023) .943 -.837 (.787) .836 (.801)

Across

Dual, two-object Within

Across

.929 (.008) .931 (.006) .881 (.056) .855 (.032) .888 (.004) .851 (.041) .813 (.079) .922 (.021) .888 (.055) .850 (.113) .871 (.876) .890 (.878) .776 (.869) .749 (.785) .774 (.808) .730 (.792) .644 (.780) •861 (.885) .797 (.891) •728 (.891)


One further issue examined concerned possible interdependencies between the component decisions in the dual-judgment conditions of Experiments 1-4. The power of the tests for interdependencies was limited by the small number of observers in individual experiments. To permit stronger tests, we reexamined interdependencies by comparisons of the data combined across corresponding conditions of the four experiments. When observers made dual within-domain judgments to one object (Experiments 2 and 3), accuracy was significantly higher than expected on the basis of the judgments being made independently: .837 vs..799, t(9) = 2.61, p < .05. In contrast, when observers made dual cross-domain judgments to one object (Experiments 1-4), accuracy was significantly lower than expected on the basis of the judgments being made independently: .814 v s . . 8 4 0 , t(19) = -3.68, p < .005. Similarly, in dual two-object judgment conditions (Experiments 1, 3, and 4), accuracy was less than expected, both when observers made dual within-domain judgments--.795 vs..851, t(15) = -3.22, p < .01--and, particularly marked, when they made dual cross-domain judgments--.708 vs..844, t(15) = -8.44, p < .001. This pattern of results is consistent with the operation of both domain-based and object-based constraints on selection. It suggests that dual judgments across domains and across objects are negatively interdependent such that when information from one domain and from one object is derived for judgment, derivation of information from the other domain and from the other object is systematically impaired. In contrast, dual judgments relating to same-domain attributes of one object tend to be positively interdependent.

Object-Based Selection The present experiments conf'Lrm the existence of object-based selection, consistent with the findings of Duncan (1984, 1993; Duncan & Nimmo-Smith, 1996) and Vecera and Farah (1994), as well as with the findings of other studies that used a different paradigm (e.g., Baylis & Driver, 1993). Duncan and Nimmo-Smith (1996) defined object-based attention as follows:

"Attending" to an object is a state in which multiple visual subsystems converge to work on its different properties, making them concurrently available for report and control of behavior . . . . Objects compete to enter the "attended" state, giving rise to interference when concurrent discriminations concern different objects ("divided attention") . . . . the key consideration in divided attention is simply the number of relevant objects in a display, not the nature or similarity of discriminations to be made. (p. 1076) Recently, Duncan (1996) attempted to account for object-based selection within the framework of his integrated competition hypothesis. According to this hypothesis, Of the many brain systems responding to visual input, perceptual, motor, cortical, subeortical, many and perhaps most are competitive Within each system . . . . representations of different objects may be mutually inhibitory . . . . Between systems, however, competition is integrated. As an object gains ascendancy in one system, this ascendancy tends also to be transmitted to others. "Attention" is the state that emerges as different brain systems converge to work on the same dominant object• (pp. 551-552) •

.

.

In summary, Duncan proposed a strong "late" object-based theory of selection according to which the different attributes of an object are selected together for control of behavior. Although attentional selection is "late," Duncan's (1996) framework allows for "early" modulation of visual responses by behavioral context, that is, priming: "Competition is controlled.., by advance priming of units responding to one kind of object rather than another" (p. 552). For example, when searching for target objects (e.g., fruit) of a particular color, "units selectively responsive to that color are preactivated in one or more brain systems in which color is coded. Inputs with the desired color gain a competitive advantage in the primed system; as such an input gains ascendancy in that system, it tends also to take control of others" (p. 552). The present findings are in accord with Duncan's (1996) integrated competition view. However, the domain-based effects observed in the present experiments question the strong assumption that all object attributes are selected together. Instead, the present findings argue that an object may be selected, and become reportable, on the basis of some dominant attribute(s), before all attributes become available---where selection is more strongly integrated for attributes from the same domain than for attributes from different domains. This idea would make adaptive sense because there are situations in which objects would need to take control of behavior before all object properties are fully analyzed; for example, a fast approaching object would need to trigger an evasive action before information about the object's shape, color, and so forth is fully integrated with the movement information. Besides conferring an adaptive advantage (of increased behavioral flexibility), a system operating along these lines would also be simpler. According to Duncan's (1996) "strong" object-based selection account, the selection system would somehow need to know that all object attributes are available. In contrast, according to a "weak" object-based selection account, selection could take place as soon as any object attributes have gained dominance, perhaps with selection involving a temporally extended process that passes other object attributes as they become available.

Domain-Based Selection As discussed in the introduction, domain-based selection effects would be predicted by both Allport's (1971; see also Treisman,

DOMAIN-BASED VISUAL SELECTION 1969) analyzer theory and the dimension-weighting account of Mtiller and his colleagues (Found & Mtiller, 1996; Krummenacher et al., 2000; Miiller et al., 1995). According to the analyzer theory, dual within-domain judgments were expected to incur an accuracy loss relative to dual cross-domain judgments because the former judgments compete for the same analyzer. Conversely, the dimension-weighting account predicted that dual within-domain judgments would show superior accuracy to dual cross-domain judgments because there is a limit to the attentional weight that can be allocated at any one time to the various dimensions on which an object is defined. The present Experiments 1-3 showed a clear within-domain dual-judgment advantage, emphasizing the importance of domain boundaries for visual selection. This resuk is consistent with the dimension-weighting account but inconsistent with the analyzer theory. Mtiller and his colleagues introduced the notion of dimensional weighting to account for the two findings in visual search for singleton (odd-one-out) feature targets under conditions of uncertainty as to the target identity: a cross-dimension search cost coupled with a dimension-specific intertrial effect. For crossdimension search cost, search RTs were increased when the target dimension was unpredictable on a trial (cross-dimension search) relative to when the target feature value was unpredictable (withindimension search; see also Treisman, 1988), without there being an RT cost for within-dimension search relative to a control condition in which both the target dimension and feature value were certain. For dimension-specific intertrial effect, detection of a target on a given trial was delayed when the target dimension changed from the preceding to the current trial but not when the target feature value changed (with the dimension remaining constant). Miiller and his colleagues reasoned (a) that target detection is based on dimension-specific feature contrast or saliency mechanisms (e.g., Wolfe, 1994), which signal the presence of a target within a given dimension (without specifying its feature value), and (b) that dimension change incurs a cost because attentional weight must be shifted to the new dimension to amplify the target's saliency signal within this dimension above the detection threshold. Recent positron emission tomography (PET) work by Corbetta, Miezin, Dobmeyer, Shulman, and Petersen (1990, 1991) is broadly consistent with the idea of attentional weighting of dimensions (see also Pollmann, Weidner, Mtiller, & v o n Cramon, 2000). The participants of Corbetta et al. (1990, 1991) had to perform discrimination tasks, either within a single prespecified dimension or across several dimensions, whereas their cerebral blood flow was scanned by means of PET. Corbetta et al. compared two conditions. In the first, participants could allocate their undivided attention to a constant dimension, such as motion (i.e., motion velocity). In contrast, in the second, divided-attention baseline, condition, participants could not predict the target dimension with certainty. Corbetta et al. (1990, 1991) found that, when participants could consistently allocate their attention to a single dimension (e.g., motion), the blood flow to the task-relevant cortical processing area (V5/MT) was increased, and target detectability was enhanced relative to the baseline condition. This pattern of results is consistent with the idea of relatively early attentional modulation or weighting of dimension-specific input modules. The domain-based selection effects revealed in the present study are perhaps best interpreted within the dimension-weighting framework, s That is, the finding of an added cross-domain cost

1349

when dual judgments were directed to one and the same object (Experiments 2 and 3) points to a capacity limitation in how multidimensional features are bound into a coherent object representation-that is, in terms of the dimensional weighting account, a limit to the total weight available to be allocated to the object's dimensions.

Interrelationship Between Object-Based and Domain-Based Selection The results of Experiments 2 and 3 suggest that the effects of domain-based and object-based selection operate independently of each other. In Experiment 2, domain-based selection was manifest, although arguably, there was no need for object-based selection (because only one object was in view); in Experiment 3, the domain-based and object-based effects were statistically noninteractive (i.e., additive). A speculative account of how domain-based and object-based selection processes interrelate is as follows: Domain-based selection, considered to be a form of segmentation, is assumed to occur first, followed by object-based selection. Objects become available for selection as soon as a responsecritical attribute domain drives segmentation of the display into separable entities (e.g., segmentation driven by a weighted domain, d 1, allows one object, o x, to be passed on for further processing, making the domain-specific attributes of o 1 available for report). In the case of dual judgments directed to separate objects, if the response-critical attribute domain is the same for both objects (e.g., dO, then a switch from one domain to another (e.g., from d~ to d2) is unnecessary, only a switch from o 1 to 02. In contrast, if the attributes involved are from different domains, then domain switching becomes necessary, resulting in a domain-based processing cost. If only one object is response-critical (e.g., o 0, but the relevant attributes are from different domains (d I and d2),

s Support for domain-based selection also comes from several other sources, notably Boucart and Humphreys (1992, 1994) and Kanwisher, Driver, and Machado (1995). For example, Kanwisber et al. (1995), who were concerned with repetition blindness (RB), asked their participants to make dual-color, dual-shape, or dual color-shape judgments to two lateral objects presented briefly. When the same dimension was to be judged for both stimuli, performance was impaired only by repetition along that dimension (but not repetition along the unreported dimension). However, when different dimensions had to be judged for the two stimuli, performance was affected by repetition on both dimensions. Kanwisher et al. (1995) concluded that "when subjects attend to one dimension for both stimuli, they can ignore the other dimension so successfully that it has absolutely no impact on their performance" (p. 331), whereas attending to "the color of one object and the shape of another simultaneous object results in both dimensions being accessed for both objects" (p. 303). However, scrutiny of the data of Kanwisher et al. reveals that attending to two separate dimensions did incur a performance cost relative to attending to a single dimension even when RB was not operating (e.g., see the "non-repeat" data in their Table 4). Note that analysis of the data of the present Experiments 1, 3, and 4 for RB effects, along the lines of Kanwisher et al.'s (1995) analysis, failed to produce any evidence of RB operating in the two-object within-domain and cross-domain judgment conditions. Furthermore, cross-dimension costs were obtained even when there was only one stimulus to be judged and when there was only one object in the display (Experiment 2). Thus, even if RB had been operating in the present experiments, it could not account for the whole pattern of results.

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then a domain-based switching cost is still incurred. This proposed scheme of domain and object switches required in the various dual-judgment conditions can be presented as follows: two-object (across): d I --~ o I ~ d 2 ~ 02 two-object (within): d 1 ~ o I ~ 02 one-object (across): d I ~ o I ~ d 2 one-object (within): d 1 ~ o 1 This scheme is in accord with Duncan's (1996) integrated competition view but permits that all object attributes not necessarily be selected together. Rather, by this "weak" object-based selection account, an object may be selected on the basis of some weighted (response-critical) attribute(s) before all attributes become available. Importantly, object selection is based on a mere spatial object array rather than a spatially invariant object representation, with the array being formed by domain-based segmentation processes--a position consistent with the findings of Kramer et al. (1997). 6 The above scheme by itself does not explain how segmentation driven by a weighted domain allows one of two objects to be selected, because the segmentation process would make domainspecific descriptions of both objects available. A possible solution might work along the lines proposed by Cave and Kosslyn (1989). They suggested that object selection involves a shape-specific mental image representation that is used as a template for comparison against the perceptual representation. The appropriate image may be constructed in advance on the basis of knowledge of which object is to be expected. Applied to the present experiments, the appropriate images might be those of a horizontal rectangle and a vertical rectangle that are consistently to be judged in terms of specific attributes (specified by the instruction at the start of a particular judgment condition). Such "images can interact with incoming perceptual information to select some parts of the input and filter out others," with "visual parsing [perhaps being] accomplished, at least in part, by top-down activation within the visual system" (Cave & Kosslyn, 1989, p. 159). The finding of a twoobject judgment cost would thus be due to the "difficulty of parsing a second target object": "When attention to both objects is required, subjects must first activate the representation for one object and then switch and activate the other" (Cave & Kosslyn, 1989, p. 160). According to the present account, such a switch could occur as soon as the judgment-relevant attribute(s) of object o I has been accessed. Conclusion To conclude, previous work has tended to support object-based selection without necessarily excluding support for domain-based processes. The present set of experiments confirms object-based visual selection while at the same time demonstrating domainbased visual selection. The results favor a domain-based account in terms of dimensional weighting rather than in terms of independent analyzers.

6 Alternatively, domain-based selection might occur after object-based selection. That is, "once the object selection mechanism has chosen all the information from a particular region, perhaps the mechanisms that analyze the selected information can shift from form analysis mode to color

analysis mode, with a cost associated with each mode shift" (Kyle Cave, personal communication, February 2, 1999). Further work will be required to decide which type of selection operates first. References Allport, D. A. (1971). Parallel encoding within and between elementary stimulus dimensions. Perception & Psychophysics, 10, 104-108. AUport, D. A. (1980). Attention and performance. In G. Claxton (Ed.), Cognitive psychology: New directions (pp. 112-153). London: Routledge & Kegan Paul. Baylis, G. C., & Driver, J. (1993). Visual attention and objects: Evidence for hierarchical coding of location. Journal of Experimental Psychology: Human Perception and Performance, 3, 451-470. Boucart, M., & Humphreys, G. W. (1992). Global shape cannot be attended without object identification. Journal of Experimental Psychology: Human Perception and Performance, 18, 785-806. Boucart, M., & Humphreys, G. W. (1994). Attention to orientation, size, luminance, and color: Attentional failure within the form domain. Journal of Experimental Psychology: Human Perception and Performance, 20, 61-80. Bravo, M., & Blake, R. (1990). Preattentive vision and perceptual groups. Perception, 19, 515-522. Cavanagh, P., Arguin, M., & Treisman, A. M. (1990). Effect of surface medium on visual search for orientation and size features. Journal of Experimental Psychology: Human Perception and Performance, 16, 479-491. Cave, K. R., & Kosslyn, S. M. (1989). Varieties of size-specific visual selection. Journal of Experimental Psychology: General, 118, 148-164. Cheal, M., Lyon, D. R., & Gottlob, L. R. (1994). A framework for understanding the allocation of attention in location-precued discrimination. Quarterly Journal of Experimental Psychology, 47A, 699-739. Corbetta, M., Miezin, F. M., Dobmeyer, S., Shulman, G. L., & Petersen, S. E. (1990). Attentional modulation of neural processing of shape, colour, and velocity in humans. Science, 248, 1556-1559. Corbetta, M., Miezin, F. M., Dobmeyer, S., Shulman, G. L., & Petersen, S. E. (1991). Selective and divided attention during visual discriminations of shape, colour and speed: Functional anatomy by positron emission tomography. Journal of Neuroscience, 11, 2382-2402. Downing, C. J. (1988). Expectancy and visual-spatial attention: Effects on perceptual quality. Journal of Experimental Psychology: Human Perception and Performance, 14, 188-202. Duncan, J. (1984). Selective attention and the organization of visual information. Journal of Experimental Psychology: General, 114, 501517. Duncan, J. (1993). Similarity between concurrent visual discriminations: Dimensions and objects. Perception & Psychophysics, 54, 425-430. Duncan, J. (1996). Cooperating brain systems in selective perception and action. In T. Inui & J. L. McClelland, Attention and performance XVI. Information integration in perception and communication (pp. 549578). Cambridge, MA: MIT Press. Duncan, J., & Nimmo-Smith, I. (1996). Objects and attributes in divided attention: Surface and boundary systems. Perception & Psychophysics, 58, 1076-1084. Egeth, H. E., Virzi, R. A., & Garbart, H. (1984). Searching for conjunctively defined targets. Journal of Experimental Psychology: Human Perception and Performance, I0, 32-39. Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16, 143-149. Eriksen, C. W., & Hoffman, J. E. (1973). The extent of processing of noise elements during selective encoding from visual displays. Perception & Psychophysics, 14, 155-160. Eriksen, C. W., & Yeh, Y.-Y. (1985). Allocation of attention in the visual field. Journal of Experimental Psychology: Human Perception and Performance, 11, 583-587.

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Pollmann, S., Weidner, R., Mtiller, H. J., & yon Cramon, D. Y. (2000). A fronto-posterior network involved in visual dimension changes. Journal of Cognitive Neuroscienee, 12, 480-494. Posner, M. I. (1980). Orienting of attention. Quarterly Journal of Experimental Psychology, 32, 3-25. Posner, M. I., Snyder, C. R. R., & Davidson, B. J. (1980). Attention and the detection of signals. Journal of Experimental Psychology: General, 109, 160-174. Tbeeuwes, J. (1992). Perceptual selectivity for color and form. Perception

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Received September 30, 1998 Revision received May 28, 1999 Accepted July 22, 1999 •

Dimension-Based Visual Attention Modulates Dual

Dimension-Based Visual Attention Modulates Dual

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